Exotic Plant Management in California Annual Grasslands

TWENTY TWOExotic Plant Management in California Annual GrasslandsJOSEPH M. DITOMASO, STEPHEN F. ENLOE,MICHAEL J. PITCAIRNCalifornia grasslands have experienced tremendous changesin vegetative composition over the last few centuries, withsignificant declines in native perennial vegetation and concomitantincreases in aggressive, non-native, weedy species.The changing grassland composition is the result of numerousprocesses that are reviewed elsewhere in this volume,including many plant introductions, primarily from Europeand Asia (Heady 1977). Burcham (1956) characterized fourwaves of invasion that were dominated by European annualgrasses and shallow-rooted winter annual forbs. In the lasthalf century, a fifth wave of annual invaders, characterized bya later season phenology and deeper rooting pattern, hasmoved across the grasslands with unrelenting progress. However,individual species from all of these waves of invasion stilldominate large grassland areas, with wide population fluxesfrom year to year, depending upon environmental conditionsand local- and landscape-scale processes (Pitt and Heady 1978).As observed in every wave of invasion, probably the moststriking shared characteristic of invaders in California’s valleyand coast range grasslands has been the success of the annuallife history. This chapter will focus primarily on annual grasslandsystems occupying the coastal ranges, central valley, andSierra Nevada foothills. To be consistent with terminologyelsewhere in the book, annual grassland systems will bereferred to as “valley grassland” systems. In contrast, some ofthe more dominant invasive species in coastal prairies areperennial grasses and shrubs (see D’Antonio et al., Chapter 6).Similarly, perennial species represent the vast majority ofinvasive non-native species in wetland or riparian areaswithin grassland settings.Over 73% of the major invasive non-native species in valleygrassland are winter annuals, about 11% are biennials, and16% typically act as perennials, although members in bothof these lesser groups can sometimes act as biennials orannuals. Although some woody species, such as Himalayablackberry (Rubus armeniacus) and the native junipers(Juniperus spp.), can encroach into grasslands, they are notconsidered as significant a threat as herbaceous species invalley grasslands. In some coastal range grasslands, includingsome that are annual-dominated, it is not uncommon forinvasive shrubs, such as the brooms (Genista monspessulana,Cytisus scoparius, Spartium junceum) or gorse (Ulex europaeus),to invade these systems (see Chapter 6). Of the 44 most commonlyencountered invasive valley grassland species in thestate, about 80% belong to either the sunflower (Asteraceae)or grass (Poaceae) family, with an equal distribution betweenthe two families (Table 22.1). Other important families ofnon-native plants in these grasslands include the Brassicaceae,Geraniaceae, and Fabaceae.Of the interior valley grassland invasive plants recently listedin the “California Invasive Plant Inventory” published by theCalifornia Invasive Plant Council (Cal-IPC 2006), eight speciesare classified in the “High” concern category (Table 22.1). Theseare considered the species having the greatest impact on valleygrassland habitats and include yellow starthistle (Centaureasolstitialis), artichoke thistle (Cynara cardunculus), Scotch thistle(Onopordum acanthium), barb goatgrass (Aegilops triuncialis),red brome (Bromus madritensis ssp. rubens), downy brome (Bromustectorum), and medusahead (Taeniatherum caput-medusae).For close-ups showing distinctive features of yellow starthistle,barb goatgrass, and medusahead see Figures 22.1 and 22.2. Allof these species belong to either the sunflower or the grassfamily. Among the remaining species occurring in valley grasslandon the Cal-IPC list, 48% are listed as “Moderate” in theirimpacts, and 34% are categorized as “Limited.”Among the species listed as “High” in their impacts,downy brome and yellow starthistle are considered the twomost invasive species in the 17 western states, with 56 and15 million acres infested, respectively (Duncan and Clark2005). Medusahead is considered the ninth most invasivespecies in the western United States, with an estimatedinfestation of 2.4 million acres. Among the most problematic281

TABLE 22.1Most Common Non-native Invasive Species in California Valley and Foothill Grasslands, Including their GrowthForm and Cal-IPC ClassificationCommon name Scientific name Family Growth habit Cal-IPC categoryFennel Foeniculum vulgare Apiaceae Perennial HighItalian thistle Carduus pycnocephalus Asteraceae Winter annual ModerateSlenderflower thistle Carduus tenuiflorus Asteraceae Winter annual LimitedWoolly distaff thistle Carthamus lanatus Asteraceae Winter annual Moderate alertPurple starthistle Centaurea calcitrapa Asteraceae Annual to perennial ModerateMalta starthistle (tocalote) Centaurea melitensis Asteraceae Winter annual ModerateYellow starthistle Centaurea solstitialis Asteraceae Winter annual HighSquarrose knapweed Centaurea virgata var. Asteraceae Perennial ModeratesquarrosaRush skeletonweed Chondrilla juncea Asteraceae Biennial ModerateBull thistle Cirsium vulgare Asteraceae Biennial ModerateArtichoke thistle Cynara cardunculus Asteraceae Perennial ModerateSmooth catsear Hypochaeris glabra Asteraceae Winter annual LimitedCommon catsear Hypochaeris radicata Asteraceae Winter annual ModerateScotch thistle Onopordum acanthium Asteraceae Biennial HighBristly oxtongue Picris echioides Asteraceae Annual or biennial LimitedTansy ragwort Senecio jacobaea Asteraceae Biennial LimitedBlessed milk thistle Silybum marianum Asteraceae Winter annual LimitedDyer’s woad Isatis tinctoria Brassicaceae Biennial ModerateCalifornia burclover Medicago polymorpha Fabaceae Winter annual LimitedRose clover Trifolium hirtum Fabaceae Winter annual ModerateBroadleaf filaree Erodium botrys Geraniaceae Winter annual Not listedShortfruited filaree Erodium brachycarpum Geraniaceae Winter annual Not listedRedstem filaree Erodium cicutarium Geraniaceae Winter annual LimitedCommon St. Johnswort Hypericum perforatum Hypericaceae