Control Invasive Plants

Giant Hogweed Removal

The Coastal ISC promotes Integrated Invasive Plant (IP) Management. Integrated IP management involves using a combination of appropriate control methods to reduce the weed infestation to an acceptable level, and requires monitoring these activities to evaluate the effectiveness of the control strategy.

The best control option depends on a number of factors including; knowledge of the potential damage of the weed, relative abundance of the plant species, characteristics of the site, the cost of the control method, and the environmental impact of the weed and the control option. 



Prevention, early detection and eradication of weed species is the most economical and effective means of invasive plant management. It is important to ensure new weed species or vegetative reproductive plant parts are not introduced into a new area.

Invasive plants can be spread in the following ways:

  • Contaminated seed, feed grain, hay, straw, mulch
  • Movement of unclean equipment and vehicles across uncontaminated lands
  • Livestock and wildlife
  • Spreading gravel, and road fill that contains seed
  • Nursery industry
  • Recreation
  • Water, and wind transportation

Once invasive weeds are identified, it is important to take action to ensure that they do not spread to uninfested areas. This includes managing activities on grasslands and seeded pastures to maintain healthy plant communities, promoting low growing vegetation, avoid practices that disturb the soil and promote the dispersal of weeds, and reseed disturbed sites as quickly as possible.

Measures to prevent invasive plant spread:

  • Ensure vehicles and equipment are clean of invasive plants and seed
  • Minimize soil disturbance in all construction and maintenance activities
  • Promote the establishment of a health plant community
  • Limit the movement of weed-infested soil or gravel
  • Use certified weed-free seed mixes or vegetation in disturbed areas to provide competition for any new weeds
  • Treat new infestations quickly – work with local invasive plant groups to deal with new infestations.
  • Contain neighbouring infestations and restrict movement of invasive plants from adjacent lands. Roadways, railways and waterways are often corridors for invasive plant spread and should be monitored for invasive plant establishment.

Protect areas that are not infested. Educate yourself and others about weeds in your area. Prevent soil disturbance whenever possible. Re-seed disturbed soils to prevent weeds from establishing. Keep machinery and vehicles clean. Do not move weed infested soil or gravel.

Mechanical Control

Mechanical control usually refers to the mowing or mechanical cutting of an invasive plant infestation to limit seed production. With mowing, timing is essential. Invasive plants must be removed before the plants go to seed in order to be an effective method of control. Plants should be cut as close to the ground as possible and may have to be treated more than once in a growing season to achieve desired results.

Benefits of mechanical control:

  • Works well for areas that have favourable terrain that is accessible.
  • Can be used in environmentally sensitive areas.
  • Reduces seed production.
  • Most effective on annual or biennial plants.

Limitations of mechanical control:

  • Plants must be mowed before they produce seed.
  • May not be suitable in some environmentally sensitive areas.
  • Not suitable for steep slopes or rocky, unstable terrain.
  • Will not always kill plants, but will decrease seed production for that year.
  • Perennial plants require several cuttings as they flower multiple times through the growing season.
  • Plants need to be cut as close to the ground as possible.
  • Non-target vegetation (natural forbs and shrubs) may be impacted.
  • Must be done repeatedly to exhaust seed bank in the soil.

Manual Control

Manual invasive plant control usually refers to hand-pulling or digging. Manual control works well for dealing with single plants or small infestations that can be eradicated with a small amount of labour. It is most effective if invasive plants are shallow rooted and the soil is loose or moist. One should be aware this type of control may not be effective for invasive plants that also reproduce by roots and rhizomes. In these instances, limited hand-pulling or digging may actually increase the size of the infestation.

Benefits of manual control:

  • Can be used in environmentally sensitive areas.
  • Can be used to manage small patches or individual plants.
  • Works best in moist, loose soils.
  • Persistent pulling can even effectively manage some creeping perennials.

Limitations of manual control:

  • Labour intensive
  • Limited to small infestations.
  • Many invasive plants reproduce through rhizomes and extensive lateral root systems, so manual control is not effective
  • Plants must be pulled before seeds are set.
  • Pulling must be done repeatedly to exhaust seed bank in the soil.

Cultural Control / Competition

Cultural control and competition includes re-vegetating, irrigating or fertilizing to encourage the establishment of a healthy ground or crop cover to resist invasive plants. When natural vegetation or soil is disturbed, cultural control can be an effective tool in invasive plant management. Re-vegetated or intensively managed plant communities can offer competition for invasive plants. In some cases where invasive plant species are found in soils deficient in sulphur, fertilization of these sites can help to create competition of natural plant communities, or cultivated crops to decrease the invasive plant population (i.e. ox-eye daisy infestations).

Quit often, the removal of invasive plants results in exposed bare ground. In these cases, cultural control (i.e. re-vegetation) should be used as part of a long-term management strategy. Re-vegetation can assist in preventing the return of an invasive plant or the introduction of new invasive species in an area.

Some factors for consideration when re-vegetating sites include:

  • Viability of vegetation
  • Seeding rate (for seed mixtures)
  • Soil moisture conditions
  • Physical properties of the soil.

