Invasive Plants of Southwestern B.C.





            The following pages detail several invasive plants that are jeopardizing the ecological balance in southwestern British Columbia. The purpose of this site is to inform public of the growing concerns associated with invasive (a.k.a exotic, alien, noxious) plants. Specifically, this site describes key features of ten common invasive plants, emphasizing identification and control techniques. Hopefully, the following information will be of use to community groups and individuals who are interested in taking an active response to the escalating problems associated with invasive plants.




Langley Environmental Partners Society: lepsgen@tol.bc.ca

Paul A Graham: p.graham@telus.net

David R Clements: clements@twu.ca



Invasive Plant Fact Sheets Invasive Plants Introduced (top)


English ivy

Himalayan balsam

Japanese knotweed

Purple loosestrife

Reed canary grass

Scotch broom

Tansy ragwort

Yellow archangel


1.0  Invasive plants introduced

      1.1 What are invasive species?

      1.2 Why are these plants a


      1.3 How did they get here?

      1.4 How pervasive are invasive

           plants in southwestern British



2.0 Measures against invasive plants

      2.1 Education and awareness

     2.2 Legislation

     2.3 Control

          2.3.1 Physical Control

          2.3.2 Chemical Control

          2.3.3 Biological Control

3.0 Management Strategies

     3.1 Upstream

     3.2 Random

     3.3 Social

     3.4 Population


4.0 Getting Involved


5.0 Definitions


6.0 Links and






1.0  Invasive plants introduced


1.1 What are invasive species? top


            Invasive species consist of plants, animals, fungi and other organisms that exhibit extraordinary reproductive success when introduced to foreign environments. These exotic species compete with and dominate native plants and animals, often forming monocultures. The following is one definition of an invasive plant:


an alien plant spreading naturally (without the direct assistance of people) in natural or semi-natural habitats, to produce a significant change in terms of composition, structure or ecosystem processes


1.2 Why are these plants a problem? top


Invasive species usually lack natural predators and disease and take advantage of favorable ecological conditions in which to grow. In essence, alien species encounter few of the factors that regulate their population in native or natural environments. Consequently, invasive plants often run rampant, spreading at phenomenal rates, making control efforts difficult and eradication especially challenging. If an invasive or exotic species becomes naturalized and spreads throughout the system, getting it out of that system [can be] like trying to unscramble an egg (McKnight, 1991). Alien invaders often change community species composition and structure, reducing natural diversity of native plant and animal communities. Impacts range from economic to strictly environmental effects; some are a serious threat to humans. The following list illustrates selected detrimental impacts:


        1. replacement of diverse systems with single species stands of aliens

   eg. Himalayan blackberry forms dense, impenetrable stands that prevent animals from accessing watering holes in the Lower Mainland


        2. invasion that poses a direct threat to native fauna

   eg. Lantana camara threatens to remove the breeding site of an endangered bird, the dark-rumped petrel (Pterodromo phaeopygia) in the Galapagos Islands


        3. alteration of soil chemistry

   eg. Gorse (Ulex europaeus) acidifies surrounding soils, preventing native species from growing (Lower Mainland)


·       4. alteration of soil processes

   eg. Himalayan balsam takes over stream riparian areas, facilitating bank erosion in the fall when this annual dies back (Lower Mainland)


·       5. invasion leading to plant extinction

   eg. Rhododendron is threatening the survival of rare Atlantic bryophytes in woodlands in the west of Ireland


        6. alteration of fire regime

   eg. Melaleuca quinquenervia has increased the frequency of fires in Florida, damaging native vegetation


        7. alteration of hydrology

   eg. Reed canary grass forms large monocultures along riparian areas, resulting in increased water run-off and erosion (Lower Mainland)


1.3 How did they get here? top


            Unfortunately, records of alien plants are limited, and we can only guess as to the specific means of introduction for these invaders. What we do know is that since the seventeenth century, European settlers transported thousands of plants to North America for wide-ranging purposes including medicinal, agricultural, and ornamental use. Once introduced, invasive plants can become established beyond the point of initial introduction, essentially escaping into foreign environments. Although only a small proportion of plants behave “invasively”, the consequences stemming from these select are often far-reaching.


1.4 How pervasive are invasive plants in southwestern British Columbia? top


            The optimal growing conditions of this coastal region have enabled several invaders to thrive, especially in disturbed areas. Consequently, many exotics have become naturalized in southwestern B.C., threatening streams, wetlands, and other sensitive environments. This problem is amplified when one considers the diversity of our region that is at stake including an array of native plants, wildlife, and salmonid species. 


