Invasive Species

The restoration of any ecosystem is likely to include control of invasive plants, and tallgrass prairie and savanna are no exception to this rule. Invasive plants tend to colonize disturbed areas, and restoration invariably involves some form of disturbance.

When people refer to invasive plants they usually mean invasive exotic or alien species. In other words species that have been introduced outside of their natural range and which, in the absence of natural controls spread rapidly and displace native species. But not all invasive plants are exotic. Under some circumstances a native plant can cause the same problems, especially when our concern is for a small area that we are trying to manage. In such cases any unwanted plant might be considered to be “invading” our restoration site, even though technically these may simply be weeds, rather than invasive species.

Some of the main invasive exotic woody plants of Ontario tallgrass ecosystems are:

  • Scot’s pine (Pinus sylvestris)
  • Black locust (Robinia pseudoacacia)
  • Honeysuckles (Lonicera spp.)
  • Common buckthorn (Rhamnus cathartica)
  • Autumn olive (Elaeagnus umbellate)
  • Russian olive (Elaeagnus angustifolia)

Although highly prolific, Scot’s pine can simply be pulled by hand or cut above the root. Other woody species generally require cutting followed by application of a strong herbicide such as Garlon 4 or a high concentration glyphosate solution to the stump.

There is a wide range of herbaceous and graminaceous invasive plants that challenge tallgrass restoration practitioners. Among the worst of these are:

  • Spotted knapweed (Centaurea biebersteinii)
  • Canada thistle (Cirsium arvense)
  • Swallowworts (Cynanchum spp.)
  • Sweet clovers (Melilotus spp.)
  • Smooth brome (Bromus inermis)

Although burning can control many invasive plants once a tallgrass ecosystem has become well established, a few of the most invasive species are tolerant of fire. For example, large quantities of sweet clover may appear after a site has been burned for the first time. Fortunately, burns of successive years can bring this plant under control. These plants can be pulled by hand from sandy soils before they go to seed, but generally this is impractical on larger sites. Mowing regularly can be effective on some species when they are concentrated in patches, however this can be labour-intensive and take several years.

Applying glyphosate when plants are in flower, followed by a second application after several weeks, can be effective on many invasive herbaceous plants (this is not the case with many woody species). However, since this chemical kills most plants that it comes in contact with, spraying can have some unintended consequences. Although highly labour-intensive, wicking of individual plants may be necessary to avoid killing non-target species.

Ontario’s pesticide by-law restricts the use of chemical herbicides. Where chemical use is allowed, licensed applicators may have to be hired. Practitioners are advised to investigate how the law relates to their situation before proceeding.

For information visit the Ontario Ministry of Environment website:

Chemical use is one tool in integrative pest management and ideally should be considered a last resort. Unfortunately in some situations and with some species it is the only practical and effective means available.

For further information see:

Kaufman, S.R. and W. Kaufman. 2007. Invasive Plants: Guide to Identification and the Impacts and Control of Common North American Species. Stakpole Books, Mechanicsburg, PA.

Or visit the Ontario Invasive Plant Council website: