Description:

RUSSIAN KNAPWEED — Centaurea repens L.

Asteraceae — (Sunflower family)

Russian knapweed is perennial, forming dense colonies by adventitious shoots. It reproduces by seeds and creeping, horizontal roots. Roots, which are both vertical and horizontal in the soil, may or may not be black with a scaly appearance. The ridged stems are erect, rather stiff, openly branched, eighteen to thirty-six inches tall. Young stems are covered with soft gray hairs or nap. Leaves of newly emerging plants are toothed and covered with fine hair, giving them a blue-green color.

Lower leaves are deeply lobed, two to four inches long; upper leaves entire or serrate, narrow to a sessile base. The upper leaves are small and narrow with broken edges. Leaves attached midway up the stem have slightly toothed margins, while basal leaves are deeply notched. Cone-shaped flowering heads are ¼ to ½ inch in diameter, are thistle-like, solitary, terminal at the tip of leafy branchlets. Flowers are white to lavender. Many pearly involucral bracts form with rounded or acute papery margins.

Russian knapweed is a native of Eurasia, probably introduced in North America about 1898. It is now widely established in the Western United States. This species forms colonies in cultivated fields, orchards, pastures and roadsides.

Flowering occurs from June to September and seed is produced in late summer to early fall.

Growth Habit: Perennial herb, up to 3` tall, erect, may be in dense clumps. Grayish color.

Leaves: Alternate, simple, of several types: Upper leaves – small, narrow, unbroken edge; Stem leaves – intermediate in size, slight toothed margins; Basal leaves – deeply notched.

Stems: Numerous branches, each ending with a single flower.

Flower: Single, terminal, lavender, thistle like, scaly seed head.

Roots: Dark brown to black and heavily scaled.

Seeds: Flattened, ivory colored, retained in cup-shaped seed heads.

Other: Leaves and stems covered with short stiff hairs giving plant an appearance of knap. Spreads by seeds and creeping rootstocks.

(Courtesy of Weeds of the West)

Control Strategies:

Russian knapweed is a serious noxious weed and is very difficult to control or eradicate once it becomes established. It grows in cultivated fields, along ditch banks, fence rows, roadsides, and in waste places. Russian knapweed roots can reach depths of over eight feet and the plant is also very poisonous to horses. Russian knapweed can tolerate tillage very well, so normal farming practice serves only to spread the weed. It is very resistant to 2,4-D, but controllable with Tordon, Transline, Redeem, Milestone, and Banvel. Glyphosate mixed at the rate of 2 gallons of herbicide per 100 gallons of water and sprayed to wet will work, but the same herbicide applied with a boom sprayer provides little control. Maximum amounts of herbicide are needed to provide long term control. Most infested land will need to be treated annually to maintain control. One pint of Tordon per acre in the fall after frost works good and will let the grass grow next spring. It will need to be repeated annually. Two quarts of Tordon per acre will give long term stand reduction, but after six or seven years, the knapweed will be back unless there are follow up treatments. Where Tordon can't be used, Redeem, Transline, and Curtail can be used. They cost over twice as much per acre and the duration of control is less than with Tordon. Milestone looks like it will fit between Tordon and Transline for duration of control and will be cheaper than any of the other products. Russian knapweed is often found in mixed stands with Hoary cress. None of the previously listed products will control the Hoary Cress. A tank mix with Escort or Telar will control the Hoary cress while the other herbicide gets the Russian knapweed

Russian knapweed spreads slowly, but relentlessly. Over 25 years Russian knapweed on Muddy Creek spread from 27 acres to over 350 without anyone noticing until it was actually mapped. There are 850 sections of land in Fremont County that have Russian knapweed on them some where. Of those infested sections, 450 have less than 0.1 acres of Russian knapweed. Those sections will be much worse in 25 years unless that little patch of weeds can be controlled today. If you have small patches of Russian knapweed, control them immediately while the cost is low to protect the rest of your land from infestation.

To learn about biological control agents on Russian Knapweed visit this link.