2.0  Measures against invasive plants top


2.1 Education and awareness top


            Introducing plants into new environments has and continues to be a largely intentional practice. Unfortunately, we humans do not always consider the impacts that alien plants can have in foreign habitats. Consequently, the impact of an invasive weed may go well beyond the intended effect of the introduction (Rossman, 2001). Once established, invasive plants have proven to be robust and pervasive. Awareness is the key to preventing unintended environmental disaster caused by invasive species introductions. People need to learn the difference between native and exotic plants, the fact that alien plants can harm native plants, and that seemingly harmless activities, such as agriculture or gardening can result in ecological disaster. Education is a tool used to inform the uninformed. Through activities such as teaching school children, holding community outreach events, and creating informative materials, people will begin to understand their role in invasive plant prevention and management.


2.2 Legislation top 


Another perspective to address the issue of invasive plants is the legal approach. This method entails advocating for laws and quarantines against invasive species to prevent new species from entering countries while simultaneously increasing awareness of those species already established. This type of lobbying can prove to be onerous, especially if governments fail to see the need for concern. However, all levels of government in Canada are beginning to recognize the importance of noxious weeds, responding in some cases through publishing material to help foster public awareness for these plants.


2.3 Control top


There are three primary means of control: biological, chemical and physical - all with numerous advantages and drawbacks. Control methods must consider environmental impacts, efficiency, costs, and effectiveness on a particular species. Since each species is susceptible to different control methods and approaches, appropriate timing and management strategies are absolutely essential for obtaining results. Additionally, control methods must be extremely intensive and efficient as virtually all invasive species in a localized area must be destroyed to make any significant gains (in most cases). Some suggest the aim of control is to limit the number of plant propagules in the long term to a tolerable level to human activities (Groves, 1989). This approach excludes the possibility of eradication as a management aim. However, there are cases where eradication is a feasible goal. Perhaps a more desirable perspective suggests that containment programs use a mix of control efforts that are as species specific and environmentally benign as possible – but that get the job done (McKnight, 1991). The bottom line is that all successful control techniques require stringent planning, effective strategies, and long-term monitoring.    

2.3.1 Physical Control top


Physical control can be divided into two main categories: mechanical and manual.  Mechanical control includes practices that use machinery to accomplish species removal. Examples include using excavators to remove large sections of blackberries and cultivating agricultural fields with farming equipment. Mechanical methods are only practical when the control area consists of a monoculture and where machinery does not destroy sensitive habitat. Native plant communities become jeopardized when machinery cannot distinguish between these and exotic species. Manual control includes methods such as hand-pulling, cutting and digging. Unlike mechanical control, manual control is species specific. Although labor-intensive, manual efforts are often effective, especially in sensitive ecological areas. Planting indigenous species is another method accomplished manually which aims to effectively compete with and shade out invasive species. Once established, native planting can provide successfully long term control. Overall, physical control can provide practical success, especially in isolated ecosystems.    


2.3.2 Chemical Control  top      


Chemical control uses pesticides, herbicides and other chemicals to control the growth and development of invasive plants. Chemical control can be expensive and require repeated treatments. Once treatment ends, exotic species may resume growth and proliferation. Additionally, this treatment is sometimes environmentally unsound, adversely affecting the surrounding environment of the treatment area (i.e. invertebrates, small mammals, microorganisms, native plants). Chemical control may be effective in limiting newly discovered infestations which have yet to spread, assuming the herbicide quickly breaks down in soil or water (eg. glyphosate or 2, 4-D). It is suggested that chemical control of already widespread invasive plants in nature reserves is often expensive, usually ecologically undesirable and rarely, if ever, effective in the long term, unless integrated with other methods of control (Groves, 1989). Logically, this would be applicable to situations existing in the Lower Mainland as well, especially in riparian zones of wetlands.


2.3.3 Biological Control  top     


Biological control is based on the principle that every plant has natural enemies in its native habitat and many of these are very host-specific. Accordingly, this control method acts to introduce the natural predators, sometimes referred to as biological agents, of invasive species. In many cases, this method proves both safe and successful, with long-lasting effects. The negative aspects of biological control are that it involves a reduction rather than eradication of spread, it is slow acting, difficult to predict, and is unsuitable for a mixture of weeds. On the positive side, once classical biological control agents are successfully established, this type of control is permanent, requiring little or no further efforts (Fowler and Holden, 1994). Biological control can also access areas where terrain restricts other control techniques. Consequently, biological control provides a cost-effective method for controlling invasive species.     


3.0  Management strategies top


After considering different control methods, it is essential to consider the management strategies of these control mechanisms. Management strategies describe the approach taken to controlling specific invasive species, taking reproductive, geographic, and other related information into consideration. The following pages describe both management strategies and control mechanisms for a variety of invasive species that exist in the Lower Mainland. For example, Himalayan blackberry is often approached using a combination of chemical and manual methods at specific time periods. Management strategies describe how and when these techniques should be applied. Some basic categories of management are listed below (Wadsworth et al, 2000).  


