Weed:  Rush skeletonweed (Chondrilla juncea L.)

Family:  Asteraceae (Sunflower family)

Images:  Courtesy of A. Peterson, A. G. Currah, J. W. Brasher, and S. A. Dewey.

Fig. 1.  Whole plant in fruit, vouchered by Adrianne Peterson s.n., 28 Aug 2006 (RM).  Photo – A. Peterson.

Fig. 2.  Closeup of inflorescences in fruit, vouchered by Adrianne Peterson s.n., 28 Aug 2006 (RM).  Photo - A. Peterson.

Fig. 3.  Closeup of stem bases showing reflexed reddish hairs, vouchered by  Adrianne Peterson s.n., 28 Aug 2006 (RM).  Photo – A. Peterson.

Fig. 4.  Closeup of heads in flower and fruit showing involucral bracts.  Photo - Steve Dewey, Utah State University, Bugwood.org, www.invasive.org/images/3072x2048/1459567.jpg.  
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Fig. 5.  Fruits.  Photo - Steve Hurst @ USDA-NRCS PLANTS Database.  Usage - see http://plants.usda.gov/java/usageGuidelines?imageID=chju_002_ahp.tif.

Fig. 6.  Seedling grown in UW greenhouse, vouchered by Adrianne Peterson s.n., 28 Aug 2006
(RM).  Photo – J. W. Brasher. 
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Brief Plant Description:  (Paraphrased from references 2 and 8.)  The plant is a deep-rooted perennial, mostly 0.4-1.5 m tall, appearing somewhat rush-like.  The lowermost part of the branching stem bears distinctive large, spreading to slightly descending, reddish hairs.  The upper stem is hairless.  Cut surfaces of stems and leaves release milky sap.  The early-deciduous basal leaves are well-developed, runcinate-pinnatifid, 5-13 X 3.5 cm, and strongly resemble dandelion leaves.  The stem leaves are linear, 2-10 cm long and 1-8 mm wide, and often also deciduous, the upper often reduced to scale-like bracts.  The flower heads are scattered along the branches, commonly with 9-12 yellow, strap-shaped flowers per head.  The involucre of bracts is 9-12 mm high, and thinly clothed with white, curly, intertangled hairs.  The body of each pale brown to almost black fruit is 3-3.5 mm long, with five broadly rounded longitudinal ribs separated by grooves.  The tip of the fruit body bears small pointed bumps then 5 small scales at the base of a long, slender beak, which bears the pappus of hair-like bristles.  Flowering in Wyoming is expected from July-Sept.

Current Wyoming Distribution:  Sublette Co., N side of Highway 191 between Pinedale and Jackson just past Granite Hot Springs turnoff, N 43°17.149'  W 110°32.200', elevation 6357 ft. Roadside about 2 ft. from road below a cut bank in a mixed sagebrush, aspen, conifer community.  Nearly bare ground apparently recently disturbed from work on a telephone-electrical box.  Adrianne Peterson s.n., 28 Aug 2006 (RM).  This is the only known documented occurrence in WY.

We have heard six other undocumented reports of this species being found in Wyoming – four in Teton Co. in about 2006 plus two (1) in other WY counties:  Medicine Lodge Creek, near Hyattville, Big Horn Co. in about 1998 and near Alpine Junction, Lincoln Co. in about 2005.

How did it get to Sublette County?  Seeds or root fragments may have arrived in Sublette County on energy trucks.  Seeds also could have dispersed by wind from undiscovered new populations or the nearest documented occurrence in Frement Co., Idaho, which may be as little as 80 miles away.

Origin:  Mediterranean region.

Global Distribution:  Europe, Mediterranean region, Australia, Canada (3,4); thirteen US states including Montana and Idaho (5,6).

Reasons for Concern:   Rush skeletonweed is an aggressive wind dispersed colonizer that also spreads by creeping roots. It has been documented to invade cheatgrass dominated areas in Idaho and sagebrush communities without disturbance.

