PNW431 SLENDERFLOWER THISTLE ( tenuiflorus Curt.) ITALIAN THISTLE (Carduus pycnoceplwlus L.) PLUMELESS THISTLE (Carduus acanthoides L.)

Five species of thistles in the States and Canada. Musk or genus Carduus are troublesome nodding thistle (Carduus nutans L.) weeds of pastures, meadows, is widespread across the United roadsides and waste areas in States. the United States. They compete with desirable , reducing Italian, slenderflower and forage production. Their spiny plumeless thistles are three leaves and stems hinder live­ similar thistles native to dis­ stock from grazing forage grow­ turbed habitats in the Mediter­ ing near them. Slenderflower ranean region. The native range and Italian thistle infest grazing of Italian thistle includes the lands in , Oregon and Balkan Peninsula and Turkey, Texas. Plumeless thistle infes­ in contrast to the more western tations occur in the north-east­ distribution of slenderflower ern United States westward to thistle. Their ranges overlap in South Dakota and Nebraska, , southern , and south to Virginia and Ohio. In . Slenderflower and Italian the western United States it thistles have spread to Argen­ grows in Idaho, eastern Wash­ tina, Australia, and New ington, Wyoming, and northern Zealand, as well as the United California, where sites occur in States. Plm:neless thistle's native range extends from the Mediter­ Nevada, Marin, Modoc, Thistles in the Carduus genus (top) Monterey, and Glenn counties. ranean region of east­ have a plumeless pappus in contrast to Welted or curled thistle ward to Siberia and the Caucasus. the feathery (plumed) pappus of (Carduus crispus L.) infests pas­ Plumeless thistle has spread to thistles in the Cirsium genus, for tures in the eastern United New Zealand, northern Europe, example, Canada thistle.

Illustrations of flower heads, left to right: Italian thistle heads have woolly hairs on the and are clustered in groups of two to five; slenderflower thistle heads lack hairs on the bracts and are clustered in groups of five or more; plumeless thistle heads have narrow, more numerous bracts and are solitary or more loosely clustered.

A Pacific Northwest Extension Publication • Washington • Idaho • Oregon Plumeless thistle is a Class A tional spiny wings extend con­ weed in California and a Class tinuously up to the flower B weed in Washington. Neither heads of slenderflower thistle Idaho nor Oregon list it as a but are often interrupted on noxious weed. Botanists first Italian thistle. Numerous purple collected plumeless thistle in flowers borne in small heads, Klickitat County, Washington, 1 3 I 2 to I 4 inch in diameter, clus­ in 1911 and in Nez Perce ter at the tips of the branched County, Idaho, in 1948. A major stems. Italian thistle flower problem in northern Stevens heads are sometimes solitary, County, Washington, plumeless but usually grow in clusters of thistle has begun to invade adja­ two to five heads. In contrast, cent Ferry and Pend Oreille slenderflower thistle usually counties. In Idaho, plumeless forms clusters of 5 to 20 heads. thistle grows in Fremont, Lewis, Bracts surrounding the flower Teton, and Idaho counties. heads are broadly lance-shaped Oregon has not reported it. Spiny wings along the stems of slender­ and have a sharp-pointed, or flower thistle are broader than wings on short-spined tip. Slenderflower Italian thistle and are continuous to the IDENTIFICATION thistle bracts lack the cobwebby base of the flower heads. hairs found on Italian thistle Because slenderflower and Ital­ bracts. ian thistles have more similari­ and Argentina as well as the ties than differences, we United States and Canada, In both slenderflower and Ital­ describe them together and ian thistle, each flower head where it has established in point out the differences. produces two types of achenes Nova Scotia, Quebec, Ontario, Plumeless thistle follows, along (seeds), each about 3 I inch and British Columbia. 8 with differentiating characters long. Achenes produced by the for welted and musk thistle. Slenderflower and Italian central flowers in the head are cream colored, have conspicu- thistles are Class A noxious weeds in Washington, Class B Slenderflower and Italian noxious weeds in Oregon, and thistle Class C weeds in California. Both slenderflower and Italian Idaho does not list them as nox­ thistle grow erect flower stems 1 ious weeds. Present in Oregon to 6 feet tall from taprooted ro­ since the 1920s, Italian thistle in­ settes of basal leaves. Leaves are fests 80,000 to 100,000 acres of pastureland in Douglas County deeply cut into two to five pair and smaller areas in Coos, of spiny lobes. The terminal Curry, Linn, Lane, Marion, and spine of each lobe grows longer Yamhill counties. Slenderflower and more rigid than the other thistle, first collected in Oregon spines. Lower leaf surfaces are in 1939, often grows in mixed whitish with woolly hairs, stands with Italian thistle in while the upper surfaces are Oregon and California. In green and essentially hairless. Washington, both thistles were Stem leaves of both species con­ found in 1991: slenderflower tinue down the stem as spiny thistle in Thurston County and wings. Stem wings of slender­ 3 Italian thistle in Whitman flower thistle, up to I 8-inch­ 3 / - County. In Idaho, Italian thistle wide, are broader than the 16 Slenderflawer and Italian thistle grows in Lewis and Idaho inch-wide wings normally dominate some foothill pastures in counties. found on Italian thistle. Addi- southwestern Oregon.

