Managing Unwanted Vegetation in Riparian Restoration Sites

Ed Peachey and Brad Withrow-Robinson
Revised: 
June 2016

There is increasing interest and activity around the Pacific Northwest in restoring native plant communities and habitats such as prairies, oak savannas and riparian forests. Restoration management aims at restoring habitat functions and processes on sites disturbed by human activities. To do so it is often necessary to remove a non-native plant community first, then replace it with native plants which are able to provide desired habitat structure and functions. With the increasing number and size of restorations being undertaken as well as the growing diversity of groups and individuals involved, it is increasingly recognized that many farm and forestry practices, including the use of herbicides, can have a useful role in this effort.

This section is meant to provide some guidance for managing unwanted vegetation in riparian area restoration projects. In particular it will address the selection and use of herbicides suitable and labeled for riparian restoration. For a broader discussion of controlling weeds in habitat areas, please see “Weed Control Methods Handbook: Tools & Techniques for Use in Natural Areas” ( http://www.invasive.org/gist/handbook.html ) or “A Guide to Riparian Tree and Shrub Planting in the Willamette Valley: Steps to Success” ( https://catalog.extension.oregonstate.edu/em9040 ) for a more local discussion of riparian restoration approaches and methods.

Site preparation

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 hand held 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. But blackberries may be injured enough by a spring glyphosate application to prevent effective control treatments applied at the recommended time in late summer or fall. 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. Other herbicides have no activity on broadleaved plants, killing only certain grasses.

Selectivity and effectiveness is 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 needs to 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 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. So 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).

Site maintenance

Once native trees and shrubs are planted, it will be necessary to prevent growth of annual and perennial weeds for several years while the seedlings get established. Left unchecked, weeds emerging from the seed bank or encroaching from adjacent lands can rapidly reestablish. A rebounding weed community can compete with seedlings and reduce both survival and growth and also build up cover for voles, which can be very destructive to young plantings.

While tree and shrub seedlings are becoming established on a restoration site, weeds may be controlled by careful spot treatment with some of the same foliar herbicides labeled and used in site preparation, including glyphosate and triclopyr, or by using physical weed control such as mulch. The objective is generally to keep an area near each transplanted seedling free of weeds to reduce competition and also limit cover for voles, thus preventing their damage. Spot spray treatments with foliar herbicides will likely need to be applied several times a season over a few years, adding to the cost of the restoration effort and with the knowledge that these applications bring an associated risk of injury to transplanted species. Managers need to plan their planting accordingly.

There are many herbicides commonly used in horticulture and forestry that prevent weeds from emerging and recolonizing cleared ground in site preparation. In new plantings, preemergent herbicides can provide season-long and cost-effective weed control. While many have good environmental profiles, with low impacts on humans, fish and other wildlife, currently only one has a label for use in restoration of riparian hardwood forests. Remember to exercise care and only use herbicides registered for the site!

Even after desirable trees and shrubs are well established, it may still be desirable to keep particular weeds such as blackberry or reed canarygrass in check for a much longer time in order to keep growing space open and allow expansion or reintroduction of native species. This can be done with a less frequent but regular spot spray program of approved herbicides. Mowing has limited value in preventing competition and near-ground cover for rodents, but can be effective in keeping areas open and passable.

The following herbicide summaries are provided as a quick reference of materials with labels allowing use in restoration. It is the project manager’s responsibility to consult an approved label for complete information on target efficacy, rates, use patterns and to determine if a particular herbicide is approved in their state for the intended use and site situation. Some of these herbicides have very narrow uses and the situation at a particular restoration site must be matched with the intended use as described on the label.