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| "Carduus pycnocephalus, a vigorous annual thistle, arrived in California during the 1930s and has since become a serious weed problem. It occurs in a variety of disturbed habitats and germinates rapidly and in large numbers. Hand pulling, cultivation, and grazing are all effective control measures. Biological control agents, particularly the weevil Rhinocyllus conicus and the rust Puccinia Carduii-pycnocephali, show considerable promise in controlling C. pycnocephalus. Chemicals such as Picloram and 2,4-D may be of some use in controlling the weed, but an integrated management program involving a combination of techniques will prove most effective." - from Carduus pycnocephalus, prepared by Don Pitcher April 12, 1986 (revised by Mary J. Russo) for The Nature Conservancy. | |
Italian thistle is a winter annual broadleaf weed that is similar in appearance to slenderflower thistle, Carduus tenuiflorus. The plant grows up to six feet tall with spiny-winged stems. The leaves are oblanceolate to lanceolate to 15 cm long and 8 cm wide. The leaves are pinnately divided into spiny-toothed or lobed segments with the terminal spine of the segments and lobes the most prominent and rigid. The underside of leaves and stems are covered with cobwebby hairs. About one to five pinkish, cylindrical flower heads are borne in terminal clusters. |
Thistles must be pulled in the early spring before their seeds are viable. The root must be severed at least four inches below the ground to prevent the plant from regenerating. Mowing and slashing are not reliable as the plant is able to regrow and produce seed even at a height of 3 inches. Biological control agents have limited success with Italian thistle. Insects that tested host-specific by the California Dept. of Agriculture and caused significant damage to the reproductive structures of the plant have not been utilized due to concerns about possible predation of native thistles in California. Puccinia cardui-pycnocephali is a rust apparently exclusive to Italian thistle; other rust species have been found on the plant as well. Grazing by sheep or goats (not cattle) in Australia has showed promise as well. |
HABITAT However, this weed does displace more desirable forage or cover plants. The blanketing effect of overwintering rosettes can severely reduce the establishment of other plants, as the leaves of the rosette can become erect in dense stands (Parsons 1973). If there is reasonable ground cover during the late summer and autumn the thistle will not invade a site, but it will come in following overgrazing or creation of fire breaks (Parsons 1973). Drought favors a rapid increase in the thistle population. On soils of naturally high fertility, thistle invasion can be expected at an earlier stage than on poorer soils. Thistles will invade basalt soils earlier than granite soils, and granite soils before sedimentary soils (Wheatley and Collett 1981).(Goeden and Ricker 1978). C. pycnocephalus has been rapidly spreading on rangelands previously dominated by alien annual grasses (Evans et al. 1979). This is partly due to its germination requirements and timing. C. pycnocephalus germinates at temperature and moisture regimes and in seedbed environments which would inhibit the germination of the alien annual grass species that presently dominate California grasslands. The seeds start to germinate in the fall with the first effective rain. Seedlings grow through the winter as rosettes and produce flowering stalks in the late spring before the summer drought. REPRODUCTIONCarduus pycnocephalus reproduces only by seed. It prefers soils of high fertility, and its seedlings establish best on bare or disturbed sites (Wheatley 1971, Parsons 1973). C. pycnocephalus seeds are mucilaginous, unlike most other thistles. The mucilage is abundant and adhesive enough to aid in seed dispersal (Evans et al. 1979). C. pycnocephalus seeds exhibit polymorphism, with brown seeds that have less mucilage and germinate at lower temperatures than silver seeds. The brown seeds do not usually dehisce from the seedheads, and this may be important in the establishment of these seeds in the seedbed litter (Evans et al. 1979). GERMINATION AND GROWTHSeed germination rate in Carduus pycnocephalus is very high, ranging between 83-96%. The seeds germinate at a wide variety of constant and alternating temperatures. The greatest diurnal fluctuation that supported optimum germination was 10° C for 16 hours and 35° C for 8 hours in each 24-hour period. Even with freezing temperatures during the daily cold period, germination was optimum if warm-period temperatures were from 5 to 20° C (Evans et al. 1979). No after-ripening is required, and seeds can germinate either rapidly or after a long dormancy period. Seeds of C. pycnocephalus exhibit rapid germination (within 2 weeks) at optimum temperatures (Evans et al. 1979). Bendall (1974) found that 85% of Carduus pycnocephalus seeds produce germination inhibitors, but they are readily leached. The length of time the seeds can survive in the soil is not known but appears to be at least 8 years (Parsons 1973). C. pycnocephalus can germinate at a variety of soil depths. Generally it does poorly on the surface of a bare seedbed, but on the surface of clay soils it shows 70% germination. At a depth of 0.5-2.0 cm germination is highest, but some seeds germinate to a depth of 8 cm (Evans et al. 1979). Seeds buried 1.3 cm deep show the highest percentage emergence, whereas 20 to 25% of seeds buried 5 to 10 cm deep remain dormant. The growth of C. pycnocephalus is favored more by the addition of nitrogen than by phosphorus or potassium. High pH (6.5) also favors growth (Bendall 1975). SEED DISPERSAL Information source - The Nature Conservancy Element Stewardship Abstract for Carduus pycnocephalus |
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