2 along the wavy leaf margins. surrounding the flower heads Alternate stem leaves are taper to a sharp spine on the deeply and irregularly lobed, outer bracts, which spread out­ and bear scattered hairs, mainly ward or curve backwards. The 1 along the prominent veins on I 8-inch-long achenes (seeds), the underside. Each lobe tip light brown at maturity, are ob­ supports three spines. Stem long, and may appear flattened, leaves extend as spiny wings straight or curved. Unbranched down the furrowed, yellowish (not plumed) hairs form the green stems. Additional wings white pappus that attaches to line the stems up to the base of the achene, hence the name, the flower heads. Solitary plumeless thistle. These flower heads form at the end of unplumed pappus hairs distin­ branches; on younger branches, guish Carduus thistles from heads may cluster loosely in Cirsium thistles such as Canada groups of two to five. The and bull thistles, which have 3 Plumeless thistle flower heads are 1Wt globe-shaped heads, about I 4 feathery pappus hairs. as tightly clustered at the tips of the inch in diameter, have rose­ branched stems as slenderflower or purple flowers. Occasional Plumeless thistle may hybridize Italian thistle heads. plants have white or cream col­ with musk thistle, producing ored flowers. The numerous, plants with intermediate char­ ous lengthwise grooves, are narrowly lance-shaped bracts acteristics. 3 - sticky, and bear a plume of I 8 5 to I 8-inch-long, dirty white, minutely barbed hairs. This plume or pappus usually falls off readily. The outer ring of flowers in the head produces achenes that are smooth, darker, non sticky and lack a pappus.

Some confusion in identification results because slenderflower and Italian thistle can hybridize. This interbreeding is limited be­ cause the two species have dif­ ferent chromosome numbers and because slenderflower thistle is mainly self-pollinated.

· Plurneless thistle Plumeless thistle grows erect stems, commonly 4 to 7 feet tall, that branch repeatedly from the middle of the upward. Basal leaves form a rosette over a stout fleshy taproot. Rosette leaves, usually 4 to 8 inches long, have yellowish spines Plumeless thistle grows 4 to 7 feet tall.

