Rush Skeletonweed, a new invader threatening our area

Rush skeletonweed is rapidly advancing from the Idaho border where it has long been established along the I-90 corridor towards the Blackfoot watershed and points beyond. Rush skeletonweed is native to Asia, the Mediterranean and North Africa, and was first reported in the United States near Spokane, Washington in 1938. A small infestation was found in Sanders County, Montana in 1991 and has subsequently been reported in Lincoln, Flathead, Ravalli, Lewis and Clark, Beaverhead, Missoula and Treasure Counties as well.

This plant spread rapidly throughout the wheat-growing area of southeastern Australia once introduced, causing significant yield reductions. It can form dense monocultures on rangeland, altering soil moisture and nutrients, and reducing forage for cattle and wildlife by outcompeting native and desirable plants. It is listed as a Priority 1B noxious weed in Montana.

This plant is a perennial or biennial, with rigid flowering stems as high as three feet tall. A milky sap will exude from stems when broken, much like leafy spurge. Plants start out as rosettes that look much like dandelions until flowering stems develop at maturity and rosette leaves wither. Rosettes will produce one or more flowering stems with numerous branches. Upper stems mostly lack hairs, but typically have dense, bristly, downward pointing hairs at the base. Stem leaves are often absent, giving the skeletal-like appearance, but when present resemble a smaller version of those dandelion-like rosette leaves.

The root is a deep, slender taproot that becomes somewhat woody with age and can penetrate soil to depths of six to nine feet or more. Buds near the top of the taproot and side-branching roots can produce new plants. Easily fragmented roots can also produce new rosettes, or plants. Rush skeletonweed flowers from July until frost and each flower head consists of seven to 12 bright yellow flowers that resemble petals.

Rush skeletonweed is primarily dispersed by seeds, and an established plant is capable of producing 15,000 to 20,000 seeds. These small, lightweight seeds can spread via wind and water and can also attach to animals, clothing and vehicles. Rush skeletonweed can also reproduce vegetatively from fragmented taproots. Chicory is similar to rush skeletonweed and dandelion, but has rosette leaf lobes that are not always opposite, as in rush skeletonweed. Other lookalikes include yellow sweet clover and prickly lettuce.

Like most invasive non-natives, this plant thrives in disturbed soils along roadsides, croplands, irrigated grain fields, semi-arid pastures, rangelands and residential properties. It grows in well-drained, sandy or gravelly soils in climates with cool winters and hot, relatively dry summers. It is tolerant of a wide variety of environmental conditions, including rainfall less than 10 inches to more than 50 inches in cold winter climates. Severe infestations are less common on heavy clay soils.

Rush skeletonweed is a high priority species for Montana due to its limited presence here and our close proximity to large infestations in Idaho. Furthermore, rush skeletonweed will have potentially devastating impacts on agriculture in Montana if becoming well-established.

Management priorities for rush skeletonweed include monitoring, early detection and rapid treatment of newly discovered plants. Hand-pulling is an option for small infestations, and requires control two to three times per year for a minimum of five years. Fall or spring applications of herbicides (picloram, aminopyralid, or clopyralid) may provide control for a year or more with the level of effectiveness dependent upon the presence of, and percent cover of, competitive perennial grasses and other desirable vegetation.

Three biocontrol agents of rush skeletonweed have been established in the western United States including a rust fungus, a gall mite and a gall midge. Research has shown the rust fungus to be extremely host-specific, preferring only certain genotypes of rush skeletonweed. This type of host specificity may be a limiting factor in biocontrol success due to the presence of three rush skeletonweed genotypes in the western U.S.

Prescribed burning is not recommended for rush skeletonweed areas, unless a rigorous revegetation plan along with additional follow up treatments are implemented. Planting competitive crops can help suppress the weed. Integrating various preventative management techniques with monitoring, reseeding and herbicides can help reduce the potential impacts of rush skeletonweed.

If you think you have found rush skeletonweed, contact your extension agent or county weed coordinator immediately.

Identification tips:

Deep-rooted, perennial forb in the sunflower family (Asteraceae) (Fig. 1)

Grows one to four feet tall and produces milky latex when broken

Highly branched stem, few leaves, or "skeletal" appearance

Stiff, downward-pointing hairs near the base of stem (Fig. 2)

Small, yellow flowers at ends of stems, either individually or clusters (Fig. 3)

Overwinters as rosette which resembles common dandelion