Diffuse knapweed (Centaurea diffusa) is an invasive plant species, notorious for its prolific seed production and disrupting natural vegetation. In this article we will talk about how to control diffuse knapweed in your yard.
Diffuse knapweed Description
Diffuse knapweed belongs to the sunflower family. This invasive plant is commonly called white knapweed, tumble knapweed and spreading knapweed. The multi-branched weed is protected with spines.
Diffuse knapweed flowers in late spring or summer (usually from June through October).
It is an annual, biennial or short-lived invasive plant, 8-40 in. tall, upright stem with a ball-shaped appearance. Diffuse knapweed has deep and long fibrous taproots. The plant can germinate in summer or winter through seeds.
How to Identify diffuse knapweed
Basal grayish-green hairy leaves, 4-8 in. long. Stem leaves are less divided, alternate, stalkless, slender lobes on the stem which disappear near flowers. Leaves appear bract-like near to flower.
Diffuse knapweed flowers are white in color (sometimes pink or lavender) with urn-shaped heads.
Seeds of diffuse knapweed are achene, light brown to black, have plumes and are 0.08-0.12 in. tall.
Distribution and Habitat
Diffuse knapweed is native in western Asia and Southeastern Europe. The plant species was first recorded in North America in an alfalfa field in 1907 and arrived in Canada in 1936.
The plant species arrived accidentally in North America and Central Europe through contaminated hay (lucerne). The weed invades several states in eastern, western and east coast regions of the U.S.
In Colorado, the worst infestations of diffused knapweed occur along the Front Range in Larimer, Boulder, Douglas and El Paso counties. Severe infestations also occur in Archuleta and La Plata counties. A 2005 survey conducted by the Colorado Department of Agriculture found over 138,400 acres infested with diffuse knapweed and over 10,000 acres infested with spotted knapweed.
Due to its aggressive behavior, it is considered a restricted or noxious weed in 13 states of the U.S and 4 Canadian provinces.
Diffuse knapweed can tolerate a wide range of habitat and environmental conditions. Diffuse knapweed prefers to flourish in semi-arid or arid habitats. The plant rapidly colonizes near roadsides, natural grasslands, woodlands, plowed or burned areas, well managed or disturbed rangelands, open forests, overgrazed areas, waterways, industrial areas, gravel pits and especially on dry sites.
Life Cycle of Diffuse Knapweed
Diffuse knapweed produces low-lying rosettes, and may stay in this form for several years until favorable conditions arrive. The stem starts to appear in May. The growth of the rosettes is complete by the end of fall after passing dormancy, and bolting takes place in early spring.
If the growth is not completed in the fall, then the bolting process shifts to next year. The floral buds appear in early June and flowering takes place in July and August. Seed maturation occurs in mid-August. The dates can vary according to weather conditions. Each plant can produce up to 40,000 seeds which can stay viable for several years.
Medicinal value and other uses
- Diffuse knapweed can be grazed by sheep, cattle, goats, mule deer, white-tailed deer and elk at the rosette stage in early spring and winter. It is not poisonous and the young rosette has nutritional value, which can be used as forage in drought.
- Deers, birds and chipmunks eat the seeds of diffuse knapweed. The pollens and nectars of the plant are a source of food for honey bees.
- However, the spines of mature plant species can cause injury in the mouths of grazing animals and can damage their digestive tract. The plant should be handled with gloves, otherwise they can cause rashes and abrasions on the skin.
- The extract of diffuse knapweed has been reported for its antimicrobial activity whereas the medicinal value of the weed is still under study.
Why is diffuse knapweed invasive?
- Diffuse knapweed should be controlled before its establishment in rangeland. The weed can aggressively grow in both summer and winter and rapidly disrupts native biodiversity. After infestation, it forms dense mats (1-500 plants / m2) of vegetation and replaces native plant species. The displacement of native vegetation eventually affects the wildlife depending on the native plants.
- Due to deep taproots, the weed can survive heavy fire and can reestablish in favorable conditions. The continuous seed production and sequential emergence lead to the formation of monotypic stand and site dominance by the weed.
- Mature diffuse knapweed plants have spines. Grazing mature plants can cause irritation or can damage the mouth and digestive tract of wildlife.
- Each plant produces prolific seeds (up to 18,000 seeds) during the growing season which facilitates the reproduction of the weed in massive numbers, and in return reduces land value. According to reports, the economic loss was estimated about $ 1-3 million per year only in Washington, USA.
- This invasive plant increases soil erosion and causes depletion of soil moisture. This reduces the biological activity of microbes and decreases the fertility of the soil. The estimated crop losses in dry land can reach up to 88%.
- Diffuse Knapweed shows phytotoxic (growth-inhibiting) effects on companion plants and reduces the plant growth and seed germination rate of native plant species. Due to the greater negative impact, the invasive plant is included in the restricted weed list of Arizona and enlisted in the noxious weed list of Mexico, Ontario, Manitoba and British Columbia.
How does Diffuse Knapweed spread?