3 BIOLOGY AND ECOLOGY Slenderflower and Italian thistles grow as annuals or, less frequently, as biennials. They commonly germinate in the fall, overwinter as rosettes, flower in late spring, and pass the sum­ mer drought as seeds. Spring gerrninants may flower during the same growing season, as they do not require a cold period before flowering. Plurneless thistle is more typi­ cally a biennial, although it also Plumeless thistle stems also bear behaves as a winter annual. A spiny wings. small percentage of spring Spiny wings line the stems of Italian gerrninants flower that year, but thistle, but are often interrupted just Welted and musk thistle most remain as rosettes and below the flower heads. flower the following summer. Welted thistle plants are less These thistles flower from May branched than plurneless thistle t6 July, or later if sufficient soil thistle heads are transported by and regularly bear heads in moisture is present. means of their sticky surfaces; clusters as opposed to single outer seeds are transported in heads in plurneless thistle. All three species reproduce the old spiny heads. Dormancy Welted thistle heads are smaller solely by seed. Seeds mature of the nondispersing outer 5 ( I 8 inch), and the sterns are of­ and disperse shortly after flow­ seeds varies from 2 days to 3 ten bare for a short distance be­ ering. Plurneless thistle seeds years. Outer seeds often germi­ low the heads. Welted thistle mature in 1 to 3 weeks. These nate within the head after it leaves have fewer, more shal­ thistles produce large numbers falls to the ground. Because low divisions than plurneless of seed, especially the earliest their outer seeds are dormant, thistle, which has deeply and emerging slenderflower and slenderflower thistle and Italian sharply segmented leaves and Italian thistle plants, which thistle persist longer than bears strong marginal spines. flower continuously until they Lower surfaces of welted thistle deplete the soil of moisture. leaves are woolly, while those of plurneless thistle have only a Slenderflower thistle and Italian few hairs on the midrib. thistle heads average two or three outer seeds and 11 or 12 You can easily distinguish musk inner seeds. The inner seeds thistle from the other Carduus have a water soluble germina­ thistles by its larger flower tion inhibitor that allows rapid heads and nonhairy leaves. Its germination (2 days) when 1 heads, 1 I 2 to 3 inches in diam­ moisture is sufficient. In addi­ eter, have purplish, reflexed tion to having a pappus, inner 3 bracts up to I 8 inch wide at seeds are aided in dispersal by a the base. Frequently nodding in mucilaginous coating that appearance, solitary musk serves as an adhesive. Seeds Italian thistle, left, has fewer flower thistle heads arise on sterns that shed at maturity from the center heads per cluster than slenderflower are leafless in the upper part. of Italian and slenderflower thistle, right.

