Montana Field Guide

PLANTS: An annual with erect, branched, and winged stems that grow 10–80 cm (Lesica et al. 2012). Plants are scabrous and gray-tomentose (Lesica et al. 2012).

LEAVES: Basal leaves form a rosette are are deeply lobed and approximately 5 cm wide and 20 cm long (Sheley and Petroff 1999). Stem leaves are entire, linear-oblanceolate, 5–15 cm long, short-petiolate, and have thin woolly hairs (Sheley and Petroff 2009; Lesica et al. 2012).

INFLORESCENCE: Corymbiform. Yellow flower heads are solitary on open leafy stems (FNA 2006). Involucres are ovoid and 13–17 mm tall (Lesica et al. 2012). The outer bracts are pale green and armed with a stout, straw-colored spine of 1-2 inches (2.5-5 cm) long (Sheley and Petroff 2009; Lesica et al. 2012). The spines radiate out into a star-shape. The inner bracts are lanceolate with swollen tips (Lesica et al. 2012). Corollas are 13–20 mm long. Pappus is absent or of white, unequal bristles of 2-4 mm long (FNA 2006). Fruits (cypselae) are 2–3 mm long and of two types: outer fruits are dark brown without a pappus and inner fruits are mottled white/light brown and with a pappus (FNA 2006).

Flowering mostly from summer through autumn, but year-round in frost-free coastal areas (FNA 2006).

Yellow Starthistle is a winter annual with bright yellow flowers. The flower head (involucral) bracts are tipped with stout, straw-colored spines of 2.5-5.0 cm (1-2 inches) long (Sheley and Petroff 1999). The spines radiate from the flower head in a star shape. In the western U.S. and depending upon the growing conditions plants can be 10 cm to 1.5 meters tall (Sheley and Petroff 1999).

Grasslands, rangeland, pastures, woodlands, roadsides, and wastelands (FNA 2006; Lesica et al. 2012).

Yellow Starthistle’s competitive success is directly related to its rapid growth and ability to capture resources (Sheley and Petroff 1999). It is capable of invading rangelands throughout the western U.S (Sheley and Petroff 1999). In the Pacific Northwest rangelands with deep, loamy soils, south-facing slopes, and 30.4 to 63.5 cm (12 to 25 inches) of precipitation that peaks in spring or winter are most vulnerable. It competes well in areas dominated by perennial grasses, such as bluebunch wheatgrass (Elymus spicatus), Idaho fescue (Festuca idahoensis), and Sandberg bluegrass (Poa secunda). It appears to not compete well with sagebrush (Artemisia spp.) unless the soil is already disturbed. However, Yellow Starthistle growth rates can decline as its density increases. Perennial grasses can compete against yellow starthistle invasions, particularly species that can initiate growth in the fall.

In deep-soiled rangelands Yellow Starthistle roots grow fast and deep (Sheley and Petroff 1999). Where cheatgrass (Bromus tectorum) or other annual grasses dominate their rooting structure is more shallow. Yellow starthistle can outcompete annual grasses until its density limits growth or they can co-dominate. Where soils are shallow, cheatgrass has a competitive edge over Yellow Starthistle.

Whirlwinds, birds, humans, and other animals aid in the dispersal of Yellow Starthistle seeds. Seeds get dispersed by whirlwinds and human activities, such as driving or sticking to clothing (Sheley and Petroff 1999). Ring-necked Pheasants and California Quail consume whole seeds, which occasionally pass through their digestive tract in a viable form (Sheley and Petroff 1999). House and American Finches shell their seeds, which leaves them unviable (Sheley and Petroff 1999).

POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus bifarius, Bombus centralis, Bombus fervidus, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus griseocollis, Bombus impatiens, Bombus insularis, and Bombus suckleyi (Thorp et al. 1983, Johnson 1986, Colla and Dumesh 2010, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).

Yellow Starthistle produces two types of seeds: those with a parachute-like plume (i.e., a pappus of bristles) and those without. Most seeds have plumes and are dispersed by the wind. Plumeless seeds stay within the flower head and mostly disperse within two feet of the parent plant between November and February. Using an analogy, this builds up a slow moving invasion front.

LIFE CYCLE [Adapted from Sheley et al. in Sheley and Petroff 1999]
Yellow Starthistle are winter annuals, whereby seeds germinate in the fall. However, spring germination has been observed. Based on laboratory tests germination occurs at a fast rate. Plumed seeds germinate within 16 hours under optimal temperature (68 degrees Fahrenheit (20 degrees Celsius)) with unlimited moisture. After 48 hours, almost 75% of the plumed seeds and 44% of the plumeless seeds have germinated. This quick response implies that seeds can quickly take advantage of optimal conditions while many species cannot. Under dry or salty conditions germination rates are lower. Other studies found that about 95% of seeds are viable with 10% remaining viable for 10 years. In heavily infested stands the seedbank approaches 13% of total seed production with most being plumeless. A single plant has the potential to produce 150,000 seeds; however, a study near Walla Walla, Washington found plants produce 20-120 seeds. Seed production is dependent upon plant density and moisture.

