Rush skeletonweed (Chondrilla juncea L.) is a deep rooted, perennial forb infesting more than 6.2 million acres mainly in the western United States. The weed is listed as “noxious” in nine states (Figure), with dense, widespread infestations on about 3 million acres in Idaho alone.
In spite of aggressive containment and control efforts, landscape-scale spread of rush skeletonweed continues, and the rate of invasion is escalating in eastern Idaho, southwestern Montana, western Wyoming, and northern Utah.
“Rush skeletonweed is one of the most wide-spread noxious weeds in Idaho,” explains Tim Prather, weed scientist at University of Idaho. “The plant infests canyon grassland and sage steppe, which provide critical winter range for mule deer and elk. During winter months, rush skeletonweed has minimal feed value, and both wildlife and livestock are challenged to find enough forage on severely infested areas.”
Rangeland and natural areas, especially those disturbed by fire, logging, road construction or over-grazing, are susceptible to invasion. Rush skeletonweed can also encroach into cropland. “Once the weed forms dense infestations the ecological progression to desirable perennial grassland seems to be stopped,” says Prather.
Rush skeletonweed is capable of reproducing by seed or lateral roots. Flowers are self-fertile, and a mature plant can produce up to 20,000 seeds. A pappus (tuft of hair on top of the seed) facilitates wind dispersal over long distances. Seeds can also attach to animals, vehicles and other vectors. Germination occurs in fall or spring based primarily on moisture conditions, with few seed remaining viable for more than a year in soil. Rush skeletonweed has a taproot that can reach depths of 7.5 feet, enabling it to thrive under a variety of climatic conditions. Root segments as small as one inch are capable of producing a new plant.
Prevention, early detection and control are important components of an integrated management program on rush skeletonweed. Kim Goodwin with Montana State University coordinates a regional project to safeguard western Montana against rush skeletonweed invasion. “The weed is characterized by landscape-scale spread,” she explains, “So we are piloting a regional approach with county-level safeguarding, risk reduction, and control plans in southwestern Montana.”
Jeremey Varley, Lemhi County Weed Superintendent in Salmon, Idaho is one cooperator in the regional effort. “The county has been controlling rush skeletonweed since 1999, but wind driven seed complicates our management program,” explains Varley. The weed is often widely dispersed on steep terrain that is difficult to access.
Control efforts include monitoring, early detection and treatment of newly invading plants. “We try to inventory infested areas and spot-treat plants each year,” explains Varley. “Our preferred herbicide application is Milestone® specialty herbicide at 7 fluid ounces per acre (fl oz/A) either alone or mixed with 2,4-D at 16 fl oz/A. We get good rush skeletonweed control, and treated areas don’t have a flush of cheatgrass (Bromus tectorum) the year following Milestone or Milestone plus 2,4-D.”
Tordon® 22K at 32 fl oz/A was applied on Bureau of Land Management land since Milestone was only recently approved for use by the agency. Milestone is more selective on desirable broadleaf plants than Tordon 22K. Tolerance of tree and shrubs to herbicide treatments can be found on the product label or within the Invasive Plant Management Guide for Natural Area Managers (http://bit.ly/milestoneguide).
Brad Gamett, Weed Superintendent for Butte County, Idaho agrees that either Milestone or Tordon 22K provide good control. “Most of our herbicide applications on rush skeletonweed are spot treatments to small infestations or individual plants, rather than broadcast applications over large areas. The problem is that we tend to get seedling germination or miss rosettes in non-treated areas with spot applications, so we continually have to monitor sites.” A broadcast application around established plants rather than just a spot treatment to individual plants is recommended to control seedlings.
Both Gamett and Varley often wait until grass cures since rush skeletonweed will remain green later in the summer, making plants easier to locate. “Earlier in the season (May) or late fall would be a more optimal time to apply herbicide, but we miss too many rush skeletonweed plants because they are difficult to see,” explains Gamett.
Research trials conducted in Washington support results reported by on-ground managers. Studies show that either fall or spring applied herbicides provide good control of rush skeletonweed for a year or more following treatment (see Table). Although Tordon 22K at 32 fl oz/A was more consistent across sites, Milestone at either 5 or 7 fl oz/A provides more selective control with less injury to desirable grasses and forbs.