4 plumeless thistle in the soil seed inhibit germination of associ­ tion controls these annual- bien­ bank, up to 7 years in one ated annual grass species. In nial thistles. report. Although outer seeds Douglas County, Oregon, these represent only 14% of total seed two thistles found an ideal envi­ In areas where the thistles are production, 60% of the seeds in ronment in hill pastures man­ abundant on pasture or range­ the soil were outer seeds. aged for subterranean clover. land, the best defense is a coor­ Subterranean clover is a winter dinated program combining In Argentina, two factors lim­ annual that reseeds best if most grazing management, perennial ited plumeless thistle to a small of the vegetation is grazed off in grasses, biological control, seed bank despite heavy seed late summer. Such preparation mowing, and pasture and fertil­ production. First, the seeds had creates an open soil surface that ity management. Researchers in no dormancy and germinated thistles quickly invade. When Douglas County, Oregon, readily when conditions were the thistles germinate with the showed that maintaining a favorable; 90% of plumeless first good rains in the fall, the good stand of grass is the best thistle seeds germinated within slower-growing clover offers no way to prevent invasion by Ital­ the first year of sowing. Second, competition. Plumeless thistle ian and slenderflower thistles. heavy feeding by birds, rodents invasion of bluegrass meadows To start a program on hillside and insects depleted the number in northeastern Washington rangeland, select one or more of seeds in the litter and soil. may relate to low soil fertility grasses that germinate quickly (low nitrogen levels in cleared and grow rapidly, then broad­ As weeds of uncultivated land, forest soils), season-long graz­ cast sow in early fall so that slenderflower, Italian and ing, and digging by gophers seed will be in place to germi­ plumeless thistles primarily and ground squirrels. nate when the rains come. In threaten forage production for addition to preventing thistle livestock and wildlife on range­ invasion, a good grass stand land and pasture. As they CONTROL may drastically reduce the invade natural vegetation in Prevent spread and establish­ number of thistles that germi­ parks and along roads, their ment of these thistles by using nate and survive in already spiny presence may restrict rec­ certified seed, clean hay, bed­ infested pastures. In severe reational activities. Dense ding, and equipment, and by cases, apply a selective herbi­ stands of slenderflower and keeping vehicles, grazing ani­ cide during winter or early Italian thistle in hillside pas­ mals, and pets out of infested spring to give the newly seeded tures in southwestern Oregon areas. Seed inspectors in Cali­ grass an advantage during the limit grazing and other live­ fornia have found Italian and first season. stock management, such as slenderflower thistle seeds in gathering of sheep. Activities commercial ryegrass, tall fescue Annual overseeding of grass in livestock bedding areas and that disturb the soil, weaken and meadow fescue seed. They other disturbed sites also helps competitive vegetation, and traced several infestations of prevent thistle invasion. Grass allow light to the soil surface plurneless thistle in California to does not survive long in pas­ speed thistle invasion and a shipment of Harding grass seed tures managed for clover. thicken thistle stands. Expect imported from Argentina in 1973. Increase nitrogen fertilizer to thistle problems to follow dis­ Italian thistle found in a Whit­ maintain grass with the clover. turbances such as heavy con­ man County hay stackyard prob­ tinuous grazing, burning and ably carne in hay from Idaho. The only biological control trampling. In California and agent currently available is Oregon, Italian and slender­ Learn to identify these thistles Rhinocyllus conicus, a seed head flower thistle have primarily so you can take prompt action weevil. Widespread, it occurs in invaded annual rangelands. to prevent seed production. Dig all major infestations of plume­ Their ability to invade may be or spray small infestations, and less, slenderflower or Italian tied to their potential to germi­ mark the location to check for thistle in the Pacific Northwest. nate when temperature, mois­ additional plants from later ger­ If you are using biological con­ ture and seedbed conditions minating seeds. Regular cultiva- trol, avoid mowing thistles at 5 bud stage when you can dam­ tected by falling into cracks or west Weed Control Handbook, an age weevil populations. Mow­ hoof prints find ideal conditions annually revised extension pub­ ing in strips may concentrate for growth following the fire. lication available from the weevils on remaining thistles as extension bulletin offices of weevils move from the wilting Apply phenoxy-type herbicides . Oregon State University, Wash­ buds of cut thistles. when these thistles are most ington State University and the susceptible, during spring or University of Idaho. Land managers in Douglas fall when seedlings or rosettes County regularly use fire to are actively growing. Phenoxy The first author acknowledges remove dense stands of mature herbicides applied to all support of the Washington State thistles and to create a good exposed parts of plumeless Department of Agriculture and environment for subterranean thistle just before flowering the Washington State Noxious clover to germinate and grow. reduce seed production. For Weed Control Board in prepara­ Thistle seeds that are exposed to chemical control recommenda­ tion of this bulletin. the flame burn, but seeds pro- tions, refer to the Pacific North-

By Cindy Roche, M.S., formerly Washington State University Cooperative Extension Coordinator, Department of Natural Resource Sciences; and Larry C. Burrill, M.S., Oregon State University Extension Weed Specialist. Photos by Roche, Burrill, and D.C. Swan. Line drawings by Cindy Roche.

Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disre­ gard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pes­ ticides in their original containers and keep them out of the reach of children, pets, and livestock.

Pacific Northwest cooperative Extension bulletins are joint publications of the three Pacific Northwest states-Washing­ ton, Oregon, and Idaho. Similar crops, climate, and topography create a natural geographic unit that crosses state lines. Since 1949, the PNW program has published over 350 titles. Joint writing, editing, and production have prevented du­ plication of effort, broadened the availability of faculty specialists, and substantially reduced costs for the participating states.

Issued by Washington State University Cooperative Extension, Larry G. James, Interim Director; Oregon State Univer­ sity Extension Service, 0. E. Smith, Director; University of Idaho Cooperative Extension System, LeRoy D. Luft, Direc­ tor; and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30,1914. Cooperative Extension programs and policies comply with federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, gender, age, disability, and gender preference.

Published December 1992 1.00/0/1.00 PNW431