The rosette stage extends through winter and spring. Large plants can eventually contain up to 26 deeply lobed leaves, but the size and lobing pattern may vary. The best diagnostic characteristic is the large triangular lobe at the leaf tip. Rosettes are usually prostrate when growing in open sites, but the basal leaves grow upright when plant densities are high or where shaded by competition. During May and June plants bolt (produce erect stems), sending up tall, highly branched stalks that produce spiny flower heads and broad canopies.
Plants generally mature in late summer, long after most other annuals have senesced. The stems of the mature plant are ridged, and the entire plant is covered with soft, appressed hairs. The rigid spines that surround the flower heads shatter in late fall and winter. Under some conditions, yellow starthistle behaves as a biennial, surviving a second winter and regrowing the following spring.
Seed maturation coincides with the progressive loss of the bright yellow pigment of the developing flowers. As flower color fades, the seed quickly develop. Two types of seed are produced. These can be distinguished by color and presence or absence of hairs. The lighter-colored seed have bristly hairs on top and are produced in the central portion of the flower head. They disperse quickly after maturity. Darker seed usually lack hairs and occur in the outer portion of the flower head. They persist in the flower head until the heads disintegrate in midwinter.
Once dispersed into the soil seed bank, the seed remain until subsequent germination or loss of viability. Although most seed germinate the following year or are eliminated by predation and pathogens, seed buried deep in the soil can survive for ten or more years.
Tillage
Tillage with appropriate tools is an excellent means of controlling yellow starthistle. The equipment used should vary according to soil conditions and stage of plant growth. If the soil is loose and plants are small, spring or spike tooth harrows are usually sufficient to destroy seedlings and young plants. As the plants grow, disking is recommended to turn the soil and uproot deep-rooted plants. If the plants are very large, it may be necessary to mow them first to ensure that equipment does not get clogged from excessive vegetation.
Mowing
Mowing is effective for managing yellow starthistle stands, provided it is well-timed and repeated once or twice per season. Mowing during the early flowering stage (i.e., with about 5 percent of the flower heads in bloom) reduces canopy size, seed production, and plant density. However, when done during vegetative or early bolting stages, mowing can increase yellow starthistle stands by reducing competition from resident vegetation.
Mowing is most effective when soil moisture is low and no irrigation or rainfall follows. When soil moisture is low a single mowing may be sufficient, but plants should be monitored for regrowth and mowed again if significant growth and flowering occurs. Generally, follow-up mowings are necessary and should be done once flowering resumes, approximately four weeks later. All mowing should be completed before seed mature. Flower pigmentation is a good way to monitor seed development. When the flowers are bright yellow and have not faded to a straw color, seed have not yet matured. If in doubt, open a flower and check for mature seed. Mature seed are about 0.10 inch long, light to dark brown, plump, and appear dry when squeezed. Mowing after seed have matured removes the spiny canopies but does not diminish the seed bank. Mowing at this stage may actually aid seed dispersal because seed heads can be dragged by the mower or discharged by the mower's chute.
Biological Control
Biological control is the use of natural enemies to control pest populations.
Yellow starthistle is not a problem weed in its area of origin in southern
Eurasia because of an abundance of coevolved natural enemies that attack
the plant. At least 42 herbivorous insect species attack yellow starthistle
in Italy and Greece. When weeds are introduced into a new area, they
typically escape all or most of their natural enemies. The relative
absence of natural enemies is a major factor contributing to the increase
of exotic plant populations to pest levels. Biological control seeks
to introduce host-specific natural enemies from a specie's area of origin
to the new area of infestation. Five insects that attack flowers
or seed heads have been introduced from Greece and are established in the
western United States for biological control of Yellow Starthistle.
These include three weevils: the bud weevil, Bangusternus orientalis; the
hairy weevil, Eustenopus villosus; and the flower weevil, Larinus curtus.
Also introduced are two fruit flies: the peacock fly, Chaetorellia australis;
and the gall fly, Urophora sirunaseva. The larvae of all five insects
feed in the flower head and reduce seed production. These insects
are highly host specific and will not attack commercially important or
native nontarget plants. The biological control program for yellow
starthistle is still in its early phases, and additional time and research
will be required to determine whether yellow starthistle can be controlled
by these or other imported natural enemies. To date, results in Calaveras
have been promising!
Livestock Grazing
Controlled grazing by ruminant animals is an effective method for managing large stands of yellow starthistle in annual grassland. Livestock routinely graze yellow starthistle before it becomes spiny, and studies have shown that it is an acceptable component of a ruminant's diet, with crude protein levels as high as 13 percent at the bolting stage. Because of its late spring growth it can be an important source of green forage in annual grassland.
Horses, however, should not graze yellow starthistle. Prolonged ingestion by horses (86-200 percent of a horse's body weight) can lead to the fatal nervous disorder equine nigropallidal incephalomalacia, or "chewing disease."
Cattle, goats, and sheep were tested separately as vegetation management "tools" in densely infested annual grassland over a period of several years using intensive grazing management timed to specific stages of yellow starthistle development. The type of livestock used to control yellow starthistle appears to be less important than timing of grazing. Availability of animals and size of infestation should determine what type of livestock are used for managing stands.