Perennial ModerateBarb goatgrass Aegilops triuncialis Poaceae Winter annual HighSilver hairgrass Aira caryophyllea Poaceae Winter annual Not listedSlender oat Avena barbata Poaceae Winter annual ModerateWild oat Avena fatua Poaceae Winter annual ModerateBig quakinggrass Briza maxima Poaceae Winter annual LimitedLittle quakinggrass Briza minor Poaceae Winter annual Not listedRipgut brome Bromus diandrus Poaceae Winter annual ModerateSoft brome Bromus hordeaceus Poaceae Winter annual LimitedRed brome Bromus madritensis Poaceae Winter annual Highssp. rubensDowny brome (cheatgrass) Bromus tectorum Poaceae Winter annual HighHedgehog dogtailgrass Cynosurus echinatus Poaceae Winter annual ModerateOrchardgrass Dactylis glomerata Poaceae Perennial LowTall fescue Festuca arundinacea Poaceae Perennial ModerateMediterranean barley Hordeum marinum Poaceae Winter annual ModerateHare, smooth and Hordeum murinum Poaceae Winter annual Moderatewall barleyItalian ryegrass Lolium multiflorum Poaceae Winter annual ModerateMedusahead Taeniatherum Poaceae Winter annual Highcaput-medusaeSquirreltail fescue Vulpia bromoides Poaceae Winter annual Not listedRattail fescue Vulpia myuros Poaceae Winter annual ModerateBellardia Bellaria trixago Scrophulariaceae Winter annual Limitedor biennial

FIGURE 22.1 Close-ups of flowerheads of two common invasive Centaurea species in California.Left: C. melitensis (tocalote or Malta starthistle). Right: C. solstitialis (yellow starthistle). Note that thespines on C. solstitialis are approximately twice the length of those found on C. melitensis.invasive plants in California’s valley grasslands, the two mostcommon species are yellow starthistle and medusahead (seeFigures 22.1, 22.2). Downy brome, despite its widespreadwestern impact, has caused significant problems only in thenortheastern (Modoc Plateau) part of the state, so it will notbe discussed further here.ImpactsInvasive non-native plants in California grasslands can havesignificant ecological and economic impacts. Although amore thorough treatment of ecological impacts can be foundin Chapter 6, a number of key impacts will be discussed here.Since the California grasslands have historically played amajor role in livestock production, weed impacts have oftenbeen strongly associated with that industry. Impacts includeinterference with grazing practices; reductions in forageproductivity or quality; increased costs of managing and producinglivestock; reduced animal weight gains; reduced qualityof meat, milk, wool, and hides; and livestock poisoning.Medusahead, for example, is of low value because of its highsilica content (George 1992). This reduces the forage qualityand makes it less palatable to livestock and wildlife comparedto other forage grasses. In areas heavily infested withmedusahead, livestock carrying capacity can be reduced byas much as 75 to 80% (Major et al. 1960; Hironaka 1961;George 1992). One of the more recent animal issues to cometo light is the impact of many invasive plants on the horseindustry, especially where small acreage development isoccurring. Invasive forbs such as yellow starthistle, Russianknapweed, houndstongue, and tansy ragwort can result inpoisoning and death to horses when consumed at high levels(Cordy 1954).Direct impacts of grassland weeds on humans and qualityof life include increased allergens in the air, sickness ordeath from inadvertent consumption of poisonous plants,damage to recreational equipment, and injury or discomfortfrom physical contact with spiny thistles such as milk thistleand yellow starthistle or abrasive parts such as the awnsof ripgut brome (Bromus diandrus). With increased use ofgrasslands for recreational activity, including hiking andbiking, direct impacts on humans are now much moreprevalent (DiTomaso 2000).In addition to impacts on humans and animals, many invasiveplants may alter ecosystem structure and functionalprocesses, including hydrologic, fire, and nutrient cycles. Structuralchanges in invaded plant communities typically causereduced native species richness and diversity and changes incanopy structure (Belcher and Wilson 1989; Parmenter andMacMahon 1983; Rikard and Cline 1980; Wallace et al. 1992).In one study reported on by DeLoach (1991), the number ofplant species present in California grasslands increased by 35%following biological control of common St. Johnswort (Hypericumperforatum). When large scale conversions of vegetationallife history strategies and phenology occur, the potential forfunctional changes to hydrology, nutrient, and fire cycles probablyincreases. This has been observed with the late-maturing,deeply rooted forb yellow starthistle when it invades the earlymaturing,shallow-rooted annual grass communities. Yellowstarthistle has been shown to deplete soil moisture reservesand alter water cycles in annual grasslands (Enloe et al. 2005).This could cause large annual economic losses in water conservationcosts in California. In Siskiyou County, for example,it was estimated that the potential water loss due to yellowstarthistle would be more than 100,000 m 3 , or 26,400,000gallons, per year (Enloe 2002). The depletion of soil moistureEXOTIC PLANT MANAGEMENT 283

widespread problems. While many weeds, such as yellowstarthistle, are well beyond the scope of EDRR in California,there are still many species to which this approach can beapplied locally and regionally. In grasslands these includeScotch thistle, woolly distaff thistle, artichoke thistle, andspotted knapweed. EDRR may also be integrated into adaptivemanagement strategies with little conflict.Mechanical ControlA number of mechanical methods are used to control herbaceousgrassland weeds, including hand labor, mowing, andcultivation techniques. In many cases these techniques arenot practical or cost-effective, but there are situations inwhich they can be used very effectively. They also can beused effectively and with little training for volunteer-basedstewardship programs or “weed pulling days.”Hand LaborHand labor methods for weed control in grasslands includehand pulling and tools such as weed whips, sling blades,clippers, shovels, hoes, mattocks, and Weed Wrenches.There has been little published research