Management Alternatives

Fertilizer. Fertilization generally increases the growth and reproduction of noxious weeds. This is not the case for hawkweeds when they are growing with other plants because hawkweeds are adapted to nutrient poor soils. One study done in England found nutrient addition reduced hawkweed populations over three years in plots grazed by rabbits and plots protected from grazing. The study ranked the severity of the decline according to the nutrient combinations added as: +N < +NK < +NP < +NPK. The decline was more rapid and greater where grazing was excluded because rabbits also impacted the perennial grasses. There were increases in perennial grass dry weight production where nutrients were added in the absence of rabbit herbivory. It is believed that fertilization reduced hawkweed populations in response to increased competition from perennial grasses and forbs. Annual fertilizer applications of 300 pounds/acre nitrogen, phosphorous, and potassium (15:15:15) are recommended to reduce hawkweed populations where competitive plants are growing with hawkweeds. On rangelands where other weeds that increase under fertilization are present, control measures to reduce these weeds should be applied before fertilization.

Herbicides.^1/ The hawkweed populations can be temporarily suppressed using phenoxy-type herbicides such as 2,4-D, aminopyralid, clopyralid, dicamba, and picloram. Application of 2,4-D is most effective at 1.5 to 2 quarts per acre (1.5-2 pounds active ingredient/acre) early in the growing season when hawkweeds are in the rosette stage of growth. Aminopyralid is effective when applied at 4 to 6 ounces/acre (Milestone®) to plants in the bolting stage of development. Clopyralid applied at 1.5 pints/acre (Transline® 0.5 lb a.i./acre) and picloram applied at 1 to 2 pints/acre (Tordon®) can suppress hawkweeds for up to six years. Dicamba at 2 quarts/acre (Clarity®) provides good control when applied to rosettes. Glyphosate applied at 2 quarts/acre will control hawkweeds where re-vegetation is planned. Addition of a surfactant in the spray solution will raise the efficacy of all herbicides by increasing adherence to the hairy stems and leaves of hawkweed.

Tillage. Hawkweed is not problematic in cultivated fields because crop competition and herbicidal control of weeds prevents its invasion in these areas. However, tillage alone will spread stolons and rhizomes and should be combined with crop rotation, nutrient management, and pest control practices. Tillage equipment used on sites with hawkweeds should be cleaned before use on weed-free areas to prevent weed spread.

Hand Pulling. Because hawkweeds have their perennial buds at the soil surface, hand pulling may be effective if rosettes and stolons are removed. Hand pulling may be practical where it can be repeated often on small patches, where there are competitive desirable plants in the community, and when combined with fertilization.

Mowing and Grazing. Mowing hawkweed is ineffective for reducing populations because the low-lying rosettes, stolons, and rhizomes grow below most mower blade height settings. Mowing infestations when plants bolt to produce flowers will prevent seed production but may encourage vegetative spread. Grazing animals eat the flower heads of hawkweed, but the rosettes are not usually eaten. Therefore, grazing may have a similar effect on hawkweed populations as mowing. The disturbance associated with grazing may help spread hawkweeds. In fertilizer experiments, the decline of hawkweed in fertilized plots was greater where grazing was excluded than in plots that were grazed. Prescribed grazing to maintain grass vigor will help prevent hawkweed invasion and spread. Over-grazing can make areas more susceptible to invasion.

^1/Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.

Biological Control. As of 2007, no insect species have been approved for release in the United States for control of orange or meadow hawkweeds. Host specificity testing has been conducted for five insect species. Two of these species have been dropped from consideration, testing continues for two of the species, and testing has been completed for the hawkweed gall wasp (Aulacidea subterminalis). Results indicate it is sufficiently host-specific for field release. Several other insects are being considered. In addition to the insects, two fungi are being tested in New Zealand.

Animal feeders of hawkweeds include nematodes (Tylenchus heiracii), Hemiptera (true bugs), Lepidoptera (caterpillars), Hymenoptera (ants, wasps, and bees), rabbits, and sheep. Parasites include basidiomycete and deuteromycete fungi.

Re-vegetation. Re-vegetation is an important management practice on disturbed sites under the threat of invasion by hawkweed, such as forest clearings and overgrazed meadows, and where dense populations of hawkweed have excluded competitive desirable plants. The development of a shading canopy is important to sustainable management of hawkweeds because hawkweeds are not shade tolerant. Shade from plant litter buildup will also reduce the competitiveness of hawkweeds. Seeding of competitive plants should be combined with herbicide application to reduce the competitiveness of hawkweed during establishment of the desirable plants, and fertilizer should be included to increase the competitiveness of desirable seeded plants.

Species selected for re-vegetating disturbed sites and hawkweed infestations should be appropriate for management objectives, adapted to site conditions, and competitive with the weed. Management objectives will determine if introduced or native species are seeded and the combination of species in the seed mix. The environmental conditions of the site including precipitation, soil texture and depth, slope and aspect, will affect species establishment. Refer to Montana Plant Materials Technical Note 46, ‘Seeding Rates and Recommended Cultivars,’ and Extension Bulletin EB19 ‘Dryland Pasture Species for Montana and Wyoming’ for seeding rate guidance and re-vegetation species selection. State and area resource specialists can help determine the most appropriate, site-specific species mix and timing of seeding.

Integrated Pest Management. Integrated pest management is the application of two or more management alternatives so they are complimentary in weed suppression, increase the longevity of control procedures, and improve crop production, or conservation of resources. Fundamental to any integrated management of hawkweeds is competitive desirable plants. On small hawkweed populations, aggressive herbicidal control should be combined with cultural practices that strengthen the competitiveness of the plant community. In areas with large scale infestations, herbicides should be used to eradicate small satellite populations and to reduce spread along the invasion front of the parent population. Annual fertilization can be used to reduce the parent population over time where competitive plants are present. On disturbed sites, pastures, and rangeland where competitive plants have been lost, re-vegetation following herbicidal control of hawkweeds and fertilization will improve the longevity of the control application. On crop and hay land in rotation, tillage followed by herbicide treatment of regenerating hawkweed plants will be more effective than either treatment applied alone.

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Last Modified: 10/11/2007