Yellow Starthistle Information
Mech control
-hand
-tillage
-mowing
Cultural control
-grazing
-burning
-re-vegetation
Biocontrol
-insects
-biocontrol table
-plant pathogens
Chem control
- risks
--spray
--water
--toxicology
--herbicide
resist
--effects
-herbicides
--preemergence
--postemergence
--late season
--pre- & post-
--imazapic
--clopyralid
--picloram
Management
Postemergence
A limited number of postemergence herbicides are registered for use in rangelands, pastures, and wildlands. They include 2,4-D, dicamba, triclopyr and glyphosate. These postemergent herbicide treatments generally work best on seedlings. However, they are not effective for long-term management of starthistle when used in spring, as they have no soil residual activity and will not control yellow starthistle plants germinating after application. Since yellow starthistle has the ability to germination throughout winter, spring and into summer whenever moisture is available, achieving control with a single application is almost impossible. A treatment following the first flush of seedlings opens the site up for later flushes. Waiting until later in the rainy season to apply a postemergent herbicide allows a greater number of seedlings to be treated, but larger plants will require higher herbicide rates and may not be controlled (DiTomaso et al. 1999c). As a result, repeated applications of broadleaf selective postemergence herbicides are often necessary (DiTomaso et al. 1999b). This increases the risk of drift to non-target species and is expensive.
Thus, the most effective way to use postemergence compounds for starthistle control is to incorporate them into later stages of a long-term management program. In particular, they are effectively used to spot-treat escaped plants or to eradicate small populations in late season when starthistle is easily visible but has yet to produce viable seed. By using spot applications in late season, total herbicide use and expenses can be reduced because only small sections or individual plants are treated. It is important to note that plants should only be treated when not exposed to severe stress. Drought stress can especially reduce the efficacy of most herbicides.
Growth regulator herbicides, including 2,4-D, dicamba and triclopyr, will cause damage to late season broadleaf species, including desirable natives.
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2,4-D can provide acceptable control of yellow starthistle if it is applied at the proper rate and time. Treating plants in the rosette growth stage provides better control than later applications. Amine forms are as effective as ester forms at the small rosette growth stage, and amine forms reduce the chance of off-target movement.
Application rates of 0.5 to 0.75 lb ae (acid equivalent)/acre will control small rosettes. Applications made later in the season, when rosettes are larger or after bolting has been initiated, require a higher application rate (1 to 2 lb ae/acre) to achieve equivalent control (DiTomaso et al. 1999b, Northam and Callihan 1991, Whitson and Costa 1986) [see Tables A, B, C, D]. 2,4-D is a growth regulator selective herbicide and will control other broadleaf plants, but generally will not harm grasses. It has little, if any soil activity. Drift from 2,4-D applications is common, particularly from the ester formulations. 2,4-D is a restricted use pesticide, requiring a permit for use. -
Dicamba is very effective at controlling yellow starthistle at rates as low as 0.25 lb ae/acre (Callihan and Schirman 1991). When yellow starthistle rosettes are small, about 1 to 1.5 inches across, the 0.25 lb ae/acre rate works well, but higher rates (0.5 to 1.0 lb ae/acre) are needed if plants are larger (Northam and Callihan 1991). Applications made in late rosette to early bolting stages have provided excellent control, although earlier treatments are better.
Dicamba is also a growth regulator selective herbicide that controls many broadleaf plants, but generally will not harm grasses. Its soil activity is very short. Like 2,4-D, it is also available in both an amine and ester formulation. Drift from dicamba applications is common, especially from the ester formulation. Dicamba is a restricted use pesticide, requiring a permit to use. -
Triclopyr at 0.5 lb ae/acre provides fair to complete control of yellow starthistle seedlings. Larger plants require higher rates, up to 0.75 or 1.5 lb ae/acre (DiTomaso et al. 1999b, Northam and Callihan 1991). Higher rates can give almost complete control (Callihan et al. 1991), but are too expensive and may be above labeled rates. Like 2,4-D and dicamba, triclopyr is a growth regulator herbicide with little residual activity. It is foliar-absorbed and active on broadleaf species, and typically will not harm grasses. Triclopyr is formulated as both an amine and ester. The ester formulation is more sensitive to drift than the amine form. Triclopyr does not seem to be as effective as either dicamba or 2,4-D on older starthistle plants.
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Glyphosate controls yellow starthistle at 1 lb ae/acre (DiTomaso et al. 1999b). Good coverage, clean water, and actively growing yellow starthistle plants are all essential for adequate control. Unlike the growth regulator herbicides, glyphosate is non-selective and controls most plants, including grasses. It has no soil activity and has an excellent toxicoloty profile (Giesy et al. 2000). A 1% solution of glyphosate also provides effective control and is used at this concentration for spot treatment of small patches. Glyphosate is a very effect method of controlling starthistle plants in the bolting, spiny, and early flowering stages at 1 to 2 lb ae/acre. The use of glyphosate may not be desirable in areas where desirable perennial grasses are present unless starthistle plants can be directly treated without contacting non-target vegetation.