comparing handlabor to other weed control methods in the California grasslands,so most available information has been translatedfrom agricultural systems or is anecdotal in nature. Handlabor is widely used for controlling small weed patches butis difficult and expensive to use on large infestations. Handlabor is also more commonly used where volunteer help isavailable and in follow-up control programs where few plantsremain after several years of intensive management (Sheleyet al.1998). The relative success of hand labor in grasslandsis dependent upon removal of a plant’s growing points. Forannuals and biennials, severing the plants below the crown(i.e., cutting or breaking plants off a few inches below the soilsurface) is all that is necessary. For creeping perennials,removing the vertical and lateral roots or rhizomes is essentialfor success. The difficulty of doing this in most soil typesis immense, with the exception of moist, sandy soils. Therefore,hand weeding techniques are typically more effective onannual and biennial species and less effective on perennials,which often regenerate from adventitious buds on deep lateraland vertical roots. Plant height is also important, as lowgrowingrosettes are generally more difficult to remove byhand pulling than plants with bolted or elongated stems aslong as the soil remains moist. These factors result in optimalcontrol timing, which is when plants reach the late boltingto early bud stage before soils become too dry. This timingalso often coincides with the end of the germination periodfor many winter annual weeds that dominate Californiagrasslands. This reduces the potential for new cohorts toemerge following the disturbance caused by hand labor.A benefit of hand removal is that desirable species, if present,can be left in place. In Marin County, repeated handpulling with a Weed Wrench not only proved to be aneffective method for French and Scotch broom control insmall infestations, but also encouraged native plant recovery(Alexander and D’Antonio 2003a). It was also more effectivethan mowing.Mowing and ClippingMowing is a common vegetation management techniqueprimarily used along roadsides and right-of-ways throughoutCalifornia. Its main purposes include maintaining the safetyrecovery zone or “clear zone,” keeping visibility high, andreducing fuel loads to prevent wildfires. Mowing has alsobeen used for weed management in both the interior andcoastal California grasslands. Although not generally effectivefor weed eradication, mowing has primarily been used toreduce seed production of both exotic grasses and forbs, andproper timing is critical for its effectiveness. Mowing tooearly in the spring increases light penetration without removinga significant proportion of weed biomass. This generallybenefits weedy species by stimulating rapid recovery ofgrowth while soil moisture is still abundant. Mowing toolate in the summer does not prevent seed production andmay serve to work the seeds down into the seed bank betterand disseminate weed seeds to new areas.The optimum time for mowing most annual species is inthe flowering stage before seed development. This generallyresults in the greatest reduction of seed production. However,when soil moisture is plentiful following mowing, the effectivenessof control may be greatly reduced. For example,mowing diffuse knapweed (Centaurea diffusa) under adequatesoil moisture resulted in compensatory growth and ultimatelygreater seed production compared to plants in an unmowedarea (Sheley et al. 1999). Although mowing is more oftenused as a tool for control of noxious annual weeds, it can successfullycontrol some biennial and perennial weeds (Benefieldet al. 1999; Tyser and Key 1988). Repeated mowing on perennialbroadleaf species can prevent seed production, reduceroot carbohydrate reserves, and give advantages to desirableperennial grasses.Properly timed mowing has been demonstrated to be asuccessful tool for the control of yellow starthistle (Benefieldet al. 1999). However, the growth form of the plants is criticalfor success. If plant architecture is characterized by profusebasal branching, then mowing tends to make the growthform prostrate, and the result is limited control. However, ifthe growth form is primarily elongated stems with little basalbranching, such as those found within dense cover, mowingcan be very effective (Benefield et al. 1999). Consequently,mowing for yellow starthistle control is best employed wherecompetition for light results in elongated yellow starthistlestems.Mowing or clipping for vegetation management has alsobeen shown to shift species composition in California coastalprairie from exotic annual grasses to exotic forbs (Hayes andHoll 2003b) or mixes of native and exotic forbs (Maron andJefferies 2001). In pastures, mowing may reduce grass canopyEXOTIC PLANT MANAGEMENT 285

TABLE 22.3List of Biological Control Agents Against Grassland Weeds, Approved for Release in CaliforniaWeed Scientific name Biological control agent Distribution Year of intro Infestation ControlCommon St. Johnswort Hypericum perforatum Agrilus hyperici Established widely 1950 Moderate UnknownChrysolina hyperici Unknown 1945 Unknown UnknownChrysolina quadrigemina Established widely 1946 Heavy ExcellentChrysolina varians No establishment 1952 Absent UnknownZeuxidiplosis giardi Established limited 1950 Light PoorDiffuse knapweed Centaurea diffusa Bangasternus fausti Established widely 1994 Moderate GoodLarinus minutus Established widely 1995 Moderate GoodSphenoptera jugoslavica Established widely 1980 Heavy UnknownUrophora affinis Established widely 1976 Light PoorUrophora quadrifasciata Established limited 1990 Slight PoorSpotted knapweed Centaurea maculosa Agapeta zoegana Established limited 1993 Light Poor(C. biebersteinii)Cyphocleonus achates Established limited 1993 Light PoorLarinus minutus Established limited 1995 Light GoodTerellia virens Established limited 1995 Light PoorUrophora affinis Established widely 1976 Light PoorUrophora quadrifasciata Established limited 1990 Light PoorSquarrose knapweed Centaurea virgata Bangasternus fausti Established widely 1996 Moderate Excellentvar. squarrosaCyphocleonus achates No establishment 1995 Absent NoneLarinus minutus Established widely 1997 Heavy ExcellentSphenoptera jugoslavica Established limited 1998 Moderate FairTerellia virens No establishment 1998 Absent NoneUrophora affinis No establishment 1998 Absent NoneUrophora quadrifasciata Established widely 1998 Light PoorPuncturevine Tribulus terrestris Microlarinus lareynii Established widely 1961 Heavy ExcellentMicrolarinus lypriformis Established widely 1961 Heavy ExcellentTansy ragwort Senecio jacobaea Longitarsus jacobaea Established widely 1969 Heavy ExcellentPegohylemyia seneciella Established widely 1966 Light PoorTyria jacobaeae Established widely 1959 Light GoodMediterranean sage Salvia aethiopis Phrydiuchus tau Established limited 1976 Light Unknown

