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
Late-season starthistle control
In all late season treatments, except with glyphosate, a surfactant should be added (DiTomaso et al. 1999b). Because late-season control typically occurs during the warmer spring or early summer months, amine formulations of 2,4-D, dicamba, and triclopyr are more appropriate than ester forms. Triclopyr provides only marginal control in the bolting stage and poor control at later growth stages. Glyphosate, dicamba, and higher rates of clopyralid and triclopyr are effective with plants in the bolting stage (DiTomaso et al. 1999b).
If late-season yellow starthistle control is necessary, the best time to treat with glyphosate is after annual grasses or broadleaf species have completed their life cycle, but prior to yellow starthistle seed production (<5% flowering). Control is less effective when mature plants show physical signs of drought stress. When clopyralid was previously applied in late winter or early spring, glyphosate can be used in a broadcast or spot treatment follow-up program to kill uncontrolled plants before they produce seed. It can also be used to prevent the proliferation of potential clopyralid-resistant plants. Broadcast treatment with glyphosate is not recommended when desirable perennial grasses or broadleaf species are present.
Pre- and postemergence
The most effective herbicides for season-long control of yellow starthistle are those that provide postemergence control of seedlings and rosettes, as well as soil residual activity for at least a couple of months until spring rainfall is completed. Of the compounds that have these characteristics, clopyralid (Transline®, Stinger®) and picloram (Tordon®) are the most effective and are the least injurious to grasses. Picloram is not registered in California. Other combinations of clopyralid are also effective including clopyralid + 2,4-D (Curtail®) and clopyralid + triclopyr (Redeem®).
Imazapic and imazapyr
Both imazapic (Plateau®) and imazapyr (Stalker®, Arsenal®, Chopper®) are branched-chain amino acid inhibitor with the same mode of action as chlorsulfuron, metsulfuron, and sulfometuron.
Imazapic is not yet registered in California. It is a broad spectrum herbicide often used in perennial grass re-vegetation efforts. Although it has some activity on yellow starthistle, results are very inconsistent (Shinn and Thill 1998, 1999, 2000a, 2000b). The use of imazapic alone is not likely to provide effective control of yellow starthistle.
Imazapyr is also a broad spectrum herbicide with both pre- and postemergence
activity on yellow starthistle (Northam
and Callihan 1991). Unlike the growth regulator compounds, imazapyr will
cause significant injury to grasses.
