Silybum marianum

SILYBUM MARIANUM can be distinguished from other California thistles by its distinctive white-mottled leaves. There is only one species of SILYBUM established in California. "The seed leaves (cotyledons) are large from 1/2 to 3/4 inch wide and 3/4 to 1 inch long, rather thick, succulent and light green. The first leaves are very conspicuously white-netted along the veins and have short yellowish prickles" (A.H. Lange et al., 1983).

SILYBUM MARIANUM is mainly confined to high fertility soils. It frequently establishes on river flats, sheep camps, around stock yards and any other area of higher than normal soil nitrogen levels, especially if the area has been disturbed.

Each terminal head of the plant produces approximately 100 seeds; 10 to 50 heads are produced per plant (Young et al. 1978). Seed weight is approximately 22 mg (Wheatley 1971). "From this we can calculate 10-50 grams of seed produced per plant. With a conservative average density of 2 plants/square meter theoretical seed production reaches 500 kg/ha" (Young et al. 1978).

Seed dispersal is the only means by which the milk thistle spreads. The seeds are equipped with a large pappus which allows effective spread by wind (Parsons 1973). Spread can also be attributed to their presence in grain and fodder (Wheatley 1971). Other means of dispersal include water, mud, agricultural produce, vehicles, machinery and animals (Parsons 1973).

The seeds of SILYBUM MARIANUM germinate in the fall after the first rains. "Plants develop slowly through the seedling stage, becoming flat rosettes by late autumn/early winter. Growth is rapid in late winter and early spring producing large cabbage-like plants up to 3 feet in diameter from which center stems develop in spring. Flowering commences in late spring and continues into early summer" (Parsons 1973).

The seedlings prefer disturbed soils which provide suitable bare areas for litter-free germination. Therefore, sheep camps, rabbit warrens, cultivated fire breaks, roadsides, overgrazed pastures and the like are ideal propagation sites. Seedlings do not establish in perennial pastures if the soil is well covered with vegetation during late summer and autumn. Litter seems to be a highly important inhibitive factor in the germination ecology of milk thistle seed. Because its germination is reduced by accumulations of grass litter, milk thistle is not adapted as a landscape dominant in areas where there is a continuous ground cover provided by existing vegetation. This can also hold true for annual rangelands if they are managed properly (Young et al. 1978). However, if there is an absence of pasture or litter cover in late summer and early fall, infestations of thistle may develop. This occurs especially in periods of drought which reduce the persistence of many pasture species (Michael 1968).

SILYBUM seed has the potential to remain viable in the soil for up to 9 years. The percentage of germination varies from year to year and can be less than 50% (Parsons 1973). In an article on germination requirements of this species, written in 1978 by Young et al., the following results were recorded.

"One month after harvest, milk thistle seeds had afterripening requirements related to germination temperature that limited germination to 10-20 C degrees. The time required to satisfy afterripening requirements was dependent on germination temperature. Generally the higher the incubation temperature during germination, the longer the afterripening requirement (up to a maximum of 5 months). Once afterripening requirements were satisfied, milk thistle seeds germinated over a temperature range of from 0-30 C degrees. Optimum germination occurred with 16 hour cold periods of 2-15 C degrees alternating with 8 hour warm periods of 10-30 C degrees. Emergence of milk thistle seedlings decreased with increased burial depth, but substantial emergence occurred from a depth of 8 cm. Germination on the surface of the soil or litter was greatly reduced compared to that with slight soil or litter coverage."

Once SILYBUM has found a niche it is a competitive thistle and it tends to establish in tall dense patches that eliminate other plant species either by shading or by competition for moisture and nutrients.

In areas of continual disturbance, eradication of SILYBUM is virtually impossible until the factors which cause the disturbance are removed. SILYBUM will stay localized in these areas unless disturbance becomes more widespread. Over-grazing and fire are two factors which encourage the spread of SILYBUM in large areas.

Seed dispersal is the only means by which the milk thistle spreads. The seeds are equipped with a large pappus which allows effective spread by wind (Parsons 1973). Spread can also be attributed to their presence in grain and fodder (Wheatley 1971). Other means of dispersal include water, mud, agricultural produce, vehicles, machinery and animals (Parsons 1973).