Carduus pycnocephalus can be distinguished from other thistle species by its relatively small and few terminal flowerheads and narrow phyllaries with copious tiny, firm, forward-pointing hairs, especially on the midrib (Hitchcock and Cronquist 1973).
Comments: It displaces desirable forage or cover plants, but more commonly colonizes disturbed habitats where interspecific competition is less intense (Goeden 1974). It is most abundant in coastal areas and occurs as a weed of pastures, ranges, roadsides, rural areas, fallow cropland, railroad rights-of-way, field margins, and ditchbanks (Goeden and Ricker 1978).
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 previ- ously 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.
Carduus 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 GROWTH
Seed 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
C. pycnocephalus does not reproduce vegetatively, but its seeds are well equipped for dispersal by wind because of the large pappus and relatively small size. The distance that seeds can be spread by wind is not known, but it is at least several hundred meters. Seeds are also spread when infested pastures are cut and the hay fed to animals on clean areas. Seed dispersal by water and on animals and machinery is less important (Parsons 1973). Ants may also play a role in dispersing the seeds (Uphof 1942).