permit for release on yellow starthistle in California. The rustwas originally collected from yellow starthistle in its nativerange of Turkey in 1978 (Woods and Villegas 2004). Pucciniajaceae var. solstitialis is an obligate parasite of yellow starthistleand is associated with the thistle over a wide area, at leastfrom Spain to Turkey (Savile 1970). It completes its life cycleon a single host plant and has all five spore forms. It causesnonsystemic foliar infections that can reduce fresh and dryweights of inoculated yellow starthistle in controlled studies(Bruckart 1989; Shishkoff and Bruckart 1993). Spores, however,may not persist over the winter.Despite considerable research on host specificity of Pucciniajaceae var. solstitialis and nontarget plant safety at the USDA-Agricultural Research Service (ARS) facility in Ft. Detrick,Maryland, little is known about the plant-pathogen interactionunder field conditions, including information on themost effective inoculation timing window to maximize infectionrates and foliar damage. Furthermore, while the rust hasbeen shown to reduce root biomass, it is unknown whetherthis is due to a decrease in lateral root production or to theability of yellow starthistle to produce deep roots. Such informationmay significantly impact our capability to predictthe effect of the pathogen on yellow starthistle’s ability todevelop tolerance to water stress. Moisture levels have beenshown to correlate directly with seed production. If rootdepth is limited by pathogen stress, this may subsequentlyimpact plant growth, seedhead production, and ultimatelyreproductive output. In addition, nothing is known on howthe Puccinia rust will interact with the established biologicalcontrol insects or how it will change the competitive abilityof yellow starthistle compared to other grassland vegetationunder different environmental conditions. Results of thesestudies can greatly improve the capacity to predict the potentialeffectiveness of the Puccinia rust under a number ofabiotic and biotic conditions.Chemical ControlHerbicides are an important method of weed control in grasslandsystems. Herbicides can be applied to grasslands by anumber of methods, including fixed-wing aircraft, helicopters,ground applicators, backpack sprayers, and rope wickapplicators. Herbicides registered for use in grasslands of thewestern United States are listed in Table 22.4, along with pertinentinformation on each compound. Some of these products,including picloram and metsulfuron, are not registeredin California. Of these compounds used in grasslands, theauxinic, or growth regulator, herbicides have played the mostimportant role in broadleaf weed control.For large-scale use, herbicides are typically considered to bethe most economical option. The most widely used grasslandherbicides in California are those that have postemergenceactivity (DiTomaso 2000). These include 2,4-D, triclopyr,dicamba, clopyralid, chlorsulfuron, and glyphosate. Ofthese, clopyralid, 2,4-D, triclopyr, and dicamba are growthregulator herbicides that are selective on broadleaf speciesand have little activity on grasses. Clopyralid also has excellentpreemergence activity and is highly effective for control ofyellow starthistle and other members of the Asteraceae. However,it may have some negative impacts on some native plantswithin the Asteraceae, Fabaceae, Polygonaceae, and Apiaceaeand short-term impacts on Violaceae (Reever Morghan et al.2003). Chlorsulfuron is an amino acid inhibitor and is alsovery effective on most broadleaf species, particularly membersof the mustard (Brassicaceae) and figwort (Scrophulariaceae)families. It has both preemergence and postemergence activityon most species, but only preemergence activity on yellowstarthistle. Like the growth regulator compounds, chlorsulfuronis fairly safe on most grasses. Glyphosate is a nonselectivearomatic amino acid inhibitor and provides excellentcontrol of annual and perennial grasses and broadleaf weeds,but it will also damage desirable plants.Aminopyralid is a new growth regulator herbicide registeredin California in 2006. It has about three times the activity ofclopyralid on yellow starthistle. In addition to its activityagainst yellow starthistle, aminopyralid has a broader spectrumof selectivity and has also been shown to be very effectiveon knapweeds, many other thistles, and fiddlenecks(Amsinckia spp.) (Kyser et al. 2007). Both products will injurenative legumes during the growing season, but some can beused safely when treatments are made after senescence or duringthe dormant phase of perennial legumes. Another newlyregistered herbicide (not yet in California), imazapic, hasproven to be very effective on medusahead, downy brome,ripgut brome, barb goatgrass, and other annual grasses, withoutsignificantly injuring seedlings of many native perennialgrass or broadleaf species (Kyser et al. 2007).Timing of herbicide applications can determine the effectivenessof the treatment. For yellow starthistle control, thebest timing for application of clopyralid and aminopyralidseems to be between December and the end of March,depending on the location (DiTomaso et al. 1999b). Fall orspring applications of imazapic can be effective for the controlof annual grasses, depending on the location. In areaswith snowpack, spring applications may be more desirable,but in typical Central Valley or foothill conditions, fall applicationshave proven successful. For perennials, timing ofapplication can depend on the herbicide. Chlorsulfuron cancontrol perennial species with spring, summer, or fall treatments(Drake and Whitson 1989; Whitson et al. 1989; Younget al. 1998), whereas glyphosate is best applied in spring,when invasive plants are at the late bud to early floweringstages (Waterhouse and Mahoney 1983; Young et al. 1998).Herbicides are generally applied as broadcast treatmentsover the entire field or directed (spot) applications to controlearly weed invasions or to prevent the spread of small infestations.However, it is also possible to achieve selective controlof a particular weed with otherwise nonselective or relativelynonselective postemergence herbicides by employing a ropewick or wick applicator. These can be either hand-held orvehicle-mounted boom wipers. As a benefit, this applicationmethod reduces the potential for herbicide drift and290 POLICY AND MANAGEMENT

