Yellow starthistle is identified as one of the most serious weeds in the Western U.S. Although it is an annual plant, it spreads rapidly and produces an extensive amount of seed. Its control is of paramount importance because it threatens the desirable vegetation in rangeland, natural areas and roadways. Learn how to slow down this invasive plant.
I. Appearance and Characteristics
Centaurea solstitialis, the yellow star-thistle, is a species of thorny plant in the genus Centaurea, which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other places. It is also known as golden starthistle, yellow cockspur and St. Barnaby’s thistle (or Barnaby thistle).
Centaurea solstitialis is an annual herb from the family Asteraceae. During the vegetative stage it forms a rosette of non-spiny leaves, between 5–20 centimetres (2–8 inches) in diameter. As the summer approaches, it produces a flowering stem up to 1 metre (3+1⁄2 feet) in height. The leaves at the base are lobed and range between 5–8 cm (2–3 in) in length, while the ones on the stem are unlobed and smaller. Between May and October, the stem produces numerous spinous flower heads (capitula) about 12–16 millimetres (1⁄2–5⁄8 in) across, containing between 10–50 yellow flowers, with spines between 1–2.5 cm (3⁄8–1 in). Flowers within the capitula are pollinated by insects and each capitula produces 10–50 seeds, some with and some without a pappus. It is an annual semelparous species, and will die after reproduction is completed, normally by the end of the summer.
- Similar species
Similar species include purple star-thistle (Centaurea calcitrapa), sulphur star-thistle (C. sulphurea), Maltese star-thistle (C. melitensis), and rough star-thistle (C. aspera). Bachelor’s button (C. cyaneus) is a relative.
- Toxicity
Grazing of the plant by horses can cause nigropallidal encephalomalacia or “chewing disease”, a neurological condition. The disease generally follows consumption of 60–200% of the horse’s body weight over an extended period of a month or more, or 2.3–2.6 kilograms (5.1–5.7 pounds) of star-thistle per 100 kg (220 lb) body weight per day. Though star-thistle is most dangerous when it is the only plant available or is delivered as a contaminant in dried hay, horses may develop a taste for it and seek it out. Many other grazing species, including mules and burros, are not affected.
- As an invasive species
The introduction of C. solstitialis in North America probably occurred in California sometime after the start of the California Gold Rush, as a fodder seed contaminant in imported Chilean-harvested alfalfa seed, also known as Chilean clover (Trifolium macraei). Star-thistle has been introduced throughout North and South America, Africa, and Europe.
In California, yellow star-thistle was dispersed into agricultural fields and immediately took hold in the state’s areas with a Mediterranean climate. Human factors, such as mowing, land grading for development and roads, domestic animal grazing, and disturbance of the soil surface for agricultural tillage and wildland firebreaks have and continue to contribute to the successful thriving and spread of this plant. Yellow star-thistle is now a very common sight in vacant lots and fields, along roadsides and trails, in pastures and ranch lands, and in parks, open-space preserves and natural areas.
After the turn of the 20th century, Spain, France, Italy, and perhaps Turkestan were also likely sources of the invasion’s seed in California. Since its introduction to California in the mid-19th century, it has become a large-scale invasive species (noxious weed or invasive exotic) throughout 23 U.S. states. It currently dominates over 15,000,000 acres (61,000 square kilometres) in California alone.
By 1970, yellow star-thistle had reached 23 U.S. states. According to the U.S. Forest Service, as of 2006 the plant has been reported present in 41 of the 48 contiguous U.S. states, with the only exceptions being Maine, Vermont, and five of the Deep South states (Arkansas, Louisiana, Mississippi, Alabama, and Georgia). The plant is considered an invasive species in six of the 41 states: California, Oregon, Washington, Idaho, Utah, and New Jersey.
II. How to Grow and Care
Sunlight
Yellow star-thistle thrives best in conditions of full sun exposure, which means it requires a minimum of six hours of direct sunlight a day to achieve optimal growth and floriferousness. While yellow star-thistle can endure some variations in light, significant deviations from full sun can result in reduced vigor and sparse blooming. Adaptive traits such as the plant’s deep taproot system, allow it to be resilient in intense sunlight by accessing deep soil moisture to sustain itself. Ideally, yellow star-thistle should be planted in outdoor areas that are not shaded, as it is not commonly an indoor plant. Light intensity influences yellow star-thistle’s robustness, promoting dense foliage and more vibrant flowers.
Temperature
Yellow star-thistle is mainly grown in the Mediterranean region and Southwest Asia. It is cold-resistant and dislikes intense heat, but nevertheless enjoys warm climates. Optimal conditions for growing them include a relatively dry environment and temperatures between 15 to 25 ℃. It is adapted to grow in hardiness zones 3-8.
Yellow star-thistle is tolerant to drought and shouldn’t be planted in soils that are prone to hold water. If it is planted in the garden it can grow on its own, absorbing rain water after its roots are fully developed. But be careful – an excess of water will cause the stem to become weak and fall over. Another danger of excess moisture includes the development of root rot, which can be detrimental to yellow star-thistle.
Watering
Originating from the Mediterranean, yellow star-thistle thrives in dry conditions and has adapted to conserve water effectively, making it drought-tolerant. Its preference for less frequent hydration aligns with its native arid climate; thus, watering every week suffices to maintain its health. As an outdoor plant that often faces competition for resources, yellow star-thistle’s root system is designed to maximize water uptake during scarce rainfall periods.
Soil
Yellow star-thistle is adapted to various soil types. It can grow in both heavy clay and rapid draining sand. However, for the most favorable growing conditions, it is best to use fertile, loose and well-drained sandy soils. A mix of garden soil, a bit of sand and wood ash is the best potting substrate.
In order to prevent pests and diseases from occurring (or recurring), yellow star-thistle should not be planted on the same plot for many consecutive years; when it comes to potted plants, substrate should be replaced annually.
Fertilizing
Yellow star-thistle is tolerant to infertile soils, but adequate nutrition will promote blossom. Before sowing, generally an appropriate base fertilizer is applied (preferably an organic or mineral fertilizer) during soil preparation. Nitrogen-based fertilizers can be applied during vegetative phase to promote growth.
Half a month before bloom time, fertilizers based on phosphorus and potassium (e.g. monopotassium phosphate) can be applied to promote flowering. Try to avoid excessive fertilization, because otherwise, it will only benefit the growth of weeds, resulting in an overgrowth of plants but reduce flowering.
Planting Instructions
The root system of yellow star-thistle has one main root, making it suitable for direct sowing. Sowing should be performed in spring and fall. Sowing in spring needs to be carried out as early as possible. The suitable temperature for germination is 15 to 20 ℃, and the seedling will emerge after 4-7 days.
Before sowing, mix the seeds and fine sand in a ratio of 1:2-3, and sow them evenly. Comb the plot gently with a rake afterwards. After sowing, the covering soil should not be too thick, as it is advised to cover the seeds tightly, otherwise the seedlings might grow up unevenly.
Crush the soil with a tool to combine the seeds with the soil tightly. Finally, cover it with a non-woven fabric or a straw curtain to increase ground temperature and promote seed germination. It is best to choose a windless day for sowing. The sowing height should be reduced if the wind is strong.
Pruning
Yellow star-thistle grows side branches naturally, and too many of them will cause the flowers to grow smaller. Producing larger flowers can be achieved by cutting off some terminal buds. During the mid-summer, climate conditions are usually unfavorable for growth of yellow star-thistle, and the leaves start to wither and turn light green or yellow. At this point, the plant can be cut off to a third or to half of its height. When the weather becomes cool, it will rejuvenate and grow new leaves and flowers.
During winter it is advised to cut off the roots of yellow star-thistle that have perennial roots below the ground, to ensure that the plant survives cold temperatures and frosts.
Propagation
Yellow star-thistle is mainly propagated by sowing and division. Its seeds are scattered easily, so it’s best to harvest them in batches, when the inflorescences turn yellowish-brown after the petals have fallen. The seeds should be sown in spring or fall.
If the sowing is performed in fall, it needs to be at a place protected from low temperatures, or planted indoors in a seedling tray and then transplanted in spring. It has strong viability and can self-propagate. The seeds successfully germinate on their own with enough water.
Perennial species can also be propagated by division. Division includes digging out a cluster of plants 2-3 years old, and separating them with a sharp knife, each cluster preferably containing 3-5 small buds. Individual clusters serve as new planting material. Division is generally carried out during spring or fall. The soil must be kept moist before the new roots have properly grown.
III. Uses and Benefits
Although the spines make the plant a pain to deal with, it produces a light honey.
IV. Harvesting and Storage
You can harvest the flowering branch of yellow star-thistle. It can be done when the petals on the outer circle of the inflorescence are fully expanded. Use sharp garden shears to harvest them when the temperature is low in the morning. Put them in a vase with clean water in time to avoid water loss.
V. How to Control
Chemical control
Most herbicides used for controlling yellow star-thistle are registered for range lands, right-of-way, and other non-crop areas. Many auxin-like or growth-regulator herbicides are used for post-emergence control, including , aminopyralid, clopyralid, dicamba, picloram and triclopyr. In California, aminopyralid, which has both post- and preemergence activity, is the most commonly used, most effective after an early summer prescribed burn.
Alternatively, glyphosate may be used. Pre-emergence herbicides used for yellow star-thistle control include chlorsulfuron and sulfometuron. Pre-emergence and post-emergence herbicides may be used effectively together to kill growing plants as well as any new seedlings that may emerge from the copious soil seed bank often produced by yellow star-thistle. Controlled burning may also be used in conjunction with clopyralid application as an effective integrated approach to yellow star-thistle management.
Aminocyclopyrachlor + chlorsulfuron, aminopyralid, chlorsulfuron, clopyralid, clopyralid + 2,4-D, dicamba, diflufenzopyr + dicamba, picloram, and triclopyr + clopyralid for the Pacific Northwest of North America.
A yellow star-thistle biotype resistant to picloram was discovered in a pasture near Dayton, Ohio, in 1988. This biotype was determined to have cross resistance to other auxin-like herbicides, especially clopyralid. Resistance was discovered to be conveyed by a single nuclear recessive gene.
Picloram-resistant C. solstitialis has been observed which had cross-resistance to clopyralid, dicamba, and fluroxypyr, but not triclopyr or 2,4-D.
Biological control
Yellow star-thistle is sometimes resistant to removal methods such as mowing and burning, because of its long root system and the seeds’ ability to withstand fire. The plant has been the target of biological pest control programs with positive results. Seven types of seed-feeding insects have been released (one accidentally) to control the plant.
Insects
Three species of weevil in the beetle subfamily Cleoninae effectively reduce seed production in the yellow star-thistle.
- Yellow star-thistle bud weevil (Bangasternus orientalis) is a fuzzy brown weevil that lays its eggs in the flowers, and when its larvae hatch, they feed on the developing seed.
- Yellow star-thistle hairy weevil (Eustenopus villosus) is a long-snouted, hairy-looking weevil that lays a single egg inside each flower bud. The larva then consumes the seeds within.
- Yellow star-thistle flower weevil (Larinus curtus) is a brownish weevil that lays eggs in the flowers as it feeds on the pollen. The larvae then eat the seeds when they hatch. This insect has failed to establish dense populations in the United States.
Four species of tephritid fruit fly also attack the seedheads of yellow star-thistle.
- Yellow star-thistle peacock fly and false peacock fly (Chaetorellia australis and Chaetorellia succinea, respectively, the latter released unintentionally) are small nectar-feeding flies that deposit eggs into the seedheads, where their larvae consume the seeds and flower ovaries.
- Banded yellow star-thistle gall fly (Urophora sirunaseva) produces larvae that pupate within a woody gall within the flower and disrupt seed production.
- Another gall fly Urophora jaculata was released in 1969 but never established.
Fungi
A variety of the rust fungus Puccinia jaceae var. solstitialis, first released in July 2003 on a ranch in the Napa Valley, has shown promise as an agent against yellow star-thistle, dramatically damaging leaves and hampering growth. The rust causes widespread pathology in the leaves of the plant and slows its dispersal. The fungus Synchytrium solstitiale (Synchytrium of phylum Chytridiomycota) is also being considered as an agent of biological control.
Grazing
Grazing by goats, cattle, or sheep can be effective in controlling yellow star-thistle. Goats will eat star-thistle even in its spiny stage. Because yellow star-thistle growth is particularly difficult to inhibit in canyon rangelands since its remoteness limits control options, goats and other herbivores have become an excellent option to curb the plant’s spread. According to one study, grazing has decreased yellow star-thistle presence by 58% when compared to the study’s controls. Subplots also showed a 94% decrease in seed heads after only three years of experimentation.
Integrated methods
California researchers (Thomsen et al., 1996) tested mowing, controlled sheep grazing and subterranean clover plantings to control star-thistle growth. According to the researchers, subclover would help fill the void left by star-thistle populations.
They compared subclover seeding, grazing, two mowings, grazing + mowing without subclover – and an untouched control 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. Excellent yellow star-thistle control can be achieved with the combination of competing plants, mowing, and rotational grazing.