Diffuse knapweed spreads rapidly, a single strand of the plant can produce up to 40,000 seeds/m2. Seeds of diffuse knapweed can remain dormant up to several years and can reestablish in favorable conditions. The roots can regrow, even after the destruction of the above-ground parts of weed.
Seed dispersal takes place mostly through wind or by breakage of branches. Contaminated seeds or infested hay can cause long-distance spread of diffuse knapweed.
Water also facilitates the long-distance spread of diffuse knapweed by transportation through riverbanks and waterways.
The other means of long-distance weed dispersal are equipment, vehicles, humans or animals.
How to control diffuse knapweed
Diffuse knapweed is difficult to control in highly infested areas. Therefore, you should manage and prevent the weed infestation in the early stages. As a precautionary measure, monitor the site regularly to stop its establishment at the beginning. Also use certified and clean seeds.
You can control small infestations by digging and removing the root crowns. For larger infestations, chemical and biological methods are preferred. Land can be completely recovered by employing an integrated management plan for the infested site.
In heavily infested areas, a combination of treatments is applied to completely eradicate or remove diffuse knapweed. Several control methods are used according to the level of infestation to effectively remove the weed from the invaded site. According to reports, chemical and biological methods are most effective to completely remove the weed from the infested site.
Hand Pulling or Digging
In the case of early detection or small infestations, the hand pulling method can be used to remove the plant. Pull the complete seedling (along with the root system) from the ground during the first growth year.
You can also use a shovel or other tools to dig out the whole plant from the ground. Pack all the plant material in a bag without leaving any part, and properly discard this material. Always use gloves to protect yourself from any injury. Monitor the site for several growing seasons to completely remove the invasive plant.
Cutting and mowing
Mowing is not recommended to eradicate the infestation of diffuse knapweed because it will further encourage the growth of the weed in disturbed areas. However, the method can be used in combination with herbicide application for the most effective results.
The desirable plant such as grasses and forbs can be introduced on the site to create competition. Irrigation will stimulate the competitive potential of grasses. In infested rangelands, the weed can be completely eradicated by the treatment of herbicides. Desirable herbs and shrubs are introduced to prevent the regrowth and reestablishment of diffuse knapweed.
There are about 5 different weevils that can be used to control diffuse knapweed. They include Bangasternus fausti, Larinus minutus, Urophora affinis (seed head fly), Urophora quadrifasciata (seed head fly), and Sphenoptera jugoslavica (root boring/gall beetle).
Releasing the above weevils on the plant will deter the growth of the diffuse knapweed plant. They will take the nutrient the plant feeds on for themselves, causing the diffuse knapweed to die.
The female weevils feed on knapweed flowers and pollen to acquire nutrients necessary for egg production. They lay their eggs among the hairs on the opened flower heads. Each female weevil can produce between 28 and 130 eggs during her lifetime. They lay up to seven eggs in a single day. The eggs it lays have a three-day incubation period.
Larvae feed on the pappus hairs; subsequent larval instars feed on the developing seeds and receptacle tissue of the flower head. A single L. minutus larva, during its four-week developmental period, is capable of consuming all of the seeds of a diffuse knapweed head.
In areas where the weevil is well established, L. minutus larvae can readily destroy every seed head.
Mature larvae construct egg-shaped pupal chambers from seed fragments and pappus hairs within the damaged heads. Adults exit from the heads from mid-July to mid-August by chewing out a round hole at the top of the pupal cell.
Herbicides are usually the best method to control large or established infestations in areas that cannot be tilled. For diffuse knapweed, it is most effective to apply selective broadleaf herbicides in the spring. Infested areas should not be mowed until after the herbicide has had a chance to work. Non‐selective herbicides such as glyphosate (Roundup) can be used if damage to grass can be tolerated.
The timing of an herbicide application is very important to its success. Diffuse knapweed should be sprayed in the spring with selective herbicides between the time when the rosettes and seedlings start actively growing until the plant has bolted and before the bud stage.
Fall applications can also be effective if the plants are green and actively growing. Herbicides are most effective on actively growing plants with good soil moisture and warm, dry weather. Use a surfactant to enhance herbicide effectiveness. Herbicides should be used as one tool in an integrated pest management approach.
Multiple herbicides can be used to effectively eradicate the infestation of diffuse knapweed. Tordon 101® is applied at rosettes of diffuse knapweed during spring or fall. The concentration of the herbicide can vary with the level of infestation in a particular site.
Glyphosate can also be applied to control diffuse knapweed. However, the herbicide will inhibit the growth of weed during the first year of treatment. The weed will regrow in the next growing seasons. Another drawback of using glyphosate is that the herbicide does not selectively kills the targeted weed. So it can also damage the native plant species along with diffuse knapweed.
2,4-D and Dicamba
2,4-D and Dicamba can be used to control diffuse knapweed. Additional efforts are required for complete eradication of the weed from a heavily infested site. However, according to previous reports, 2,4-D and Dicamba reduce the weed infestation to such a significant amount that can be completely eradicated by 2-3 years of monitoring and hand pulling.