Rush skeletonweed or Skeletonweed [Chondrilla juncea L.][CHOJU][CDFA List: A] Photographs Map of Distribution

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SYNONYMS:Skeleton weed; naked weed; gum succory; devil’s-grass; hog bite.

GENERAL DESCRIPTION:Herbaceous perennial or biennial, with rigid, wiry flowering stems to 1 m tall, milky sap. Plants exist as basal rosettes until flowering stems develop at maturity and rosette leaves whither. Persistent flower stems can hinder harvest machinery. Several forms (biotypes) occur, differing in leaf width, branching pattern, and flowering time. Characteristics can vary between, but rarely within populations since all reproduction is by clones (vegetative and seed apomixis). Plants are highly competitive for water and nutrients. Rush skeletonweed is also a significant problem in several other countries, particularly Australia. Introduced from southern Europe. The biocontrol agents skeletonweed gall midge (Cystiphora schmidti), skeletonweed gall mite (Eriophyes chondrillae), and rush skeletonweed rust (Puccinia chondrillina) can control some infestations. All are established in California.

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SEEDLINGS:Cotyledons spatulate to oval. First leaves elliptic with backwards pointing teeth. Require a continuous moisture supply for up to 6 weeks to develop a persistent root system.

MATURE PLANT:Rosette leaves oblanceolate, 4-12 cm long, 1-5 cm wide, prostrate, and typically lacking hairs. Margins often purple-tinged and irregularly shallow-lobed, with lobes often pointing backwards towards the leaf base. Lobes opposite one another. Terminal lobe more or less sharp-pointed. Rosettes produce 1 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 often absent or bract-like, but when present resemble reduced rosette leaves.

ROOTS and UNDERGROUND STRUCTURES: Taproot slender, deep, persistent, with short lateral branches along the length. Taproots become somewhat woody with age and can penetrate soil to depths of 2-3 m or more. Most lateral roots are short-lived, non-woody, and less than 8 cm long, but a few lateral roots near the surface can become rhizome-like and grow laterally for 15-20 cm before turning downwards. Adventitious buds near the top of the taproot and on major lateral roots can produce new rosettes. Roots are easily fragmented, with pieces as small as 1-2 cm producing new rosettes from depths to 1 m.

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FLOWERS:July until flowering stems killed by frost (fall or winter). Flower heads axillary or terminal, sessile or short-stalked, and solitary or in interrupted spike-like clusters of 2-5. Each flower head consists of 7-12 bright yellow ligulate flowers, strap-shaped with 5-lobed corollas 12-18 mm long, and phyllaries (bracts) cylindric as a unit and in 2 unequal rows, the outer much smaller than the inner. Receptacle lacks small bracts (chaff) among the flowers. Temperatures of at least 15 ºC are necessary to induce flower production.

FRUITS and SEEDS:Achene body oblong, tapered at both ends, lacking hairs, pale to dark brown, and 3-4 mm long, with many lengthwise ribs, pointed tubercles near the top, and to 6 small scales at the apex, surrounding the point of beak attachment. Beak slender, 5-6 mm long, not including the pappus which consists of many equal, fine, white bristles about 5 mm long.

POSTSENESCENCE CHARACTERISTICS:Persistent rigid stems with clusters of flower head bracts (and sometimes seeds) on old stems distinguish rush skeletonweed from dandelion (Taraxacum officinale Wigg.) and Brassicaceous weeds such as mustards (Brassica spp.) and radish (Raphanus spp.).

HABITAT:Disturbed soils of roadsides, croplands, especially irrigated grain fields, semi-arid pastures, rangelands, and residential properties. Grows best on well-drained, sandy or gravelly soils in climates with cool winters and hot, relatively dry summers without prolonged drought. Tolerates a wide variety of environmental conditions, including rainfall less than 250 mm (10 in) to more than 1200 mm (~50 in), cold winter areas, and continental climates. Severe infestations are less common on heavy clay soils.