3.1 Upstream top


This strategy attempts to limit the impact of dispersal along rivers by tackling upstream plant populations first. There are several plants that disperse seeds and vegetative roots via waterways such as Himalayan balsam and Japanese knotweed. By removing upstream sources first, the ability for invasive plants to migrate or colonize downstream is reduced.


3.2 Random top


This strategy controls specific areas or cells at random until the maximum number of infestations have been found and treated. This can be a useful technique to eradicate pockets of infestations.


3.3 Social top


In this technique, infested areas which have the highest human population density are systematically treated first, assuming more people equates to more frequent human-plant interactions. This strategy is actually a mimic of realistic responses to a perceived problem, such as a local authority responding to complaints or notifications of the species' presence. Unfortunately, this process is often inadequate for effective control.


3.4 Population top


This strategy attempts to focus control where invasive species are more prevalent, based on the size of the plant population. From here, decisions can be made to prioritize which habitats are suitable to treat in terms of cost-benefit analysis. Other considerations may emphasize tackling more recently established populations or those infestations that can be treated with the least environmental impacts.


4.0 Getting Involved top


            Volunteer groups to control invasive species have emerged in B.C. with variable success. These groups can work on their own or alongside local or provincial agencies. The key to efficient control is for groups to establish clear goals before commencing efforts, based on careful assessment of both plants and target sites. To start, invasive aliens should be prioritized to streamline efforts. Planning must also include long-term monitoring, otherwise efforts will likely be ineffective. Another key consideration for volunteer groups is to maintain quality records and photos, which can provide useful data for research, future planning, and to measure success. Once groups become established, it is a good idea to become familiarized with plant identification (native & invasive) and control techniques.


5.0  Selected Definitions top


2,4 -D - a readily available type of synthetic herbicide that controls broadleaf plants (but not grasses) as it is absorbed into plant cells, disrupting plant growth

alternate - placed at uneven sequences upon the stem; leaves are not opposite each other but rather there is one bud or one leaf at a node

annual - a plant whose life cycle is completed in a single growing season

biennial - a plant that normally requires two growing seasons to produce its flowers and fruit, growing leaves the first year, flowering, fruiting, going to seed, and dying the next

compound leaf - a leaf whose blade is divided into several distinct leaflets

evergreen - retaining foliage year round

exotic - not native, from another area; introduced

glabrous - smooth; not rough, pubescent or hairy

glyphosate - a common synthetic, non-selective herbicide that can kill any plant including well-established trees; readily absorbed through leaves

inflorescence - a flower cluster, with a definite arrangement of flowers

lance-shaped - elongate in shape, broadest below the middle and gradually rowed toward the tip

leaflets - a little leaf, 1 of the divisions of a compound leaf

midrib - the central, main prominent vein of a leaf

monoculture - a land mass containing a single reproducing species

native plant - of indigenous origin or growth; not exotic or of foreign origin; for example the California poppy is native to California

noxious weed - plants which may cause harm to collectors; invasive exotics which may harm the ecosystem or agriculture of an area

opposite - term applied to leaves occurring in pairs at a node

ovary - the enlarged, rounded, basal, ovule-bearing part of a pistil, ultimately becoming the fruit of a plant

perennial - a plant in which the vegetative structures live year after year

pistil - the female or ovule-bearing organ of a flower; a complete pistil consists of three parts: ovary, style (when present), and stigma

pH - a logarithmic scale that measures acidity/basicity

pubescent - covered with hairs, especially if short, soft, and down-like

rhizomes - a thickened, modified stem lying on or under the ground, bearing scale like leaves, and usually producing shoots above and roots below

Rodeo - a brand of glyphosate herbicide that is deemed safe for use near water in the U.S.

rosette - a cluster of leaves which grows in a circular overlapping pattern, somewhat like the petals of a rose

Roundup - a brand of glyphosate herbicide, approved for consumer use in Canada

runner - see stolon

sepals - one of the outermost flower structures, usually enclosing other flower parts in the bud

serrate - having sharp teeth pointing forward

stamen - the male reproductive, pollen-bearing organ of flowering plants, consisting of the filament and the anther

stolon - a stem that grows horizontally along the ground surface and may form roots or shoots at its nodes; eg. runners of a strawberry plant

styles - a slender column of tissue that arises from the top of the ovary

Triclopyr - a broadleaf herbicide used to control woody plants, pasture, and rights of way

vegetative reproduction - propagation by asexual means, such as stem cuttings, leaf cuttings, layering, root division, or bulblets;  in seed plants, reproduction by means other than via seeds; produces genetically identical offspring; can be natural or artificial (cloning)

whorl - a circle of radiating leaves, flowers, or other organs around a common center, on the same plane


6.0 Links and References  top


From Fact Sheets  top

    Literature  top