Drainage Design Manual - the City of San Luis Obispo

• Removal of Herbaceous Weeds Prior to PlantingOn sites where the existing grass and herb cover is to be retained, a 1.2 x 1.2meters (4' x 4') zone should be cleared for each individual revegetation plant. Ifthe existing cover is excessively tall and dense, mowing and/or light discing canfacilitate clearing, planting and irrigation operations. Mowing should not occur ifit has been determined that removal of high, residual cover will adversely affectcertain wildlife species. While fire vulnerable dry grasses and herbs are generallydiscouraged in flood control channels, tall cover plant residues can serve toprotect delicate revegetation plants from drying winds. The choice betweenleaving or removing all or part of the existing grass and herb cover will depend onwildlife habitat issues, revegetation goals, proposed plant establishment andmaintenance practices, and long-term management goals.On sites where revegetation goals require the removal and replacement of existingcover plants, more intensive weed eradication methods can be employed toprepare the site for planting a specified cover of grasses and herbs (see discussionon grass seeding below). After all clearing, grading and disking (ripping and/orrototilling) is completed and irrigation systems have been installed, the plantingarea should be thoroughly irrigated to force germination of as many weed seeds aspossible. Once significant weed growth is established, the area can be sprayedwith a short-duration, broad spectrum, contact herbicide that will completelydegrade before planting. This irrigation-herbicide treatment should be repeated anumber of times over several months to facilitate further germination and killingof residual weed seeds. For sites where sprinkler irrigation is not possible (i.e.,sites that rely upon seasonally available precipitation) herbicides should beapplied in late October or early November after weeds emerge and begin to grow.A second spray treatment should be applied in early to mid spring depending onthe timing of germination and growth of the weed species present.In situations where chemical weed control programs are not possible, severalmechanical methods can be employed. They include repetitive mowing,cultivation, hand pulling, and light exclusion (i.e., covering the planting area withfabric or other materials that prevent germination and/or growth by excludinglight). Annual cover plants can be reduced on a site by mowing them to a heightof two to four inches in the spring when grasses begin to flower. For moreeffective control, mowing usually needs to be repeated once or twice during thegrowing season (from late March to mid June). Repeated cultivation during thegrowing season by either discing or rototilling can also effectively reduce annuals.However, cultivation is discouraged in soils that have large stores of viable weedseeds because repeated mixing of the soil can encourage their germination (i.e.,cultivation moves buried weed seeds to zones within the soil where conditionsfavor their germination). While mechanical weed control methods are valuable incertain situations, they are usually much more labor intensive and expensive thanusing herbicides, and often less effective. In many situations, they can also bemore disruptive to the environment.SLO Creek Drainage Design Manual 151 February 2003