DISTRIBUTION:Uncommon. North Coast (ce & cw Mendocino Co.), Cascade Ranges (ne Shasta Co.), northern Sierra Nevada (s Plumas, Sierra, Nevada, Placer, El Dorado, Calaveras cos.), Central Valley (n Sacramento, e Yolo, Fresno cos.), San Francisco Bay region (sc Napa, n Santa Clara cos.), South Coast Ranges (s Monterey, San Luis Obispo cos.), and South Coast (Los Angeles Co.); to 600 m (2000 ft). Previous infestations now eradicated occurred in Tehama, Butte, Solano, San Mateo, Madera, Santa Barbara, and San Diego cos.

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PROPAGATION/PHENOLOGY:Triploid. Reproduces only by clones produced vegetatively from adventitious buds on roots and asexually by apomictic seed. Seeds primarily disperse with wind, but also by water, animals, and human activity. Seeds can be highly viable, with ~90% germination the first year and are short-lived under field conditions, to ~2% the third year but often less than 6 months. Seeds lack dormancy and can germinate within 24 hours under optimal conditions. Fresh seeds germinate without light and at a temperature range of 7-40 ºC (optimum 15-30 ºC). Seed germination and new bud growth begin in fall after first rains in mild winter areas or early spring in colder climates. Seedling emergence is reduced in water saturated or heavy clay soils and during drought conditions. First year plants on deep sandy soil can produce viable seed earlier. Can develop from rosette to seed maturity in 1 month. Flowering stems are produced in early summer. One plant can produce 15,000-20,000 seeds per season.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Moderate soil disturbance, such as grain cultivation alternated with grazing on a yearly basis, encourages infestations by dispersing rootstocks. Under moist conditions, shallow burial of seed by hooves of grazing livestock appears to promote seed germination. Increasing nutrient levels of poor soils discourages infestation by increasing competition from other vegetation.

SIMILAR SPECIES:Rosette leaves of rush skeletonweed and dandelion share the following characteristics: leaves without hairs, leaf lobes pointing backward and opposite one another, milky juice exuded when torn. Unlike rush skeletonweed, dandelion has unbranched, leafless, hollow, non-persistent, fleshy flowering stems and seeds without small scales at the apex. In addition, dandelion is typically found in turf and gardens. Chicory (Cichorium intybus L.) is similar to rush skeletonweed and dandelion, but has rosette leaf lobes pointing outwards or forwards and not always opposite, and basal leaves with a few rough coarse hairs.


Prevention: Rush skeletonweed is likely to establish along roadsides and right of ways, and spread to the surrounding areas. In rangeland areas, proper grazing management and fertilization will help prevent its establishment. Where infestations are present, cattle and sheep will readily graze the rosettes and shoots until the stems become lignified. Grazing will reduce seed production, and few viable seed will pass through a ruminant digestive system. Subterranean clover (Trifolium subterraneum), an important forage source, forms dense stands, which prevent skeletonweed seedling establishment. Wheatgrass, however, has failed to prevent reinfestation after herbicides were used to control initial stands.

Mechanical: Rush skeletonweed may proliferate in wheat-fallow areas where fallow cultivation is used. Tillage will effectively eliminate seedlings and older plants. However, new plants will rapidly reestablish from severed rootstocks as small as 2 cm and from a depth of 120 cm. Mowing will reduce the number of viable seed produced. However, the plants will persist due to vegetative reproduction. Mowing after initial seed set is not recommended and will likely increase seed dispersal.

