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
Herbicides
For yellow starthistle control, herbicides are an appropriate tool on large infestations, in highly productive soils, and around the perimeter of infestations to contain their spread (Sheley et al. 1999b). Most available compounds used for starthistle control in grasslands provide postemergence activity and very few give preemergence control (see table). In a couple of cases, a herbicide can provide excellent postemergence activity and a significant period of preemergence control, e.g. clopyralid, picloram and imazapyr. Herbicides are categorized below as preemergence, postemergence and both pre- and postemergence.
Preemergence
To be effective, preemergent herbicides must be applied before seeds germinate. The long germination period of yellow starthistle requires that a preemergent material have a lengthy residual activity. Applications should be made before a rainfall, which will move the material into the soil. Because these materials adhere to soil particles, offsite movement and possible injury of susceptible plants could occur if the soil is dry and wind occurs before rain. When yellow starthistle plants have already emerged, it is possible to combine a postemergent herbicide (to control emerged plants) with a preemergent herbicide (to provide residual control of any subsequent germination) for an effective control strategy (Callihan and Lass 1996, DiTomaso et al. 1999c).
A number of non-selective preemergence herbicides will control yellow starthistle to some level, including simazine, diuron, atrazine, imazapyr, imazapic, metsulfuron, sulfometuron, chlorsulfuron, bromacil, tebuthiuron, oxyfluorfen and prometone. All these compounds are registered for use on right-of-ways or industrial sites (although not all in California), but few can be used in rangeland, pastures, or wildlands. In rangeland, only metsulfuron (not registered in California) and to some degree chlorsulfuron (not registered for pastures or rangeland in any state) provide selective control of yellow starthistle without injuring most desirable grasses.
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Atrazine (Aatrex®) is a photosynthetic inhibitor that can control yellow starthistle at rates of 1 to 1.5 lb ai (active incredient)/acre (Lass and Callihan 1994a, 1994b, Lass et al. 1993b). Since atrazine is primarily a root-absorbed chemical, applications should be made before seedlings emerge. Because of ground and surface water concerns, this product is a restricted-use herbicide and requires a permit from the county agricultural commissioner (California) for its purchase or use. It is not typically used for control of yellow starthistle, except along roadsides or on industrial sites.
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Tebuthiuron (Spike®) is also a photosynthetic inhibitor that is used for total vegetation control. It will control yellow starthistle preemergence, but will also injure other herbaceous and woody vegetation (Callihan et al. 1991).
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Chlorsulfuron (Telar®) and sulfometuron (Oust®) are registered for roadside and other non-crop uses and are very effective at controlling yellow starthistle when applied at 1 to 2 oz ai/acre. Metsulfuron (Escort®) is registered in other western states, but not California, for use in rangelands. These compounds provide excellent pre- to postemergence control of many weed species. Metsulfuron is safest on grasses, chlorsulfuron is safe on most grasses but will injure some, and sulfometuron is the most non-selective. Little postemergence activity occurs on yellow starthistle with these three compounds. The best control is achieved when applications are made before weeds emerge (Callihan et al. 1991, DiTomaso et al. 1999b, Lass and Callihan 1995b, Wilson and Costa 1986). To get the most effective control of yellow starthistle, chlorsulfuron chlorsulfuron and metsulfuron should be used in combination with postemergence compounds such as 2,4-D, dicamba, or triclopyr. Metsulfuron appears to be more inconsistent than chlorsulfuron, sometimes providing good control and other times giving poor control (Table A, Table B). When chlorsulfuron (at 1 or 2 oz ai/acre) was combined with 2,4-D or triclopyr, yellow starthistle control improved to near 90% (Table A, Table B) (DiTomaso et al. 1999b).
