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 become established. Left unchecked, weeds emerging from the weed 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.