Yellow starthistle (Centaurea solstitialis) is one of the most ecologically and economically damaging invasive plants in California. California Department of Food and Agriculture estimates that Yellow starthistle covers over 12 million acres in California. It is a serious nuisance on recreational lands, degrades the value of private property, range and timber lands, is toxic to horses and poses a major threat to biodiversity in native ecosystems.A highly competitive and invasive weed, yellow starthistle has adapted to a wide range of habitats and environmental conditions, mostly in California, Oregon, Washington and Idaho.
Yellow starthistle is a winter annual. Seeds germinate in the fall and grow into over-wintering rosettes. Under favorable conditions of temperature and moisture germination can continue through the winter and early spring. Once established the seedlings use up soil moisture and are highly competitive for soil nutrients . In a dry year plants can be 6 inches tall with 1 to 2 flower heads. In moist years plants at the same site can be 2 to 4 feet tall with several hundred flower heads.
Yellow starthistle management requires well-planned, integrated programs that maximize the effective use of all weed management strategies in combination. Long-term strategies include a combination of mowing or burning, alternative herbicides, managed grazing, biological control and establishing competition by desirable vegetation. Well-adapted, perennial grass and clover mixes can curtail its expansion.
Annual or biennial thistles reproduce only by seed. Tilling, hoeing, burning or hand pulling should be done before flowering. Cut the plant below the ground or as close to the ground as possible to prevent re-growth. Cutting or mowing is more effective later in the season when the stem core is hollow, but before flowering. At that time, the plant is least likely to regrow. If cut in the rosette/flowering stage, they will regrow easily. Mowing can wait until two days before blooming to prevent seed production. (Anderson, 2001) Mowing or hand cutting only four days after flowering will allow some seeds to mature. Plants cut after the flowers open should have the flowers removed. Put the flowers in a tight container and bury or otherwise destroy them. Timing a mow can be difficult since thistles don’t all bloom at the same time. It’s important to reestablish desirable forage or crop plants with adequate fertility soon after thistle is controlled, to provide competition to future thistle plants.
California researchers (Thomsen et al., 1996) tested mowing in combination with controlled sheep grazing and subterranean clover plantings for starthistle control. The researchers theorized that the subclover would help fill the niche vacated by the yellow starthistle populations.
They compared subclover seeding + grazing + two mowings - grazing + mowing without subclover - and an untreated area without grazing, subclover, or mowing. Thistle seed production was 130 times higher where only one mowing was done, and 1,720 times higher where nothing was done, as compared to the area that had been grazed and mowed twice. This study shows that excellent yellow starthistle control can be achieved with competition from desirable plants, such as subclover, combined with mowing and rotational grazing. Tall fescue has also showed good competition results in tests.
Beneficial Insects/ Biocontrol
The goal of biological control is not to eradicate starthistle but to reduce its competitive ability so that yellow starthistle-infested sites can be colonized by desirable plant species. In order to qualify as a biocontrol agent, an insect is only allowed to eat and develop on yellow starthistle and in some cases on a few very closely related plant species. The most important precondition for an insect to be used as a biocontrol agent is that it will die without yellow starthistle.
Starthistle is attacked by the yellow starthistle bud weevil, Bangasternus orientalis, and the yellow starthistle hairy weevil, Eustenopus villosus. Larvae and adult bud weevils feed on the flowers, thus reducing seed production. The hairy weevil adults feed on the outside of the flowers, while the larvae feed inside the flowers. As with the bud weevil, the hairy weevil also reduces seed production. PVFS is looking for a new source for these weevils, as our previous provider is no longer in business.
Natural Herbicide Control
Acetic acid in vinegar, citric acid, oil of clove and oil of cinnamon have been shown to kill plant tissue by dissolving the cell membrane, which causes the plant to dry out. Alternative Herbicides and Vinegar work best when used in the sun. A word of caution, however: vinegar in concentrations greater than 5% acetic acid, citric acid and concentrated oils may be hazardous—burning the skin or damaging the eyes—and should be handled with care. Additionally, vinegar is not registered with EPA for use as herbicide.
The following Alternative Herbicides are in stock at PVFS: Burn Out 11 and Greenmatch are registered with the EPA and OMRI approved. Alldown Green is a 25(b), “minimum risk pesticide,” exempt status with EPA. PVFS has a product called Corn Gluten Meal which acts as a preemergent germination inhibitor on seed. Though I found no information that connected this product to the control of starthistle seed germination, I think it would be an interesting trial.
Soil Conditions and Pasture Management
Some farmers report that thistles grow where soil calcium levels are low, iron is high, and phosphorus is low or complexed. (Anderson , 2001) Thistles seem to prefer soils high in anaerobic bacteria, where residue decay is poor, or the soil is compacted.
ATTRA - National Sustainable Agriculture Information Service http://attra.ncat.org/attrapub/thistlecontrol.html
Biology and Biological Control of Yellow Starthistlehttp://www.invasive.org/weeds/StarthistleBook.pdf
Yellow Starthistle ?IPM Practitioner’s Association, Eugene, OR ?http://members.efn.org/~ipmpa/Noxystar.html
Anderson, Lee. 2001. Keeping thistles at bay. ACRES USA. February. p. 26.
Byczynski, Lynn. 2003. The vinegar brouhaha. IPM Practitioner. September/October. p. 7-8.
Thomsen, Craig D., William A. Williams, William Olkowski, and Dave W. Pratt. 1996. Grazing, mowing and clover plantings control yellow starthistle.?The IPM Practitioner. February. p. 1-4.