The purpose of site preparation is to clear the planting area of existing vegetation that will get in the way of planting and weed control activities, and to kill established perennial weeds that will compete with the new planting for water, light, space and nutrients.
Many riparian restoration projects are done on former farm or pasture lands that are already infested with a robust and well-established community of invasive perennial weeds such as reed canarygrass, common tansy, blackberry and Scotch broom. These and many other herbaceous or woody perennials present both a physical obstruction and a competitive threat, which is best addressed before planting.
There are many approaches and practices that can be used for site preparation. These include mechanical strategies such as repeated tilling or mowing with tractor mounted or handheld tools, and/or herbicides. While some of these same methods may also be used after planting, most can be applied more efficiently and effectively before planting, avoiding the risk and effort of working around sensitive young seedlings.
Effective site preparation is rarely achieved in a single step, and often takes one or more years to accomplish. Factors to consider in selecting methods and deciding how much time to allow for site preparation include the weed species present, the control methods available, the most effective treatment season for each species/control method combination, if time is needed to establish a cover crop, and if time is needed for an herbicide to break down before cover crops or tree seedlings can safely be planted.
If herbicides are used in site preparation, they should be chosen according to the plant community present, the materials that are effective on those weeds and the season or growth phase at which a particular herbicide is most effective on a particular weed. When there are several problem weeds on site, effective control will often require sequential, targeted applications of specific herbicides at different times during the growing season and/or using two or more different herbicides targeting different species.
For example, a broad-spectrum herbicide such as glyphosate can be effective on both herbaceous and woody species, although the optimal timing varies dramatically from early spring to late fall according to the plants targeted. Spring applications of glyphosate can readily control most perennial grasses, but not shrubs such as blackberries. However, blackberries may be injured enough by a spring glyphosate application to eliminate the need for the recommended late summer or fall applications. Thus it may take several applications directed at different species and areas of a site to effectively treat a given a site.
Other herbicides are more selective. Some will kill woody plants like blackberries, Scotch broom and native hardwood trees and shrubs as well as broadleaf herbaceous weeds, while leaving grasses unharmed. Some herbicides have no activity on broadleaved plants, killing only certain grasses.
Selectivity and effectiveness are also a matter of the application method used. Cut stem or basal bark treatments may be most appropriate in some situations when foliar applications may not be practical such as for the control of larger woody plants, or when sensitive desirable plants are nearby (See the Forestry Section K1 in this handbook or the Weed Control Methods Handbook for more information on these application methods).
The scope of site preparation activities should reflect conditions at the restoration site since not all sites will require aggressive broadcast applications for site preparation. If the site is actively farmed and occupied by a grass-seed crop or managed pasture, it may be suitable to treat individual planting spaces or planting strips rather than do a broadcast treatment. If transitioning from an annual crop rotation, it may be best to take time to first establish a grass cover crop and then create planting spots or strips prior to planting. An established cover will help suppress new weed establishment and control erosion. The reduced spray treatment area also reduces chemical use and costs.
Since it is often necessary to remove brush and other vegetation as part of site prep, it is important to do it in a way that does not compromise your ability to kill the targeted weeds. Blackberries and other woody species are generally best controlled by late summer to early winter foliar applications, using herbicides that are moved inside the plant and translocated from the leaves down to the roots. Mowing or other disturbances that remove or significantly reduce the canopy of the plant can make such applications less effective. In such cases, the smaller "target" may be unable to translocate sufficient herbicide to kill the roots. When vegetation prevents effective spraying or other management activities, mowing or grazing should be timed to allow plants to regrow adequately prior to the planned herbicide treatment to allow effective control of the target weed species.
Most herbicides used for site preparation need to be applied directly to growing plants at the appropriate rate and time of year to be effective in weed control. Still, soil activity must be considered before seeding or transplanting native species. Some herbicides used for site preparation do have soil activity and may be absorbed by roots to injure existing native plants, or may persist for months, preventing new seedlings from emerging or harming transplanted vegetation. Therefore, it may be necessary to allow additional time for those materials to break down before planting in order to avoid injury to seeded cover crops or transplanted tree and shrub seedlings. Pesticide labels describe the amount of time needed between herbicide application and planting to avoid carryover of herbicides that will injure desirable plants (crop rotation intervals).