TABLE 22.4Commonly Used Herbicides for Grassland Invasive Weed ControlRegistered inCommon name Trade name Mode of action Weed spectrum Soil residual California Effective timing2,4-DWeedar ® ,Weedone ® andmany othersBroadleaf speciesLess than 2 weeksYesPostemergence only, fromseedling to boltingAminopyralidMilestone ®Growth regulatorCertain broadleaf families(between clopyralid andpicloram)Full seasonYesEffective both pre- andpostemergence; applied fall,winter, or springChlorsulfuronTelar ®Amino acid synthesisinhibitorMainly broadleaf speciesAt least 2 monthsYesPreemergence onlyClopyralidGrowth regulatorTransline TM Growth regulatorRemedy ®Certain broadleaf families(e.g., Asteraceae, Fabaceae,Apiaceae, Solanaceae,Polygonaceae)Most of the seasonYesEffective both pre- andpostemergence; applied fall,winter or springDicambaBanvel ® ,Vanquish ®Growth regulatorBroadleaf speciesLess than 1 monthYesPostemergence only, fromseedling to boltingGlyphosateRoundup ® , andothersAmino acid synthesisinhibitorNonselectiveNoneYesPostemergence only, fromseedling to early floweringImazapicPlateau ®Amino acidsynthesis inhibitorNonselective, but best onannual grassesFull seasonRegistrationexpected in 2007or 2008Mainly as a preemergencetreatment, postemergencecontrol with seedlings orrosettesMetsulfuronEscort ®Amino acid synthesisinhibitorBroadleaf speciesAt least 2 monthsNoFairly effective; preemergenceonlyPicloramTordon TMGrowth regulatorBroadleaf species, weak onmustardsUp to 3 yearsNoEffective both pre- andpostemergence; applied fall,winter or springTriclopyrGarlon ® ,Growth regulatorBroadleaf speciesLess than 1 monthYesPostemergence only, good onseedlings, fair on matureplants

injury to adjacent sensitive agricultural crops and can beused to selectively control invasive species around vernalpools, streams, and other bodies of water, or in areas with rareand endangered species or other desirable plants.Residual thatch can influence the effectiveness of herbicideswith preemergence activity. Some herbicides, such asimazapic, can adsorb to standing thatch or other dried debrison the soil surface, thus reducing the effectiveness of theapplication. However, this does not appear to be a characteristicof aminopyralid or clopyralid (DiTomaso et al.1999b).Although herbicides are effective for the control of invasivegrassland weeds, they generally do not provide longtermcontrol of weeds when used alone (Bussan and Dyer1999). In the absence of a healthy plant community composedof desirable species, one noxious weed may be replacedby another equally undesirable species insensitive to the herbicidetreatment (DiTomaso 2000). Thus, herbicides in grasslandsare best used as part of an integrated weed managementsystem.Cultural ControlGrazingGrazing can be an effective way of managing undesirablespecies in some grasslands both in California and elsewhere(see also Huntsinger et al., Chapter 20). The effectivenessdepends on the plant species present, the type of grazer used,and the timing and intensity of the grazing program. Undersome conditions grazing can increase undesirable nonnativespecies, or other less palatable or poisonous species. Intensivegrazing can also disturb soil and enhance weed seed germination,reduce competition from more desirable species, andincrease soil compaction (Elmore 1992). So, if grazing is to beused, it must be used judiciously with prescriptions designedfor individual sites.Successful invasive weed management can also depend onthe type of grazer and timing of grazing. For example, theforaging behaviors of both cattle and goats are conducive tothe management of yellow starthistle when plants are grazedat the bolting stage, whereas only goats are effective whenplants are in the spiny stages of growth (Thomsen et al.1993). The ideal time to graze is when the noxious species aremost susceptible to defoliation or when the impact on thedesirable vegetation is minimal (Kennett et al. 1992).Stocking rates of livestock can also be adjusted to maximizeinvasive plant management. Lower stocking rates willgenerally allow livestock to graze preferred plants and avoidless palatable species. If the invasive species is preferred, thenlower stocking rates can be effective. In most cases, however,grasslands with low cattle stocking rates have higher weedinfestations compared to areas that are more intensivelygrazed. Higher stocking densities can minimize the grazers’ability to avoid less palatable invasive weed species. This canlead to a more uniform composition of plant species andmore balanced competitive relationships among native andinvasive species (Olson 1999).High-intensity, short-duration grazing, practiced on arotational basis, is a management system widely adopted inother countries (DiTomaso 2002) that is becoming increasinglyrecommended in California grasslands (see Jacksonand Bartolome, Chapter 17). This can be logistically difficultand generally requires electric fencing to keep animals confinedto a specific area. Once grassland has been intensivelygrazed, it is allowed to recover for about a month beforebeing grazed again. This system usually leads to more uniformforage use (including many weeds) by the grazer. Inmany cases, this method has been shown to provide muchbetter control of specific weeds than