Growth usually starts in the fall following precipitation. Frost heaving can reduce seedling survival. Seedlings transition to rosettes from March to early May. The rosette stage appears to be a vulnerable period. Studies have shown that 60-75% of rosettes may die by July due to moisture stress or self-thinning. From the rosette, a flowering stalk emerges. Mature plants can die from environmental stressors, but generally populations are stable from year to year. In early autumn, plants drop their leaves and die, leaving behind a silvery-grey skeleton with cottony-white seedheads. With autumn rains, seeds germinate, and the cycle repeats.

Successful control requires integrating strategies to prevent new, eradicate or containing existing, and controlling large populations.

PREVENTION [Adapted from Sheley et al. in Sheley and Petroff 1999]
Targeting actions that prevent or reduce seed production will hinder its ability to grow and spread. Spread can be reduced through many actions, such as, a) preventing vehicles from driving through and animals from grazing within infested areas, b) thoroughly washing the undercarriage of vehicles that have travel through infested areas, c) developing educational campaigns to teach people to not pick and transport the yellow flowers, and d) encouraging landowners to frequently monitor their land for new infestations and, when found to implement effective control methods.

BIOLOGICAL CONTROL [Adapted from Sheley et al. in Sheley and Petroff 1999]
As of 1996 three weevils (Bangasternus orientalis, Eustenopus villosus, Larinus curtus) and three flies (Chaetorellia autralis, Urophora sirunaseva, Urophora jaculata) that attack Yellow Starthistle flower heads have been approved for released. They were released on plants in Canada and the Pacific Northwest, but apparently are not in Montana.

CHEMICAL CONTROL [Adapted from Sheley et al. in Sheley and Petroff 1999]
Herbicides are effective, especially when properly managed with other tactics. The herbicide type and concentration, timing of chemical control, soil properties, and other factors will determine its effectiveness and impact to non-target species. Strict adherence to application requirements defined on the herbicide label will reduce risks to human and environmental health. Many herbicides must be applied by applicators with an Aquatic Pest Control license. Consult your County Extension Agent and/or Weed District for more information on herbicidal control.

Picloram at the 0.25 pound per acre rate can provide selective control when applied to seedlings, rosettes, or plants beginning to bolt without harm to well-established grasses. It cannot be used in sandy soils, near surface waters, or in areas with a high-water table.
2,4-D at the 0.5 pound per acre rate can be effectively used in riparian areas or around homesteads.

Clopyralid at the 1.2 pound per acre rate can be applied before plants produce flower buds and in areas with high water tables.

Glyphosate at the one pint per acre rate will kill all plants and can be used in revegetation projects that need to prepare an initial seedbed for seeding desirable vegetation.

PHYSICAL & CULTURAL CONTROLS [Adapted from Sheley et al. in Sheley and Petroff 1999]
Hand-pulling is very effective because it is an annual. Pulled plants should be bagged and dessicated or preferably burned before disposing to ensure that seeds are not viable. Hand-pulling small infestations, around gardens and homesites, and within crop fields, riparian areas, and along waterways will eradicate or slowing its spread. Systematic surveys and repeated removal should be conducted every 2-4 weeks throughout the growing season.

Tilling and discing severs roots below the soil surface and will kill plants, but it should be done periodically through the growing season. This will deplete the seed source and lessen the severity for re-invasion. Once the ability for re-invasion has been controlled then desirable plants can be established.

Mowing is not effective because plants respond by growing more prostrate. Mowing in dry years might reduce seed production. Mowing before it rains will stimulate growth and increase seed production.

Burning populations will increase infestations. Fire releases nutrients and Yellow Starthistle seeds are quick to capture them, germinate, and grow.

Revegetation should follow any control method. A mix of perennial grass cover is best to interrupt the cycle of re-invasion because they compete strongly for light, nutrients, and water resources. This may require the use of particular cultivars of exotic and/or native grasses. Successful seeding will depend upon the particular grass species/cultivars, the density that establishes, environmental conditions, and the land manager’s ability to maintain grass vigor. It is recommended to not fertilize new grass seedings that are infested with yellow starthistle because it will increase the forb’s production.

GRAZING CONTROLS [Adapted from Sheley et al. in Sheley and Petroff 1999]
Grazing and grazing management can be effective at controlling Yellow Starthistle. Cattle or sheep can suppress Yellow Starthistle if grazed when plants bolt, but before they develop spines. Prior to developing spines, the crude protein content of plants ranges from 5-10%. Repeated grazing is necessary to suppress growth, but the intensity and frequency should depend upon environmental conditions. In revegetated areas, the desirable rangeland plants should achieve full recovery before being grazed, to ensure they are most competitive against Yellow Starthistle.

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides