Northern Prairie Wildlife Research Center
Perennial sowthistle is a deep-rooted, cool-season, perennial forb. It has a vertical and extensive root system and milky latex. It has an erect growth form, and the stem is 0.3-2 m tall and 0 3-10 mm thick. Stems are hollow and sparingly to highly branched above. The lower stem is glabrous to glaucous, and the upper stem and peduncles have glandular-tipped hairs. Rosette leaves are 5-15 cm long and 2-4 cm wide with a somewhat winged petiole. Cauline leaves (6-40 cm long and up to 15 cm wide) are placed alternately along the stem. Mid- to lower-cauline leaves are sessile with clasping bases and pinnately lobed with two to seven triangular lobes. Leaves are reduced upwards and progressively unlobed. Leaf margins are spinulose and vary from dentate to subentire. The inflorescence is a terminal corymbiform cluster of few to several heads. Heads are 3-5 cm in diameter and each is subtended by 38 to 50 narrow, lanceolate bracts arranged in three imbricate rows. Bracts are dark green and often have white margins. Outer bracts (6-10 cm long and about 1.5 mm wide) are pubescent and glandular. Inner bracts are typically shorter. Heads are completely ligulate with 150 to 240 perfect florets per head. The corolla is tubular and yellow to yellowish orange. Achenes (2.5-3.5 mm long and 1-1.5 mm wide) are elliptical, flattened, ribbed, and light brown to straw-colored. Each achene has a pappus of numerous white bristles that are about four times the length of the achene. Two subspecies have been described and are based on pubescence of the bracts, subsp. arvensis L. with pubescent bracts and subsp. uliginosus (Bieb.) Nyman [syn. var. glaberescens (Guenth.) Grab. & Wimm.] with glabrous bracts.
Perennial sowthistle, is native to Europe, western Asia, and Iceland. The first North American report was from Pennsylvania in 1814. It first spread from Pennsylvania to and along the coast before spreading inland. The first midwest report was from North Dakota in 1919, where it was believed to have spread southward from Manitoba. Perennial sowthistle is now scattered throughout temperate North America.
Perennial sowthistle is adapted to a wide variety of soils, but it is most common on cultivated or noncompacted soils that are low, semi-moist to moist, fine-textured, and have a pH range of 7.2 to 5.2. It does not appear to tolerate alkaline or saline conditions. One exception is the subspecies uliginosus which can be found in association with halophytic communities occupying saline depressions. Perennial sowthistle is somewhat shade tolerant but is most productive in full sunlight. Perennial sowthistle is associated with disturbance and is abundant in cultivated fields. It may occur on disturbed sites of prairies, pastures, sloughs, woods, lawns, roadsides, ditches, streams, and lake shores.
Perennial sowthistle vegetatively reproduces from root buds on lateral and vertical root stalks, crown buds, and buds on the aerial portion if they become buried by soil. The spreading rootstalk is the primary means of propagation and invasion into new areas. It also helps it to persist under cultivation. Roots are from 0.25 to as much as 1 cm in diameter. Roots will often reach a depth of 2 m. Data indicate that most root segments 5-12 cm below the surface and as short as 1-2.5 cm in length can produce viable plants. Over wintering roots can survive temperatures to -16°C. Minimum root reserves appear to occur when the plants have five to seven leaves and again when the stems elongate and the heads begin to develop. The amount of vegetative reproduction is related to the level of spring and early summer root reserves.
Perennial sowthistle begins to grow in late April or early May when soil temperatures begin to increase. Small leaves first appear 1 week after initial growth begins from buds on shallow spreading rootstalks or from the basal portion of old stems. These new leaves may at first appear reddish or purple in color. Adventitious roots begin to develop after 3 to 4 weeks. Initial root thickening occurs after the plant develops five to seven leaves and has become photosynthetically independent. Roots have reproductive capabilities at this stage. Secondary thickening of the roots begins after 3 months of growth.