Benefits of cultural control:

  • Valuable in encouraging long-term management of invasive plants.
  • Can be used in environmentally sensitive sites.
  • Can include use of native plants

Limitations of cultural control:

  • Site and soil conditions can be unfavourable.
  • Cultural control methods can be labour intensive and costly

Chemical Control

Various herbicides are approved for treatment of invasive plants within British Columbia and, used properly, can be the most effective option for certain persistent invasive plants. The type of herbicide and application method will vary, depending on the target weed species and environmental considerations.

Large infestations, infestations near water, or infestations on steep slopes may be too costly or too environmentally sensitive to control by chemical means. In these situations, it is important to look at other management options. Also, if chemical control leaves a site bare, it is important to re-vegetate the site so that control is achieved over the long-term.

Factors that can affect the effectiveness of herbicides include:

  • Invasive plants with waxy or hairy leaves may not easily adsorb the required amount chemical to kill the plant.
  • Invasive plants are usually most susceptible to herbicide during its active growth stage. This is often in the seedling stage or the bud or early flowering stages.
  • Cool or extremely hot dry temperatures may decrease translocation of systemic herbicides.
  • Soils with high organic matter or clay content may require higher rates of chemical than sandy soils.
  • Soil moisture and pH can also affect persistence and effectiveness of some herbicides.

Benefits of chemical control:

  • Effective tool for new and small infestations of invasive plants.
  • Will kill target plants
  • Can have residual control of seed-bank for future years depending on the chemical selected.
  • Less labour intensive than alternative mechanical and cultural methods.

Limitations of chemical control:

  • Precautions need to be taken to limit the effects on surrounding non-target plants
  • Limited use in environmentally sensitive areas or steep slopes.
  • May have limitations of certain soil conditions or presence of water.
  • Some concern from community groups
  • Some concern of usage on traditional First Nations lands

Selection of herbicide should depend on your target weed species, other crops or plants on site, environmental considerations, and meeting your management objectives.

Biological Control

Biological control involves using living organisms to reduce seed production and vigor of an invasive plant species. Many invasive plants came from Europe or Asia without their natural insects or diseases to keep them in check. The province of British Columbia has partnered with other agencies, provinces and Agriculture and Agri-Food Canada to introduce agents that are natural enemies of the invasive plant into North America. Before an insect is released, it must undergo extensive testing to ensure that it will not attack any other plants.

Biological control often works best on large infestations, or infestations that are near water. It is a long-term approach and often it takes many years for insects to establish and results to be seen. In some cases, a single biological control agent can adequately control an invasive plant species. However, in most cases, a variety of agents are needed to achieve control of the weed species population levels. Biological control will not eradicate the infestation directly. Rather, the agents are used to decrease the vigor and seed production of the plants in order to decrease their competitive ability. Therefore, it is important to use other weed management strategies to ensure that the infestations are contained. Some insects may already be present on site. Local weed specialists or Agrologists can assist in identifying insects present and assisting with obtaining insects for biological control.

Biocontrol has been used on Vancouver Island and surrounding coastal communities since the 1960’s. Over decades, biocontrol can be an effective means of controlling plant populations that have grown too large or dense to be eradicated. Only some invasive plants have biocontrol agents, and of those agents only a fraction are capable of establishing in the temperate West Coast climate. Invasive plants with successful biagents on Vancouver Island and surrounding coastal communities include: Canada thistle, tansy ragwort, the knapweeds, Dalmatian toadflax and purple loosestrife. Screening of additional biocontrol agents is underway. Biological control agents are not available for all invasive plant species. However, since biological control research is ongoing, this list is constantly changing. Check with local weed committee or the B.C. Ministry of Natural Resource Operations office for up-to-date information about insects and availability.

Another approach to biological controls is the inundative or bioherbicide method which usually involves native, naturally occurring pathogens applied in a manner that can cause death or significant damage to the target weed. Since indigenous pathogens are subject to natural controls, impacts much beyond the target plants are unlikely. Simply spraying cultures of the pathogen onto target weeds is not necessarily effective as conditions for infection are often very complex, involving the pathogen, the host and the environment. Additional treatments to the weed hosts to lower resistance to infection, e.g. through wounding or off-season treatment, may be necessary. Also, modification of the micro-environment by incorporating sunscreens or other adjuvants often will assist infection. The pathogens or bio-agents are usually fungi, the most common causes of plant disease, so bioherbicide formulations are sometimes called mycoherbicides. They may also be developed from bacteria, nematodes, insects or other organisms. Thorough study of candidate bioherbicides and target hosts to ensure sufficient efficacy and minimal non-target damage is essential before they can be registered for use under Health Canada’s Pest Control Products’ Act. Not many bioherbicides have been registered so far but some of them are based on generalist pathogens with multiple host ranges and may therefore be suitable for testing on invasive plants.

Benefits of biological control:

  • Uses the plants natural predators for control.
  • Can be used on environmentally sensitive areas, areas near water and steep slopes
  • Suited to large infestations
  • Reduces the vigor of invasive plants being attacked and can reduce seed production
  • Used for long-term management of invasive plants.

Limitations of biological control:

  • Biological control agents are not available for many invasive plant species.
  • Will not “eradicate” an invasive plant species.
  • Can take multiple years (sometimes 10-20) before any noticeable difference in invasive plant populations are seen.

Biological control agents can require specific site conditions for success.