Biological: Three organisms have been released for control of skeletonweed; the skeletonweed gall midge (Cystiphora schmidti), skeletonweed gall mite (Eriophyes chondrillae), and skeletonweed rust (Puccinia chondrillina). The gall midge has four to five generations per year and attacks all known biotypes of skeletonweed. Females insert eggs below the surface of rosette stems and leaves. The larvae feed on the leaves and stems at the site of egg deposition. This results in the formation of galls which are circular, slightly raised, approximately 3 mm in length, and yellowish to maroon in color. Infested plants show reduced vigor and early senescence, with decreases in viable seed production and stem size and length. Heavily infested stands may appear a purplish color from a distance. This midge and the mite are available for distribution in California. One of the best strategies for use is collecting stems with galls from an infested patch, removing any seed heads or flowers, tying the stems into teepee bundles, and placing them in the center of uninfested patches.
The gall mite has several generations per year and has been the most effective agent released for skeletonweed control. The mites attack shoot buds when plants bolt in the spring, and continue to form galls until the fall. Infested plants exhibit deformed shoot buds, and produce few to no viable seed. The mite also reduces vegetative reproduction by reducing carbohydrate reserves and preventing new rosettes from establishing from the original plant. This mite is available for use in California. After seed head and flower removal, gall infected stems may be placed against new uninfested plants, and colonization should occur.
The rust has multiple generations per year and has demonstrated considerable success in California. Large cinnamon to brownish pustules form on leaves and stems of rosettes and mature plants alike. These open lesions result in stunted, deformed plants, with reduced branching and floral bud production. Infected plants produce fewer viable seed. This rust is available in California, and can be spread by placing infected stems in patches of healthy plants. To improve the chances of infection, spray uninfected plants with water before placing the stems or time infected stem placings with expected periods of heavy dew. It should be noted that different biotypes of rush skeletonweed have shown resistance to this rust in Australia. The narrowleaf biotype is susceptible, while the intermediate and broadleaf types appear to be resistant to the fungus. This biocontrol agent is the first example of an exotic fungus successfully used in the classical biocontrol of a weedy plant. It may, however, take up to four years before results are highly visible.

Chemical: There are few herbicides registered for control of rush skeletonweed. Single treatments rarely provide long term control, and repeat applications have been proven more effective. Picloram (Tordon) and metsulfuron methyl (Escort) have been effective in controlling rush skeletonweed, but are not currently labelled for use in California. Spring herbicide applications will control rosettes and bolting plants. However, new rosettes may form from rootstocks. Tank mixes of clopyralid (Transline) and MCPA or 2,4-D have been shown to be more effective than MCPA or 2,4-D alone. Glyphosate will control rosettes, but is nonselective and will kill any desirable vegetation. New rosettes will emerge, and fluorish without competition. Table one lists herbicides, rates, and timings for effective control of rush skeletonweed.

Table 1. Herbicide recommendations for rush skeletonweed control in rangelands
Herbicide Rate Timing Remarks
Clopyralid 2-4 oz ae/A Spring, rosette to early bolting Will kill annual and perennial legumes
MCPA 1-1.5 lb ae/A Spring, early bolting and May injure legumes and any other broadleaves
2,4-D 1-2 lb ae/A Spring, early bolting May injure or kill legumes and any other broadleaves

Integrated weed management: Compatibility of herbicide programs, competitive vegetation, and biocontrol agents is critical for effective management of rush skeletonweed. Complete kill of above ground shoots with a herbicide will greatly reduce establishment of the rust Puccinia chondrillina. Likewise, all the currently labeled herbicides may kill or injure important legumes. Reestablishment strategies for legumes following herbicides has not been well documented. An integrated approach of using the rust fungus with subterranean clover has been shown to be compatible and effective in reducing rush skeletonweed populations.

Chaboudez, P. 1994. Patterns of clonal variation in skeleton weed (Chondrilla juncea L.), an
apomictic species. Australian Journal of Botany. 42:283-295.
Dodd, J. and F.D. Panetta. 1987. Seed production by skeleton weed (Chondrilla juncea L.) in
Western Australia. Australian Journal of Agricultural Research. 38:689-705.
Groves, R.H. and J.D. Williams. 1975. Growth of skeleton weed (Chondrilla juncea L.) as
affected by growth of subterranean clover (Trifolium subterraneum L.) and infection by
Puccinia chondrilla Bubak & Syd. Australian Journal of Agricultural Research. 26:975-983.
Panetta, F.D. and J. Dodd. 1987. The biology of Australian weeds. 16. Chondrilla juncea L.
Journal of the Australian Institute of Agricultural Science. 53:83-95.
Panetta, F.D. and J. Dodd. 1987. Bioclimatic prediction of the potential distribution of skeleton
weed Chondrilla juncea L. in Western Australia. Journal of the Australian Institute of
Agricultural Science. 53:11-16.

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