The level and duration of control achieved from herbicides on rush skeletonweed is dependent in part on presence of biological control agents, skeletonweed biotype, and percent cover of competitive perennial grasses. Prather reported that 18 percent perennial grass cover is the threshold needed to keep rush skeletonweed from dominating a site. Once perennial grass cover drops below 18 percent, the duration and level of rush skeletonweed control achieved with herbicides declines.
Other Management Options
Targeted grazing to suppress rush skeletonweed can be effective; however, cost and other challenges such as managing livestock distribution, increased labor needs, and fencing may limit this approach (Goodwin, personal communication).
Establishing effective biological control agents is a top priority to reduce rush skeletonweed vigor, seed production, and establishment success. Three biocontrol agents of rush skeletonweed are established in the western United States including the rust fungus (Puccinia chondrillina), gall mite (Aceria chondrillae), and gall midge (Cystiphora schmidti). Although these agents are well established, their success has been limited in reducing either rush skeletonweed populations or spread. The root moth (Bradyrrhoa gilveolella) is established on a limited number of sites in the West and control efficacy of this agent is unknown.
There are on-going studies to determine suitability of additional biological control agents, such as the root crown moth (Oporopsamma wertheimsteini), for control of rush skeletonweed, and recent genetic studies will help support collection, screening, and distribution of agents better adapted to rush skeletonweed genotypes established in the United States.
Integrating various management techniques including prevention, judicious monitoring, biological, cultural (reseeding), and herbicides is critical for managing rush skeletonweed. Maintaining desirable perennial grass, minimizing disturbance, and establishing desirable competitive vegetation on disturbed or degraded sites will help reduce susceptibility of a landscape to rush skeletonweed invasion.
John Gaskin and others conducted molecular studies on rush skeletonweed to determine origin and distribution of invasive genotypes. Results showed that 682 unique genotypes were present in the native range for rush skeletonweed (Spain to Uzbekistan), but only seven were present in North America, with two of these being genetically distinct from previously identified genotypes. Two new genotypes have been identified in the eastern United States. Results from these studies will support efforts to develop viable biological control agents on rush skeletonweed.
Full Paper Citation:
Gaskin JF, Schwarzlander M, Kinter L, Smith JF, Novak SJ. 2013. Propagule pressure, genetic structure, and geographic origins of Chondrilla juncea (Asteraceae): An apomictic invader on three continents. American Journal of Botany 100(9): 1871–1882. 2013.
Rapid and distant spread. Long distance dispersal by wind, high annual seed production, long period of seed production, and high seed survival rate contribute to spread.
Complex spread patterns. Wind-dispersed plants have diverse patterns of spread to distant sites that are often difficult to access.
Monitoring challenges. Distant and complex spread makes early detection difficult, as vast expanses of wildland must be monitored regularly for new populations.
Plants lack leaves and showy, distinct flowers allowing the plant to blend in with surrounding vegetation and making detection and eradication difficult.
Herbicide limitations. Complete control of the extensive root system is difficult on older, well-established populations. Spot-treatment programs often miss small rosettes and newly germinated seedlings, so continued monitoring and follow-up treatment are required.
Wide ecological amplitude.
The weed tolerates a variety of climatic and soil conditions allowing for invasion of diverse ecosystems. Although it grows well on disturbed sites, plants also establish on undisturbed open forest habitat types and native shrublands.
*adapted from Goodwin.
Coombs EM, Clark JK, Piper GL, Cofrancesco Jr, AF. 2004. Biological control of invasive plants in the United States. Corvallis, OR: Oregon State University Press. 467 p.
EDDMapS. 2014. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; accessed May 8, 2014.
Gamett, Brad. Butte and Custer County Weed District, personal communication.
Gaskin JF, M Schwarzlander, CL Kinter, JF Smith, and SJ Novak. 2013. Propagule pressure, genetic structure, and geographic origins of Chondrilla juncea (Asteraceae): An apomictic invader on three continents. American Journal of Botany 100(9): pp. 1871–1882.
Goodwin, Kim. Montana State University, personal communication
Prather, Tim. University of Idaho, personal communication.
Sheley RL, Hudak JM, Grubb RT. 1999. Rush skeletonweed. In: Sheley RL, Petroff J, eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press. 438 p.
Varley, Jeremey. Lemhi County Weed District, personal communication.
First Published 8/2014; Reviewed and revised 6/2017
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Tordon 22K is a federally Restricted Use Pesticide. Always read and follow label directions.