Yellow Starthistle Management For Homeowners

Yellow starthistle is a problem weed for many suburban homeowners. Yellow starthistle spines injure people and animals. The plants suppress desirable vegetation, mar the appearance of landscapes, and interfere with the use of gardens, paths and play areas. Suburban areas particularly susceptible to yellow starthistle encroachment are vacant lots, back alleys, industrial areas, railroad sidings, unpaved equipment parking lots and other unused or untended areas.

Where Did It Come From?
Yellow starthistle, a native of Eurasia, is thought to have been transported to California in hay seed. It has spread into much of the semiarid northwestern United State, including rangeland, small farms, and urban property. Its movement over long distances is largely because of human activities and travel, including animals, vehicles and various kinds of equipment.

Biology
Yellow starthistle (Centaurea solstitialis: Asteraceae) is a close relative of knapweed, artichoke, and wild chicory. It is easily recognized by its lobed leaves, yellow flowers and long needle-like spines found only on the bracts of the flower head. These spines form as the flower head forms, and fall off the flower head in the late autumn, so the plant is spiny only in the summer and autumn. During the winter the slender stems lose color, turn gray, and the flower heads are topped by white hairy tufts. The leaves have a unique, persistent, bitter taste characteristic of many member of the Centaurea, or knapweed, genus.

Growth Habits
Yellow starthistle plants emerge from seeds in the fall and mature in late spring or summer of the following year. They are annuals, that is, a yellow starthistle plant completes its life cycle within a 12 month period, flowering in only one season, and producing only one crop of seeds. Yellow starthistle seeds germinate whenever conditions are favorable, however, so small plants may be found emerging after rainy periods in spring or summer as well. If they germinate in late summer they may produce flowers, but are likely to die without producing seeds. If they germinate in early fall, they will produce large, vigorous rosettes that do not bolt or flower until the following year, when they will be the largest of that year’s seed-producing population. Yellow starthistle produces up to 150,000 seeds per plant. Most germinate the first two or three years after they ripen, but a small fraction of the seeds can remain dormant in the soil for as long as 10 years before germinating. The weed grows in a variety of conditions, including dry and shallow soil. Its rapid root and shoot growth enable it to grow faster to out-compete most other plants in semiarid environments, which is why it forms dense populations, dominating the plant community it infests.

Control Measures for Yellow Starthistle

Manual Labor
Hand weeding is one of the most important means of controlling yellow starthistle in homesites. Hoeing, hand pulling, or various weed digging devices is best in gardens and other areas where herbicides may injure desirable plants. Hand pulling works best when the plants are large enough that they will not break off to leave the crown and roots and later resprout. Hand removal by pulling or digging is appropriate when only a very few plants are found in a sparsely infested area. Digging and pulling are useful, but if the soil contains seeds from the previous year’s plants, soil disturbance from pulling or digging will favor growth of more seedlings. Mowing yellow starthistle is seldom fully effective, because mowed plants produce more side branches below mowing height and these low branches will produce seeds. Mowing this weed after its stems elongate above the mower’s cutting height will reduce the weed’s seed production, however, and can improve the appearance of an infested area. Never mow prior to seeing a few yellow blooms.

Herbicides
Short-term control of yellow starthistle in suburban areas can be achieved by using herbicide products that are labeled for that use. Repeated applications (2-3 times per year) of over-the-counter systemic broadleaf weeds in lawn grasses are effective when begun in late winter or early spring when yellow starthistle is in the seedling or small rosette stage. Such treatment only suppresses yellow starthistle for a year, but if there is a good stand of a well-adapted turfgrass, and if other good lawn management practices are followed, yellow starthistle can be reduced to the point where it warrants less frequent use of herbicides or occasional hand work. Yellow starthistle growing along sidewalks, in cracks in concrete or asphalt paving, in parking lots, and in waste areas may be best managed with a suitable herbicide. Herbicides are generally efficient where mechanical removal is ineffective, too difficult, or too time-consuming.

