With the arrival of Europeans, California’s grasslands changed dramatically. Annual grasses and forbs from the Mediterranean area were introduced both accidentally and intentionally. These species were shorter-lived and shallower-rooted than the perennial grass that they replaced. Growing numbers of domestic livestock greatly increased the grazing pressure on the range, resulting in less soil moisture use by plants. Also, the summer fires that had swept through the perennial grasslands were controlled. These changes undoubtedly favored the spread of tarweed.
By the end of winter, the tarweed plant has developed a deep taproot and about a dozen broad leaves in a rosette. Roots of tarweed go deeper than most of the winter annual grasses, reducing competition with them for soil nutrients and moisture. Penetration rates in sand of over 1.5 inches per day have been observed. From late spring until early summer the shoots elongate and branch out with bract-like leaves on woody stems that stand 1 to 2 feet tall.
By October, the plant disperses mature seeds and dies. The woody stems and thicker branches remain standing for at least one additional season.
Livestock use tarweed in winter and early spring while it is young and succulent. Use decreases rapidly as it increases in height and resin covering. It is hardly grazed at all at maturity when covered with resinous exudate, although it is still an important source of protein and moisture for ground squirrels. Summer annuals are often the only actively growing green plants, relatively high in protein, available in the summer on annual range. To discourage herbivory, summer annuals have apparently evolved mechanisms such as spines, aromatic compounds in vinegar weed, and aromatic resins as in tarweed. Few animals are able to feed on these plant in the summer.
CONTROL
Mechanical: Mowing to 4 inches in May reduced tarweed by 20%, mowing in July reduced tarweed by 90%, whereas mowing in late August eliminated all but a few prostrate plants. Density in the year following late summer mowing was reduced by 90%.
Chemical: University of California researchers, using 1.5 lb./acre of a low volatile ester of 2,4-D, found that tarweed was affected much more by the herbicide treatment before elongation (April 21) than after elongation (July 14). Because legal restrictions on herbicides are constantly changing, you should contact your Agricultural Commissioner before using any chemical control method.
Seedbank: One of the major obstacles to mechanical or chemical removal of tarweed is the seedbank of hard ray achenes that exists on sites. After five years of summer mowing, tarweed densities were about 10% of those in unmowed plots. To be successful, the use of these methods must be long-term (over five years) to totally eliminate tarweed, otherwise the pasture will be reinfested once the eradication project ends.
Fertilization: Nitrogen fertilization increase the vigor and productivity of tarweed’s competitors, making them better able to deplete soil moisture that supports tarweed survival and growth in the summer. The fertilizer should be applied in the fall to ensure that winter annuals utilize it efficiently. However, it is doubtful whether the large amounts of nitrogen fertilizer required annually to reduce tarweed density effectively (107/lb./acre) would be economical if applied to rangelands.
Annual legumes: Nitrogen fixation by annual legumes increases forage production and reduces soil moisture available to tarweed. Rose clover fertilized with single superphoshate has been shown to reduce tarweed. Lana vetch, subterranean clover, and the annual medics should have the same effect. Perennial grass: Although no studies have demonstrated a reduction in tarweed, established grass seedings should deplete soil moisture, making it unavailable to tarweed.