Alien Invasion, Part 5:

Italian Thistle

Italian thistle (Carduus pycnocephalus) is a member of the thistle tribe of the composite family Asteraceae (sunflowers). The weed is an annual thistle that sprouts and flowers in the spring and dries out in the summer. The remains of the plants can be seen in the fall and winter as colonies of erect, brown stalks and dried flowers.

Figure 4: Italian thistle (Carduus pycnocephalus)

Photo courtesy of John M. Randall/ The Nature Conservancy

This thistle starts from a ground-hugging, basal floret of deeply lobed, pointed, spiny leaves. The undersides of the leaves are thinly wooly. The emergent stalks are somewhat wooly and lined with prominent, broad spines. The stalks and branches eventually end in a single, cylindrical flower head or clusters of two to five flower heads. Its fairly small flowers and low numbers of pink-purple terminal flower heads can distinguish Italian thistle from other thistles. The narrow phyllaries at the base of each flower head have numerous, forward-pointing hairs. The plants reach a height of about one to three feet in the Elfin Forest. Elsewhere, in a more fertile habitat, the weeds can be as tall as six feet.

Italian thistle is native to the Mediterranean region of southern Europe. It is now widespread in temperate parts of the world. There are serious infestations in Australia, New Zealand, South Africa, Pakistan, Iran and non-Mediterranean parts of Europe. This pest plant is thought to have entered the U.S. in the 1930s. In the U.S. it is found only in a few places in Texas and Arkansas, but it has been spreading out of control in California. Here, it is most abundant in coastal areas.

Colonies of Italian thistle have established themselves in the Elfin Forest and large areas of the Los Osos Oaks Preserve, under the oak canopy. The weed also can be found out in the open in prairie and rocky soils of Morro Bay and Montana de Oro State Parks. The weed displaces native ground cover plants and establishes areas of only its own species. There currently are colonies of this pest in the Elfin Forest's southwest and southeast corners and several hundred yards west of the northeast corner.

This thistle reproduces by seed. It spreads preferentially to bare or disturbed soils. The wind can spread the seeds, or the seeds can stick to surfaces and spread by contact. Seeds can remain viable for up to eight years in the soil. Thus it takes at least eight years to completely eradicate the thistle from a single site. Monitoring each year for eight years is essential.

Italian thistle may be removed by pulling or digging it out of the soil. At least four to five inches of root should be removed. Otherwise, the weed can quickly re-sprout, flower and produce seeds. Thus mowing or slashing are not always effective. Plants that are cut close to flowering time can flower and seed on the cut portion. If the flower heads are destroyed, slashing is more effective than mowing. In large areas, cultivation and reseeding with perennial grasses or grazing by sheep, goats or horses can be effective.

Biological control shows promise for use in California on this pest plant. Three insects have been found to feed on Italian thistle in other countries. One has been imported for use on both milk thistle and Italian thistle. Additionally, some fungal rusts have been found to attack Italian thistle.

Herbicides used on Italian thistle have been found to provide only temporary control. Thus they are only effective when used in conjunction with other methods. Picloram (Tordon) has been used, but has a long persistence in soil, up to 18 months. It is nonselective and will kill other species. 2,4-D has been used also. However it lasts for one to four weeks, and although safe for use around grasses, it will kill other broadleaf plants.

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