season-long livestockgrazing (see review by DiTomaso 2000).High-intensity grazing of both cattle and sheep has beentested experimentally for the control of medusahead. Georgeet al. (1989a) found that two years of intensive grazing withcattle significantly reduced medusahead from 45% of thetotal species composition to only 10%. In another studyusing sheep, intensive mid-spring grazing (April/May)reduced medusahead by greater than 80% the following year(DiTomaso, Kyser and Doran, unpublished data).Prescribed BurningThere has been increasing interest in the use of prescribed firefor vegetation management in recent years, including inCalifornia grasslands (see Reiner, Chapter 18). Purposes forprescribed burning have included invasive weed control,thatch removal, nutrient release from dead, dried plant matter,and stimulation of early growth of desirable species in thespring. However, while most of these purposes are bestaccomplished with late fall burns, burning designed to controlinvasive species in California grasslands generally needsto be conducted in late spring or summer. Unfortunately,this timing coincides with the period when there is high riskof fire escapes. Moreover, air quality problems and liabilityissues can also present a problem when burns are conductednear populated areas. In areas where biological control agentsare present, burning may cause damage to these insect populations.In some areas, burning can lead to rapid invasionby other undesirable postfire colonizers with wind-dispersedseeds, particularly members of the sunflower family.The main objective of using controlled burns for invasiveplant control is to deplete the soil seed bank, destroy seedsthat are on the plant, and prevent sexual reproduction,which would in turn replenish soil seed reserves. To successfullycontrol annual species with fire, it is critical to either killplants before their seeds become viable (DiTomaso et al.1999a) or destroy the seeds before they disperse (Allen 1995;Menke 1992). For annual species, burns should be conductedwhen the target plant’s seeds are still undispersed and areexposed to direct flames in the canopy, but desirable specieshave dispersed their seeds to the ground. Seeds on the soilsurface are not generally exposed to lethal temperatures in292 POLICY AND MANAGEMENT

grassland fires (Sweet 2005). For perennial or herbaceousplants with protected meristems (e.g., rosettes or rhizomes),the burn must be hot enough to damage the vegetative reproductivetissues and prevent resprouting. For this reason, prescribedburning is rarely effective for the management ofperennials and in most cases can stimulate their growth(DiTomaso et al. 1999a). It has, however, been used to reducethe seed bank of French broom within California grasslandsettings (Alexander and D’Antonio 2003b). Fire stimulatesbroom seed germination, and the flush of seedlings is thencut, treated with an herbicide, or reburned prior to plantsbecoming reproductively mature.Annual species that are most susceptible to control withprescribed burning are those that produce seeds after the fireseason begins, have flowering structures either embeddedwithin the fuel bed or exposed to direct flames, and haveshort-lived seed banks. For control of these late-season annualgrasses and forbs, the timing of the burns is critical. Burns conductedbefore the target species have fully cured their seedsare most effective (Brooks 2001). To be successful, however,fine fuels to carry the fire should not be limiting. However, finefuels are often patchy across landscapes, and the result is amosaic burn pattern that results in variable weed control.Invasive grasses with long-awned seeds (e.g., medusahead,downy brome, ripgut brome, red brome, and barb goatgrass)rely on animal dispersal. In many of these species, the seedsremain in the inflorescence longer than most desirablegrasses, and, as a result, they are more susceptible to destructionby the direct heat of burning (Dahl and Tisdale 1975,Young et al. 1970). Effective control of medusahead with prescribedburning (more than 90%) was demonstrated as farback as 1953 (Furbush 1953) and has been demonstrated ona number of other occasions (George 1992; McKell et al. 1962;Pollak and Kan 1998; Sharp et al. 1957; Betts 2003; DiTomasoet al., unpublished). However, in some cases burning has notproven successful on medusahead. Young et al. (1972) foundthat repeated annual burning in mid-summer increasedmedusahead infestations while decreasing the population ofmore desirable annual grasses. This inconsistency is probablydue to differences in the length of flame exposure and to theheat of the burn. Other invasive long-awned annual grasses,including barb goatgrass (DiTomaso et al. 2001; Hopkinsonet al. 1999) and ripgut brome (DiTomaso et al. 1999a; Kyserand DiTomaso 2002) have also been controlled with one ormultiple years of burning, although exception can also befound (A. Levine and C. D’Antonio, unpublished data). Incontrast, downy brome and red brome are difficult to controlwith burning because their seedheads begin to shatter and theseeds fall to the soil surface before enough fuel is available(Brooks 2002; Young and Evans 1978).Late season forbs, particularly yellow starthistle, can alsobe controlled by repeated early summer burns (DiTomasoet al. 1999a; Kyser and DiTomaso 2002). Because the seed ofstarthistle survives for more than two years in the soil andgermination is enhanced by a preceding burn (DiTomasoand Kyser, unpublished data), a single year of burning willnot control an infestation. In one case, three consecutive yearsof burning were required to reduce the yellow starthistleseed bank by 99% (DiTomaso et al. 1999a). In other studies(DiTomaso and Kyser, unpublished data; Miller 2003), integratinga first year burn with a second year herbicide treatmentwas the most effective strategy.Like other control strategies, prescribed burning requires afollow-up program to prevent escaped or isolated plants fromcompleting their life cycle. Where the seed bank is shortlived,a follow-up program may take only a couple of years;in other cases, it may take longer.Since multiple burns are not usually practical or permitted,and fuel loads may