Invasive Species

Unfortunately, the stars have aligned this year for the knapweed crop.

I have received an inordinate number of calls this spring and summer about knapweed. Clearly, the climatic conditions have been suitable for a bumper crop of knapweed this year.

Most residents of the Okanagan-Similkameen are familiar with diffuse knapweed, the spiny “tumbleweed” that grows prolifically along disturbed roadsides, fence lines and other linear corridors, and significantly impacts important rangeland and wildlife habitat.

What many people do not realize is we have three species of knapweed in our region: diffuse, spotted and Russian. All three species are in bloom right now.

Diffuse knapweed is a highly branched, tap-rooted biennial or short-lived perennial that reaches heights of up to one metre. Its divided leaves are greyish green and hairy. Flower heads are broadly urn-shaped and have yellowish bracts that terminate in a slender bristle or spine.

This is the knapweed species that tends to scratch bare legs during the summer months or can cause a mild rash on forearms as you pull mature plants.

Flowers are typically white or sometimes lavender coloured. This spiny invader is unique from other knapweed species in that it breaks off at the base when it reaches maturity and dries out, and then it tumbles around in the wind. The spiny seed heads and branched nature of the plant also result in whole plants or fragments being caught in bicycle spokes or in the undercarriage of vehicles.

One plant can produce up to 18,000 seeds and they can remain dormant in the soil for up to 80 years. Because of these characteristics, diffuse knapweed has triumphed in the weed world. It has become very prolific in North America since its introduction over 100 years ago, likely via contaminated crop seed from Eurasia.

Spotted knapweed is the second most common species in our region. While not as widely recognized as diffuse knapweed, spotted knapweed deserves increased attention in our region as there are many niches it has not yet filled, most notably sites that are somewhat wetter and higher elevation than where diffuse knapweed typically grows.

It is similar to diffuse knapweed in its life cycle and overall appearance, but can be distinguished by the black-fringed bracts on the seed heads, giving the plant its ‘spotted’ appearance. The flowers are most commonly pinkish-purple, but can also be white. Spotted knapweed can cause skin irritation; hands and exposed skin should be washed with soap and water following contact with this plant.

Russian knapweed, the third species in our region, differs in its appearance and mode of reproduction. Stems are thin, stiff and branched; when young, the plant is covered with soft, short, grey hair. Russian knapweed’s well-developed root system functions as the major means of propagation and spreading and it tends to form dense patches along roadsides, riverbanks, irrigation ditches, pastures and other disturbed sites.

It can be distinguished from other knapweeds by the pointed, papery tips of the floral bracts. Its flowers are pink or purple, turning straw colour at maturity. Russian knapweed is toxic to horses and can cause chewing disease, a neurological disorder.

Since diffuse and spotted knapweed reproduce entirely by seed, the key to controlling existing infestations is to eliminate new seed production and deplete the existing seed bank.

Small infestations may be hand pulled to prevent seed production; plan to pull when soil moisture remains high enough to remove the entire taproot with ease.

Mowing or cutting at the early flower stage may also be used for controlling these knapweed species.

Physical control must be repeated over several years to deplete the seed bank.

There are biological control agents (natural insect enemies) for both diffuse and spotted knapweed, and we have seen considerable decreases in diffuse knapweed populations in the past couple decades as a direct result.

With Russian knapweed, a new biocontrol agent is being explored in our region. Cutting or removal of the aboveground portion of the plant reduces the current-year growth and may eliminate seed production.

Plants should be mowed or cut aggressively several times before plants bolt to weaken the root reserves. Plants that re-emerge are usually smaller in size and less vigorous.

For all three species, it is important to promote the growth of native plants or other desirable species to provide competition. Continued monitoring and follow-up treatments should be conducted annually to eliminate any re-infestation of knapweed.

Lisa Scott is the program co-ordinator for the Okanagan and Similkameen Invasive Species Society