The primary limitation of herbicide use is that herbicides that are most effective on yellow starthistle are also hazardous to other plant species. When used as directed by the label, herbicides useful for yellow starthistle control are not hazardous to humans, but failure to observe label instructions can endanger valuable plants and water quality.

BIOLOGICAL CONTROL

Insects and Pathogens
University and USDA researchers are evaluating parasitic insect species and fungi to determine if they can be used to reduce yellow starthistle populations without affecting other organisms. These parasites have not yet been fully successful and may never suppress yellow starthistle adequately for suburban purposes, but may help greatly.

Plant Competition
Desirable, competitive vegetation has long been known to provide the most effective, long-lasting, economical weed control. Competitive vegetation is effective against all weeds, not just yellow starthistle. Grasses are normally the choice for suburban landscape vegetation because they are well adapted to persist where yellow starthistle can invade, and because they will tolerate selective herbicides that occasionally must be used for corrective action. Even vigorous grasses will not protect every area once yellow starthistle becomes established, without the support of occasional corrective action to destroy the weed.

Yellow starthistle does not thrive in deep shade, so close-planted trees and tall, leafy shrubs will keep yellow starthistle suppressed. Yellow starthistle is seldom found directly beneath shade trees or conifers, but will thrive near those ornamentals.

When unused suburban areas such as vacant lots, industrial surroundings or equipment lots are not planted to a dense turf or other competitive vegetation, yellow starthistle invades readily. If they are planted and some care is provided to maintain grass vigor and remove occasional invading yellow starthistle plants, control of that weed is not difficult. Such areas should be planted to low-growing grasses that will survive drought stress, maintain density, but not grow so tall as to cause a fire hazard.

Lawns
Irrigated lawns normally do an excellent job of excluding yellow starthistle because they are often fertilized well, mowed closely and occasionally treated with a herbicide that selectively kills weeds. That usually keeps yellow starthistle from seriously invading a lawn.

CONTROL STRATEGIES

Long-Term, Environmental Control
The major reason that suburban properties become badly infested with yellow starthistle and other weeds is that homeowners do not follow a strategic vegetation management plan. Good plant management practices result in healthy, vigorous turf and ornamentals that will be competitive enough to resist weed invasion. This will reduce dependency on corrective measures such as herbicide applications and hand pulling. Vigorous, healthy turf and ornamentals requires attention to correct mowing, fertilization, thatch control, disease and insect control, and irrigation.

Management strategies for yellow starthistle include use of most or all of the practices previously discussed, depending on needs of the particular site. Although managing is time-consuming and complex, it can be achieved by landowners who are willing to invest the necessary time and resources. Each property owner must ultimately bear the burden for managing yellow starthistle.

Community action is needed for the most successful strategic plan. Owners of neighboring properties should cooperate to prevent large-scale seed production and invasion of yellow starthistle from adjacent land. Cooperative community action is the basis of the noxious weed law, and should result in compliance with the law in a way that is not unreasonably burdensome for anyone.

ECONOMICS OF YELLOW STARTHISTLE CONTROL FOR HOMEOWNERS

For controlling yellow starthistle in lawns, along sidewalks, on driveways, and in waste areas with herbicides, all of the chemicals needed can be purchased at garden supply stores. The cost of the materials generally varies between $5 and $25 per year. A hand pump sprayer can be purchased for between $25 and $50. A typical sprayer should last from 5 to 10 years if it is well-kept, so its average cost will be from $3 to $6 per year for purchase and repairs. The annual out-of-pocket cost of controlling yellow starthistle for a typical homeowner where the weed is prevalent may vary between $15 and $35 per year. Most of the additional cost is in labor, which can require as much as 20 hours per year to control yellow starthistle around an average suburban home.

Controlling yellow starthistle should be integrated with control of other weeds around a homesite. When that is done, the cost of yellow starthistle can cover the cost of controlling most other weeds around the homesite as well, and the benefits will likely be a home with much better landscape care overall.