Physical Control

The physical control of invasive species by digging, removal, and harvesting (Table 2) may or may not be feasible depending on the size of the invasive population, and number of available people to help with the removal (Box 1). Sometimes, the invasive species has built a large reserve of seeds in the soil, so that seedlings from germinated seeds grow to replace the removed adults in the soil disturbed by the removal activities. Often, the invasion

Table 2 Mechanisms for the control of invasive plant species

Method

Advantages

Disadvantages

Hand or machine cutting

Labor intensive although less so than some other

Most species grow back from

methods

underground parts

Root pulling

Removes underground parts of plant that may regrow

Disrupts the soil and may encourage reinvasion of the exotic species from seeds or plant fragments

Herbicide application

Easy to accomplish relative to more labor intensive methods

May kill native species

Handcutting with herbicide

Very effective removal mechanism, especially for

Uses herbicides, although in very small

application to cut shoots

woody species

amounts

(Bradley method)

Herbicide

Can be effective in the removal of certain species; less

Improper usage may harm the health of

labor intensive than other mechanisms, so that it

users and/or the environment

can be used to treat large tracts of land

Biocontrol (release of insects or

Can target the invading species only

Pests may be unpredictable; native

pathogens transferred from

species may be damaged

their continent of origin)

Box 1 Think globally, act locally: volunteer programs for invasive species control, and research data collection

Volunteers can help with the invasive species problem by volunteering their time to help in management and research projects. The control of invasive species can often be achieved by the removal of invasive species from natural communities, an effort greatly helped by teams of volunteers. The 'weed pulling organizations' are regionally coordinated, and these organizations often have websites with information for volunteers. A short search on the Internet can help volunteers locate an organization in their region engaged in weed removal activities. A few agencies for volunteers to check include The Nature Conservancy, California Invasive Plants Council, Hawaiian Ecosystems at Risk, Invasive Plant Association of Wisconsin, Midwest Invasive Plant Council, Southeast EPCC, and Texas Invasives.

Volunteers also can donate their time to help researchers collect data, e.g., the U.S.G.S. purple loosesetrife volunteer program. Volunteers in this program collect data on Lythrum salicaria around the world. See: http://www.nwrc.usgs.gov/special/purplel/

potential increases with the amount of disturbance created by the removal of invasive species, so care should be taken in physical removal programs not to disturb the sites. Especially in restoration projects, it is important to remove invasive species by hand, or the resulting vegetation will be unsatisfactory to meet the regulatory requirements of the restoration process. In such cases, the best approach is to remove invasive species immediately, before the invasives become widespread in the restoration area.

Invasive species can be snipped at the base by hand with a long handled clipper, while carefully avoiding native species (Bradley Method from Australia). The uncut native species are at an advantage since they are already established, and often regrow to shade and reduce the invasive species after the treatment. This selective clipping method is very time consuming, but can have very good results in reducing invasive species in native plant communities.

Mechanical mowing with a tractor or by hand with a scythe can be an ideal way to reduce the amount of an invasive species and encourage native species, particularly if the mowing is done in certain seasons. For example, mowing in the late spring and early summer reduced the amount of Arrhenatherum elatius and increased the native Danthonia californicum in a grassland. Harvesting by hand also can remove invasive species. Kudzu (P. lobata) is sometimes harvested for cattle forage, and the invasive Prosopis juliflora can be used for fuel wood (the famed mesquite barbecue charcoal). The downside of putting invasive species to a 'good' use after their harvest is that the transfer of biomass elsewhere can spread seeds or disseminules.

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