Like mowing, grazing can either decrease or increase yellow starthistle, depending on the frequency of defoliation and the stage of plant growth. Grazing during the rosette stages (March through April) increased yellow starthistle compared to ungrazed controls because neighboring plants were defoliated and the competition they provided was largely eliminated. Grazing during the bolting stages ( May through June), however, reduced plant densities, height, and seed production. Grazing is more effective when deferred until bolting because it allows resident plants to compete against yellow starthistle for light and moisture. Also, desiccation due to injury from defoliation is more likely with the higher air temperatures and lower humidity occurring during this period.
After initial grazing, one to three follow-up grazings at two-week intervals under rotational grazing are required to adequately suppress growth. The number of grazings required varies according to the rate of regrowth, which is a function of rainfall patterns and available soil moisture levels. Although two or more grazings are usually needed to adequately suppress plants, rotational grazing is not essential. Continuous and less intensively managed grazing will lead to reductions if livestock are present in sufficient numbers during bolting and remain for about eight weeks. Since yellow starthistle is still green and palatable during the bolting stage, animals will often selectively graze and regraze plants by utilizing it as a late-season green forage.
Grazing does not eradicate yellow starthistle, and long term management requires continued use of livestock or other weed-control practices appropriate for the site. By grazing after the earlier-maturing annuals have completed their life cycle and produced seed, plant diversity can be maintained. Also, the soil surface will be covered by sufficient residual dry matter, well above the recommended minimum for annual grassland.
Plant Competition
Management practices that encourage competitive species that already exist on the site or establish competitive species such as well-adapted legumes or grasses should be part of any control program. If yellow starthistle is controlled but its ecological niche remains unfilled, reinvasion wo invasion by another undesirable species is likely. Because yellow starthistle is so prolific and adaptable, relying only on competition from desirable plants is not recommended. Some additional control before and after seeding is generally necessary.
Dense stands of reseeding legumes have some potential for suppressing yellow starthistle. Legumes are particularly effective when combined with a program of well timed mowing or grazing. When using legumes to manage yellow starthistle infestations, it is advisable to use high seeding rates up to 40 pounds per acre and be sure that the seed are properly inoculated with the host-specific nitrogen fixing bacteria. Fertilization with phosphorus or sulfur is generally necessary in pasture soils since they are often deficient in there nutrients. In general, late-winter mowing or grazing is needed to defoliate taller grasses and should be followed by one or two late-spring or early summer defoliations targeted to yellow starthistle.
Prescribed Burning
Prescribed burning has been shown to be effective against yellow starthistle. Burns should be conducted in late spring before seed set but after annual and perennial grasses have produced seed. Since yellow starthistle is still green in late spring, there must be enough dry biomass from other vegetation to carry a fire. Without sufficient fuel the burn will not be completely effective and additional control measures will be necessary. Mowing bolting yellow starthistle plants before burning and allowing the mowed biomass to dry may increase fuel and improve the burn. Another option is to plant annual grassed in the autumn to produce additional fuel for a late-spring or summer burn.
Chemical Control
There are many herbicides available for use on yellow starthistle, including selective, non-selective, pre-emergent, and post emergent types. Because of potential drift onto nontarget areas and hazards to people from direct exposure, many herbicides require a permit from your county Agricultural Commissioner.
For some control projects, more than one application per growing season will be necessary. The long germination period of yellow starthistle, beginning with autumn rains and continuing into spring, usually prevents complete control with one application because the residual activity of preemergent herbicides may not last through the entire germination period. On uncultivated lands, skips in the spray swath and interference from surrounding vegetation make follow-up spot spraying or a broadcast retreatment necessary. Retreatment should take place before bolting as soon as skips or missed plants are evident.
Consideration should be given to other resident plants that exist in the spray zone. Herbicides may kill nontarget species that compete with yellow starthistle or provide soil cover, forage, biological diversity, or wildlife habitat. Broadcast spraying in an infested area is not always necessary because yellow starthistle stands are often patchy. Yellow starthistle skeletons that remain in the winter from previous growth are good markers for locating patches and new seedlings. Stands can also be marked with irrigation flags months before spraying and specifically targeted at the proper time.
Treatment with postemergence herbicides generally works best during seedling and early rosette stages. They are most effective when temperatures are warm, soil moisture is high, and plants are actively growing. Treatment during cold or wet conditions often fails due to poor herbicide absorption and removal by rain. Applications at later stages of growth allow a greater number of plants to be treated but may not control larger plants or may require higher rates.
Glyphospate is nonselective and controls yellow starthistle at a 1 percent solution. Control is effective even at the early flowering stage. Because glyphosphate is nonselective, treatment should occur after desirable species have completely gone to seed for the season. Broad-leaved selective herbicides such as 2,4-D, dicamba, and triclopyr control yellow starthistle and generally leave grasses unaffected. Drift from these herbicides is common: avoid spraying when wind speeds are greater than 5 MPH. Because ester formulations are very volatile, amine forms are recommended to reduce the chance of off-target movement.
Manual Control
Manual control of yellow starthistle is very effective in dealing with small, localized infestations. Small populations can be controlled and greatly reduced by hand pulling, hoeing, or cutting with scythes or weedeaters. Manual methods are also useful for spot-weeding plants that survive from other control methods.
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