Alien Invasion, Part 4:

Malta StarThistle (Tocalote)

This alien invasion article originally was written for yellow star thistle (Centaurea solstitialis) by mistake. Sometime later, two Cal. Poly. professors corrected the identification of the Elfin Forest weed to be Malta starthistle or tocalote (Centaurea melitensis), a close relative. Like the better known yellow starthistle, tocalote also is a member of the plant family Asteraceae (sunflowers). Although a scourge in its own right, Malta starthistle does not infest as much of the U.S. west as its better known cousin.

Tocalote (Centaurea melitensis)

Photo courtesy of John M. Randall/ The Nature Conservancy

Tocalote is an annual weed that reproduces by seed production. It grows one to two feet high and has multiple branches, each ending with a terminal yellow flower. The flowers are at the end of a stiff bract that sports a crown of slender spines that range from 1/8 to 3/8 inches long. The spines radiate out from the bract like in yellow starthistle, but are shorter. The basal leaves can be recognized in growing plants by their lobed structure and bluish gray-green color. Stiff hairs and minute resinous dots also cover the basal leaves. The stems are ridged and have narrow, alternate, non-lobed leaves. The plants send down a prominent taproot.

Malta starthistle is native to southern Europe. It was introduced to the U.S. in the 1700s. Tocalote is common in western California, but its distribution is as far as Washington State, Nevada and Texas. It also can be found in scattered pockets in the Midwest and eastern U.S. It also is prevalent in the southern part of the San Joaquin Valley. It usually is not found in desert areas, however.

Unlike yellow starthistle, tocalote is not toxic to horses. In fact, it is used medicinally in Spain. Seed production is highly variable. There may be 1-60 seeds per seed head and 1-100 heads per plant.

Although undocumented, the means of controlling tocalote probably are the same as for yellow starthistle. This would include the herbicide picloram, dicamba and 2,4-D. Digging or pulling is effective in small infestations. Grazing is effective prior to formation of spiny flower heads. Mowing is effective just before flowers bloom. Burning is effective when plants dry out, but only prior to seed production. A small beetle that was accidentally introduced to the U.S., Lasioderma haemorrhoidale, will feed on the seed heads, but it is not effective in controlling infestations. A heavy, early treatment with the rust fungus, Puccina centaurea, is effective, however.

The tocalote infestation in the Elfin Forest is mainly in the meadow areas. It also can be found to the east side of South Bay Boulevard. SWAP first attacked it in the Elfin Forest by manual pulling during the flowering phase in the summer of 1998. At that time the plants were often 2 feet in height with vibrant yellow flowers. Weeding took more than 12 man-days of labor. Since that time, the plants have been progressively smaller and weaker in appearance. In 2002, the fifth consecutive year of weeding, it took only 6 man-hrs. labor. Most of the plants were well less than 1 ft. high. Obviously, the bank of seeds in the soil is being depleted, and control is being established.

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