Each head only produces 20 to 80 fully developed achenes, but a typical plant can average between 3,000 and 10,000 fertile achenes. Once pollinated, seeds have the capability to germinate in only 5 days, but they reach maximum viability in 7 to 9 days. Wind is the primary dispersal agent, but seeds may become attached to animals and move in vehicles and farm equipment. Initial seed viability may be as high as 70 to 90%, but it rapidly declines. One study showed that germination percentages dropped to 19% after a 60-month period of laboratory storage. Nearly 80% of the seeds in the seed bank germinate in the first year. Optimal germination temperatures are between 25 and 30°C. Perennial sowthistle appears to have a low light requirement for germination, but presence of light will stimulate germination. Seeds require an almost continual water source for germination, although short periods of desiccation may not reduce viability. Burial depth influences seedling emergence, especially if the seeds are buried deeper than 3 cm. Germination is higher under plant cover and litter.
An intermediate number of perennial sowthistle plants occur at Pipestone National Monument (PIPE). They generally grow in patches that cover a total of 5 to 10 hectares. Perennial sowthistle is found in mid-successional sites that have been disturbed in the last 11 to 50 years. They have the long-term potential to invade and modify existing native plant communities. They may modify or retard the successional potential by decreasing plants that are the secondary successional resources. Perennial sowthistle has a minor visual impact on PIPE.
Several control options exist for perennial sowthistle. Biological, chemical, cultural, and mechanical control methods have all been used on this species with varying levels of success. Important considerations include the high vegetative reproductive capability and that the seeds do not remain viable in the seed bank for a long period. Other considerations are that many sources of new propagules surround PIPE and that seeds are widely dispersed.
The method chosen for control may have a detrimental effect on desirable plant species and may be a disturbance that will favor reinvasion by perennial sowthistle or by other exotic species. It is also important to note that many native thistles are present in the area, and proper identification is necessary to restrict control measures to perennial sowthistle.
Frequent mowing for several years may control perennial sowthistle or, at least, will reduce its vigor. Most studies indicate that patches of perennial sowthistle should be mowed at least twice a year to prevent seed dispersal and reduce root reserves. Systematic monthly mowings may be necessary to prevent flower development and to keep root reserves depleted. Cultivation may be used to control this species, however, it may result in increased abundance due to the spreading of rootstalks and the creation of a disturbance. For this technique to be effective, it is necessary to cultivate to a depth of 30 cm when the plants are in the seven- to nine-leaf stage followed by repeated cultivations at 2- to 4-week intervals throughout the summer. Smother crops may provide some measure of control by shading, however, they may also stimulate germination near the edges of the cover. Smother plants that have been used include sweetclover, alfalfa, millet, sorghum, hemp, and small grains. No studies have addressed the influences of prescribed burning on perennial sowthistle. Burning in May or June may reduce perennial sowthistle abundance and vigor on grasslands, but it will probably not be an effective means of control.
Many herbicides are not specific to this species or may not be specifically licensed for this use. It is important to read and follow all label directions. A few herbicides are available for the control of perennial sowthistle. Banvel (dicamba), 2,4-D amine or 2,4-D ester plus acid will suppress perennial sowthistle. A mixture of Banvel and 2,4-D has been affective. Control is generally greater at heavier application rates. Roundup (glyphosate) spot applied during the active growth period will control perennial sowthistle. Stinger (clopyralid) has provided effective control.
Over 50 insect species feed on Sonchus spp. in Eurasia. Three are being investigated as potential biological control agents. Tephritis dilacerata Loew (Diptera:Tephritidae) is being tested in Canada. Larvae of this species infest the heads of perennial sowthistle and form galls preventing seed formation. This fly only infests 3 to 15% of the heads of a plant, but it forces the plants to pool nutrients to the damaged heads subsequently reducing the fertility of the seeds in the other heads. This species of fly has yet to become successfully established. Even if it becomes established, it will probably not be a controlling force. A leaf-gall fly, Cystiphora sonchi (Bremi) (Diptera:Cecidomyiidae), lays its eggs in the stomata on the undersides of leaves has been released and established in Canada. However, no significant reduction of plant populations have been noted in these areas. A leaf mining insect, Liriomyza sonchi Hendel (Diptera), has been released, but results are inconclusive.
The nematodes species Meloidogyne incognita (Kofoid and White) Chitwood, Heterodera sonchophila Estonia, and Pratylenchus penetrans (Cobb) Filip & Stek. have been listed as potential controlling agents for perennial sowthistle. Thirteen species of fungus (three specific to Sonchus arvensis), aster yellow virus (Chlorogenus callistephi Holms), and a bacteria (Pseudomonas solanacearum E.F.S.) have all been documented as infecting perennial sowthistle. However, no information on levels of control provided by these species has been documented.
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