not be sufficient to allow multiple yearburns, integrated approaches are often more appropriatethan using burning as a sole control option.RevegetationThe goal of grassland management should be to improvedegraded communities and make them less susceptible tonoxious weed invasion. Revegetation with desirable andcompetitive plant species is one approach to achieving longterm,sustainable suppression of weed population growth,while providing high forage production or desirable plantdiversity (Borman et al. 1991; Lym and Tober 1997).The choice of species used in a revegetation effort is criticalto its success. Seeded species need to be adapted to the soilconditions, elevation, climate, and precipitation level of thesite (Jacobs et al. 1999). Only a limited number of specieshave proven to be aggressive enough to resist establishmentof problematic invasive species, and the proper species choicevaries depending on the location and objective. Perennialbunchgrasses are among the most commonly used species forrevegetating western rangelands and grasslands, and theyhave been shown to reduce the growth and reproduction ofweeds such as yellow starthistle (Lym and Tober 1997; Rochéet al. 1994; Dukes 2001a; Reever Morghan and Rice 2005).Some native broadleaves, including Hemizonia congesta, havea similar life cycle to and can suppress the growth of yellowstarthistle (Duke 2001a). Introduced broadleaf species such aslegumes can also be used in revegetation programs to suppressrangeland or grassland weeds. For example, Thomsenet al. (1997) and Thomas (1996, 1997) tested several legumespecies for their competitive effect on yellow starthistle.Thomsen et al. (1997) found subterranean clover (Trifoliumsubterraneum) varieties to be the most competitive when combinedwith grazing and mowing, but they did not provideadequate seasonal control of yellow starthistle in the absenceof other control options. Thomas (1996, 1997), however,used a combination of subterranean clover and/or crimsonclover (Trifolium incarnatum) as a cover crop in yellowstarthistle–infested pasture. In a completely infested field, hereported an 80–90% reduction in yellow starthistle one yearafter planting with crimson clover. However, use of legumesmay increase soil nitrogen, which can cause other potentiallyundesirable effects (see Dukes and Shaw, Chapter 19).EXOTIC PLANT MANAGEMENT 293

SIDEBAR 22.1CASE STUDIES OF INTEGRATED MANAGEMENT STRATEGIESCombination of Herbicides and RevegetationAn integrated approach combining herbicide treatments and perennial grass revegetation was tested ina heavily infested yellow starthistle grassland site near Yreka, California (Siskiyou County). The goal wasto provide ranchers and land managers with economical and sustainable management programs thatmaximized forage production or restored and preserved desired ecosystem functions, including reducingthe susceptibility of their lands to reinvasion or invasion by other noxious weeds.In this severely degraded site, a mid-February treatment with glyphosate (one treatment) and clopyralid(one, two, or three annual spring treatments) was used to provide a window of reduced competitionfor the subsequent establishment of pubescent wheatgrass drill-seeded in early March (Enloe 2002;Enloe et al. 2005). This seeding timing, while not appropriate for much of California, has been shownto work well for far Northern California (Kay and Street 1961), where sufficient rain falls after early Marchto support wheatgrass establishment.The study area was monitored for six years (Enloe et al. 2005). Clopyralid treatment significantlyreduced yellow starthistle, and glyphosate gave control of the annual grasses. This combination allowedpubescent wheatgrass seedlings to establish with a single year of treatment. Once pubescent wheatgrassseedlings survived the first year, additional applications of clopyralid to control the starthistle did notimprove their establishment. In the absence of clopyralid and glyphosate, pubescent wheatgrass establishmentwas very limited.The integrated approach gave long-term suppression of yellow starthistle and other exotic annualgrasses and forbs over the six year period. Treatments with clopyralid alone (e.g., without seeding ofwheatgrass), gave good control of yellow starthistle, but the plant community initially became dominatedby undesirable annual grasses, particularly downy brome. Downy brome, in turn, offered little competitiveresistance to starthistle reinvasion, and within a couple of years after the final clopyralid applicationthis site reverted to yellow starthistle (Enloe et al. 2005).Long-term Management Using Prescribed Burning and ClopyralidAs was previously discussed, repeated burning is generally impractical and can negatively impact air qualityas well as compromise establishment of biocontrol agents. The continuous use of clopyralid can alsohave undesired outcomes. As a result, an integrated strategy was developed combining clopyralid andprescribed burning for management of yellow starthistle (DiTomaso et al. 2003). Results of small-scaleplot studies indicate that prescribed burning stimulated the germination of yellow starthistle seed in thesubsequent rainy season. This helped to deplete the seed bank more rapidly. Thus, a first-year prescribedburn followed by a second-year clopyralid treatment gave nearly complete control of yellow starthistlein the year after the last treatment. This strategy may reduce the number of years necessary to intensivelymanage yellow starthistle and allow land managers to transition into a follow-up management andreseeding program sooner. The reverse order gave very poor control, suggesting that an integratedapproach should not end with a prescribed burn.An additional benefit of integrating prescribed burning into a yellow starthistle management programis the control of noxious annual grasses. When ripgut brome and medusahead coexisted with yellowstarthistle, burning contributed to the reduction of all three invasive species (DiTomaso et al. 2003).With the results of these small-scale studies, a large-scale integrated approach was used at FortHunter Liggett in Monterey County, California (Miller 2003; Torrence et al. 2003a, b). After two orthree years of treatment, which included a first-year burn followed by one or two years of clopyralid,

