Grazing Control, Hand Pulling, Mowing and Tilling

Grazing Control

Diffuse knapweed is more likely to be grazed when it is green and succulent. Therefore, the timing of grazing management of diffuse knapweed using sheep or goats is important and repeated grazings are needed to affect populations. The first grazing should be timed to the bolting stage of development and before spiny flowerheads form. Subsequent grazings will be determined by local conditions and the re-growth of the knapweed. Utilization levels, duration, and frequency should be keyed on the desirable forage species to maintain vigor in competition with diffuse knapweed. Controlling diffuse knapweed using sheep or goat grazing is not compatible with seed-feeding bio-control insects because the grazers consume the flowerheads the insects need for larval development.

Hand Pulling

Hand pulling that extracts the root crown is an effective method to temporarily reduce diffuse knapweed on small-scale infestations, scattered plants either as new invaders or those persisting after herbicide treatments. Pulling or grubbing the root crown is most easily accomplished when the soil is moist and a shovel is used to pry-up the tap root. When the soil is dry the plant tends to break-off above the root crown, enabling regeneration. One experiment showed that when rosettes were cut just below the ground, 38 percent of the crowns re-grew compared to 4 percent of those cut two to four inches below the ground. Pulling should be done three times during the year; first in spring, second in June when plants bolt, and third before seed dispersal to target plants missed the first two times. Flowering plants should be sealed in plastic bags and disposed of in the trash to prevent seed spread. Wearing gloves while pulling diffuse knapweed will protect against potential skin irritation from spines and phytochemicals.

Mowing

Mowing will not control diffuse knapweed but it can be used to reduce seed production. When diffuse knapweed was mowed each month from April through October, 22 percent of the plants were still growing fours years after mowing began. Viable seeds can be produced after cutting if flowers have developed. Plants mowed in the early flowering stages can re-grow and produce late-season seeds. Mowing can alter phenological development, resulting in late-season flowerheads that will be out of sync with the seasonal timing of attack by biological control insects. Mowing can also stimulate new growth. Mowing when plants have bolted to produce a flower, but before flowering, will reduce root-energy reserves and seeds produced. Repeated mowing may be needed if plants reinitiate bolting after the first mowing. Washing mowing equipment after mowing will help prevent spread of this weed where the equipment is subsequently used on weed-free areas.

Tilling

Shallow tilling that severs the tap root below the root crown has been effective in reducing diffuse knapweed populations. This treatment may, however, increase regeneration from the seed bank by exposing buried seeds. Multiple tilling or tilling followed by application of herbicide to target establishing seedlings can be used to exhaust the supply of viable seeds in the soil. Tilling is only recommended in combination with seeding perennial competitive plants. As with any mechanical treatment, equipment should be washed after working infestations and before it is used on weed-free areas to prevent weed spread.

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Last Modified: 06/02/2008