Legislative Status:  Chondrilla juncea is currently listed as a noxious weed in Arizona, California, Colorado, Idaho, Montana, Nevada, Oregon, South Dakota, and Washington (6).  This weed is not on the Wyoming designated noxious weed list, but it has been pre-emptively declared in four Wyoming counties:  Big Horn, Converse, Teton, and Washakie (8).

Control Methods:   

Given its current limited distribution in Wyoming, eradication is very feasible and should be given top priority.  Elimination of all shoots and roots to prevent both seed production and asexual spread from creeping roots is the goal.  Strategies to accomplish this include digging, hand pulling, and herbicides.  Digging and hand pulling should attempt to remove the primary root crowns and as much of the lateral roots as possible.  Rush skeletonweed roots respond to topgrowth removal by releasing new shoots from adventitious root buds.  Immediate follow-up (within a few weeks) will be required to remove new plants sprouting from missed roots.

There are several effective herbicides including picloram, clopyralid, aminopyralid, and metsulfuron for Rush skeletonweed control.  Timing of application should be in the summer before flowering to prevent seed production.  Follow-up will be required later in the fall and the following year to retreat plants from surviving rootstocks.  If infestations are detected after seed production, fall herbicide treatments to newly emerged rosettes are also effective.  Persistence will be required for several years given the persistence of Rush skeletonweed roots in the soil.

While there are some biological control agents available, they are not suited for eradication of rush skeletonweed and should be used only after eradication no longer is feasible.

Additional Notes:

There are three forms of rush skeletonweed in the US, each differing in

the morphology of their inflorescence and their susceptibility to control measures (4,5).  It is not yet clear which of the three forms was collected in Sublette Co.

Rush skeletonweed is sometimes confused(4) with (rush-) skeletonplant, Lygodesmia juncea, which has pink (occasionally white) flowers and grows in Wyoming and the surrounding region.

Most monospecific stands are in abandoned agricultural fields.  Ann Hild (personal communication) has seen populations “marching” into sagebrush stands.  This species follows fire.  Per seed rates of seed germination are low, but seeds can be 10,000/ plant and blow in the wind. 

References:

1.  Brasher, J. W., S. F. Enloe, A. Peterson, A. G. Currah, and B. E. Nelson.  2007.  Noteworthy collections for Wyoming and Colorado:  Centaurea montana, Chondrilla juncea, Echium vulgare, and Rorippa austriaca.  Madroño 54(2):210-211.

2.  Cronquist, A.  1994.  Asterales.  In:  A. Cronquist, A. H. Holmgren, N. H. Holmgren, J. L. Reveal, and P. K. Holmgren (editors).  Intermountain Flora:  Vascular Plants of the Intermountain West, U.S.A.  Volume 5.  New York Botanical Garden, Bronx, New York. 

3.  GBIF (Global Biodiversity Information Facility).  2006.  GBIF Data Portal.  Copenhagen, Denmark.  Available from www.gbif.net. (accessed Oct. 2006). 

4.  Parchoma, G. (editor).  2002.  A guide to weeds of British Columbia.  British Columbia Ministry of Agriculture, Food, and Fisheries, Open Learning Agency.  Burnaby, British Columbia, Canada.  Available from http://www.weedsbc.ca/pdf/rush_skeletonweed.pdf).  (accessed Aug 2007).

5.  Sheley, R. L. and J. K. Petroff (editors.)  1999.  Biology and Management of Noxious Rangeland Weeds.  Oregon State University Press, Corvalis.

6.  USDA, NRCS (United States Department of Agriculture, Natural Resources Conservation Service).  2001 onwards.  The PLANTS database, version 3.1.  National Plant Data Center, Baton Rouge, Louisiana.  Available from http://plants.usda.gov.  (accessed Aug. 2007).

7.  Whitson, T. D., L. C. Burrill, S. A. Dewey, D. W. Cudney, B. E. Nelson, R. D. Lee, and R. Parker (editors).  2002.  Weeds of the West.  9^th edition.  University of Wyoming, Laramie.