yellow starthistle control was excellent. At that time, a follow-up maintenance plan was implementedto prevent any potential reinfestation.This project demonstrated that yellow starthistle populations could be controlled with two years ofproperly-timed, intensive management. The most successful long-term, large-scale yellow starthistlecontrol treatment was to follow a first-year prescription burn with a broadcast clopyralid application treatmentthe next year. However, a follow-up program should be instituted immediately in order to preventinvasive plant resurgence to previous levels.Because of the ecological diversity within California, nosingle species or combination of species will be effective inproviding ecological resistance against invasive weeds underall circumstances. While pubescent wheatgrass (Thinopyrumintermedium) has proven successful for yellow starthistle suppressionin Siskiyou County (DiTomaso et al. 2000; Enloeet al. 2005), it may not be appropriate in many other areasof the state that lack summer rainfall or where native grassesare the landscape objective (see Stromberg et al., Chapter 21)Though perennial grasses have been shown to be mostsuccessful in competing with grassland weeds, a combinationof species with various growth forms can also be effective.This diversity allows for maximum niche occupation andmore sustained resource capture over the growing season(Sheley et al. 1999; Dukes 2001a, b). For example, seed mixturesof grasses with legumes improved the rate of microbialand soil structure recovery compared to grasses alone (Jacobset al. 1999). Seeding with a variety of species, however, makesit difficult to choose control techniques (such as herbicides)that will not harm one of the seeded species. This is particularlytrue if grasses are seeded with broadleaf species and theweeds that are being controlled are broadleaf species. A revegetationprogram may require initial seeding with perennialgrasses during the weed management phase, followed by subsequentseeding with desirable broadleaf species. Revegetationis generally a slow process and may take several years tobe successful. It is most successful when combined with othermanagement techniques.Developing a Management StrategyThe major elements of a grassland weed management programare preventing introduction or reinvasion of invasiveweed seed, reducing the susceptibility of the ecosystem toinvasive plant establishment, developing effective educationalmaterials and activities, and establishing a program for earlydetection and monitoring (DiTomaso 2000).An effective invasive weed management strategyshould include three major goals: (1) controlling the weed;(2) achieving land use objectives such as forage production,wildlife habitat and ecosystem preservation, protecting diversityor endangered species, or recreational land maintenance;and (3) preventing reinvasion or invasion of other noxiousspecies. All these goals are tied together with improving thedegraded grassland community and reestablish a functioningecosystem.Integrated Approaches to Weed ManagementAs previously discussed, a single method does not generallygive sustainable control of a grassland weed. A successfullong-term management program should be designed toinclude combinations of mechanical, cultural, biological,and chemical control techniques. There are many possiblecombinations that can achieve the desired objectives, andthese choices must be tailored to the site, economics, andmanagement goals. See “Case studies of integrated managementstrategies” for more elaboration (Sidebar 22.1).Even when a single control method does provide effectivecontrol over a number of years, it may not be practical. Forexample, repeated burning can be effective for the control ofsome annual grasses and yellow starthistle, but this approachis often prohibited. Thus, it may be necessary to incorporateother control methods, along with burning, into a long-termmanagement strategy (DiTomaso et al. 2006; Kyser andDiTomaso 2002).When an integrated strategy is employed, it may be importantto employ a particular sequence of approaches. ForEXOTIC PLANT MANAGEMENT 295

example, in a revegetation effort along a yellow starthistle–infestedcanal and roadside, the first step was to intensivelymanage starthistle (Brown et al. 1993; Thomsen et al.1994). The second step was to reseed with competitive, deeprootednative perennial grasses. In the final stage, nativebroadleaf forbs such as California poppy and lupines wereseeded into the system.Biological control can also play a key part in the success ofan integrated control program. For example, Huffaker andKennett (1959) reported on the success of the biological controlprogram for the management of common St. Johnswort.They noted that maximal improvement of the rangeland wasachieved when the biocontrol agent was used in combinationwith moderate timely grazing. This combination preventedthe expansion of ripgut brome in the grassland sitespreviously occupied by common St. John’s wort.In a review, Lym (2005) described the numerous situationsin which the successful use of biological control insects(Aphthona spp.) for leafy spurge (Euphorbia esula) depend onthe use of other conventional weed control methods. Integrationof Aphthona spp. with herbicides, grazing, or burninggave more rapid and better leafy spurge control that anymethod used alone.Finally, it is important to emphasize that the ultimateobjective of any control strategy is to develop a “healthy”functional ecosystem. Competitive background vegetationin grasslands will not only reduce the potential establishmentof invasive plants, but can also enhance the effectivenessof other control strategies, including biologicalcontrol (McEvoy and Coombs 1999). As an example, theseed feeding insects for yellow starthistle can have attackrates of greater than 90% (DiTomaso et al. 2006), yet reduceseedset by only about 50–60% (Woods et al. 2004). In a systemwith competitive vegetation, a reduction of this levelmay suppress starthistle to an acceptable and sustainablelevel.296 POLICY AND MANAGEMENT