Knapweed - Its cost to British Columbia

Knapweed - Its cost to British Columbia

Knapweed infestations are causing major environmental deterioration and loss of beef production in the southern interior of British Columbia. If left unchecked, the loss to ranchers and recreational users could be disastrous.

People are the major cause of spread. Awareness by all land users will reduce the impact of these weeds.

How to Identify Knapweed

There are two varieties of knapweed of concern to B.C. -- both equally harmful to B.C. rangelands.

Diffuse Knapweed

the more common of the two varieties.
has a single upright stem 20-100 cm (8-40 inches) tall and numerous spreading branches.
plants bolt in early May, usually producing one stem with numerous flower buds which bloom in July and August.
the white, sometimes pinkish flowers are surrounded by yellowish-green bracts which narrow to short, stiff spines.
over 900 seeds per plant are formed under rangeland conditions while over 18,000 seeds may be produced under irrigation.
easily recognized by the short rigid spines on the flower heads.
a biennial to short-lived perennial weed overwintering as seeds or low growing rosettes.
extremely bitter tasting but non-poisonous.

Diffuse knapweed plant

Diffuse knapweed flowerhead

Spotted Knapweed

classified as short-lived perennial weed
overwintering rosettes bolt in early May to produce one to 15 stems.
thistle-like purple flowers bloom from July to October, capable of producing over 400 seeds per plant under range conditions and over 25,000 seeds under irrigation.
easily recognized by a black-tipped fringe on the flower head.
flower head has a spotted appearance.
extremely bitter tasting but non-poisonous.
branching occurs in the upper half of the plant and these end in purple, occasionally white, flowers.
20 - 120 cm tall (8 - 48 inches).

Spotted knapweed flowerhead

Environmental Loss

Knapweed is highly competitive, capable of invading grassland sites to the exclusion of native vegetation.

Domestic animals and wildlife such as elk rely on these range grasses and herbs for up to 80 per cent of their diet. Knapweed encroachment can destroy the forage base and would result in a significant decline in deer and elk numbers.

Over 40,000 hectares (100,000 acres) in B.C. are knapweed infested, reducing forage potential by up to 90 per cent. There is a potential in B.C. for spread to 1 million hectares (2.7 million acres) of grassland range and undetermined areas of fringe forest.

If spread to its ecological limit, knapweed could infest up to 8 to 10 million hectares (20 to 26 million acres) in western Canada.

Distribution in British Columbia

Distribution of Diffuse and Spotted Knapweed in British Columbia

Economic loss

To date, knapweed has resulted in an economic loss of over $400,000 annually in equivalent hay production in B.C. If knapweed spread to its limits, that figure could climb to over $13 million annually.

A People Problem

Introduced to B.C. from Eurasia in the early 1900's, knapweed in North America has no natural enemies or parasites to keep populations in check. People help spread knapweed as it is easily caught up and transported great distances in the undercarriage and doors of:

recreational vehicles
trains
light aircraft landing at infested airstrips
logging trucks and heavy machinery

It is also spread by:

Florists, who use knapweed in dried floral arrangements.
Movement of hay from knapweed infested to non-infested areas.
Animals and birds - cattle, deer and elk may pick up the weed and disperse it, and some bird species and small rodents eat knapweed seed.
Soil disturbance provides an ideal seedbed for new knapweed infestation.
Wind severs diffuse knapweed at ground level when mature to blow around in tumbleweed fashion.

Is there any way to control Knapweed?

Successful long term knapweed control will require the combination of proper grazing management, judicious herbicide use, development of effective biological control and a high level of public awareness and responsibility.

Cultural Control

Pull small patches of knapweed before they set seed. On grasslands, sound range management is the key to success. Seed bare soil and overutilized range with adapted grass species and provide adequate water, salt and fencing to obtain proper stock distribution. Do not overutilize for the amount of forage present.

Chemical Control

Knapweeds are easily controlled on rangeland, permanent grass pasture and non-cropland with low rates of selective herbicides. Containment programs underway in B.C. are carried out by trained personnel and are designed to protect currently uninfested grassland.

Biological Control

British Columbia is heavily committed to the use of natural agents to control knapweed. These weeds are not a problem in their native countries as there are numerous biological agents (usually insects) that use knapweed as a food source. Once research has proven that the candidate agent will not feed on any other plants and approval for release in Canada and the U.S. has been granted, the agent is released to B.C. for propagation and redistribution.

Seed-reducing flies (Urophora sp.), now well established throughout infested areas, have resulted in up to 95% reduction in knapweed seed production.

Adult Urophora Fly

Further reductions in seed production have resulted from release of a moth (Metzneria paucipunctella) against spotted knapweed and a root-feeding beetle (Sphenoptera jugoslavica) against diffuse knapweed.

Root-feeding Agapeta moth
prefers moist sites Sphenoptera beetle larva results in
knapweed root damage

Other useful insects include a root-feeding moth (Agapeta zoegana) for moist sites and a root-feeding weevil (Cyphocleonus achates) which attack plants with large rosettes.

Root feeding Cyphocleonus weevil

The combination of several of these agents is expected to place enough stress on the knapweeds to significantly reduce the problem in B.C.

What can you do?

Learn to recognize the knapweed species.
When travelling in B.C's rangelands, stay on established roads. Do not drive across the grasslands.
If involved in any activity such as logging or road construction which disturbs the top soil layer, ensure that the disturbed area is reseeded immediately.
Check your vehicle and remove attached knapweed before leaving an infested area.
Report knapweed sightings in remote areas to the B.C. Ministry of Agriculture and Food or the B.C. Ministry of Forests district office.
Pull up isolated knapweed plants and remove them from the area for burial
Don't purchase knapweed-infested hay.

For More Information

Consult your local office of the B.C. Ministry of Agriculture, Food and Fisheries.

Return to Pest Management Home