Bushland Weeds Manual - Environmental Weeds Action Network

18
Chapter 3 Grass Weeds
Rhizomatous and stoloniferous grasses
Hand-weeding should only be considered for very
small infestations (less than one meter square) when
soil is moist. It is important to remove all root, rhizome
and stolon fragments or plants will rapidly regenerate.
For some species like Para Grass and Johnson Grass,
rhizomes can extend for up to two metres. However,
because of the extensive soil disturbance involved,
hand-weeding is generally not recommended in
bushland.
Herbicide can be blanket sprayed over the dense mats
commonly formed by rhizomatous and stoloniferous
grasses. A systemic herbicide that is translocated
through all parts of the plant, including the rhizomes
and stolons, must be used. Where native plants are
present, grass-selective herbicides are necessary to
avoid off-target damage. Without thorough foliage
coverage, the amount of herbicide absorbed into the
plant is not sufficient for effective control. Even with
good coverage, many rhizomatous grasses require
multiple spray treatments within a single season; only
a few rhizomes or stolons need to survive for the plant
to rapidly re-establish.
Herbicide use near waterways becomes an important
issue where many rhizomatous and stoloniferous
grasses are invasive. At present Roundup Biactive ® is
the only herbicide registered for use near water. Many
of the wetlands of the Swan Coastal Plain are seasonal
and dry up over the summer months. Luckily, summer
coincides with the active growth of most wetland
grasses and spraying can be done at this time. Grasses
that grow in permanent water bodies will need to be
either treated with Roundup Biactive ® or pulled from
the water and methods such as solarisation tried.
Solarisation is useful for controlling summer growing
(C4) grasses in highly disturbed areas. This involves
laying heavy duty plastic sheeting (black or clear) over
part or all of the infested area. With edges firmly held
down, the plastic is left in place for four to twelve
weeks before being removed. The plastic sheeting
traps the heat, increasing the soil temperature to levels
that kill plants, seeds, plant pathogens and insects
(Bainbridge 1990). The use of clear plastic results in
higher soil temperatures, whereas black plastic
prevents photosynthesis and this contributes to plant
death. The method is most effective when applied
during the hot summer months and is only really
useful in sites where the soil is moist (Tu et al. 2001).
The area may need to be blanket sprayed after any
surviving grass has regrown vigorously.
Establishing shade along bushland edges by
encouraging the regeneration of local plants can inhibit
the spread of many amenity grasses from adjacent
parklands. Kikuyu, Buffalo Grass, Couch and similar
amenity grasses are often intolerant of shade.
Monitoring and follow-up
It is important that grass control is done effectively
and that any control program does not simply enhance
the competitiveness of other serious weeds. With
control programs for grass weeds in small urban
bushlands costing up to $1000 per hectare, monitoring
their success is essential.
Monitoring provides a quantitative record of the
management program, measures the impact of the
control program on native plants and ensures value for
money from contractors. How you monitor will depend
on the changes to be detected, the type and patchiness
of vegetation and the terrain. Case study 3.4 provides a
detailed example of monitoring a Perennial Veldgrass
control program in Banskia woodland.
Regardless of the control method used, follow-up
treatment in subsequent years is usually necessary.
Plants that were missed or survived the first treatment
will need treating and soil seedbanks may take several
years to deplete. If follow-up does not take place, the
initial effort and resources will be wasted.
Key points
The information presented in this chapter highlights a
range of management issues concerning grass weeds and
how these relate to the biology of this group. Briefly:
• Invasion of many grasses is often facilitated by fire
and loss of canopy cover.
• Annual grasses are generally weeds of highly
disturbed areas, although there are exceptions
(Wild Oat, Annual Veldgrass). To control, prevent
seed set over successive years and establish
desirable vegetation.
• Perennial grasses can threaten intact bushland.
They are grouped into the tussock-forming
caespitose grasses and the mat-forming
rhizomatous or stoloniferous grasses. Management
strategies and control techniques differ for the
different growth forms.
• Recognising and treating grasses at the
appropriate growth stage is integral to a
successful outcome of any control program.
The case studies described below bring together this
information, providing an example of management in
action for an annual, a perennial caespitose and a
rhizomatous grass. A fourth case study describes the
monitoring of a Perennial Veldgrass control program
at Blue Gum Lake. Following the case studies is a
table of currently available information on the control
of grass weeds.
One aspect of their biology that has contributed
greatly to the success of the grasses is the ability to
store carbohydrate reserves in tillers and rhizomes at
the base of the plant. This strategy is taken even
further by the group of plants covered in the next
chapter. The bulbous, cormous and tuberous species
are another group successfully naturalising in south
west Western Australia. Their success, in part, is due
to an ability to die back to underground storage organs
over our long dry summers.