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Landowners Guide to Native Grass Enhancement and Restoration

    3. Selected Invasive Weeds and Their Management in Native Grasslands

by Mark Stromberg, Ph.D. Hastings Natural History Reserve, UC-Berkeley
and Paul Kephart, Rana Creek Habitat Restoration, Inc

Certainly there are many more non-native and weedy plants that can cause serious problems in native grasslands relicts than we present here. For the latest updates on weeds, consult the California Exotic Pest Plant Council, or the UC-Davis / Nature Conservancy Wildland Invasive Species team. Here are a few of the more troublesome weeds that are often found in or near California grasslands.

Yellow Star Thistle

Yellow Star Thistle (YST), or Centaurea solstitialis, is one of the very few deep-rooted, late-season plants in the California annual grasslands.Because it has a deep tap root, it can access late season soil water. Other native grasslands plants that can do this include Madia (tarweed) and Conyza (horseweed).

   The flower is shown to the left and new leaves to the right (above). YST will often be the only green, tall plant in a field of weeds in late July. YST can invade and thrive in the openings between the native grass bunches. YST plants produce thousands of seeds, and the seeds can continue to germinate for up to 5-10 years. Leaves of YST are highly poisonous to horses. Mature YST plants have long sharp needles that prevent cattle, horses and people from walking through an area. A variety of control methods are in use including fire (Hastings, 1996), grazing, mowing, herbicides and the use of host-spedific insects (Lanini, 1995, DiTomaso 2000, DiTomaso et al., 2000). Clopyralid (“Transline”) is a newly registered growth regulator, a post-emergent herbicide, that shows particular promise for control of YST without harming most other native grasses and wilflowers (DiTomaso, 1999) Yellow star thistle is spread by heavy equipment, trucks or livelstock that pick up seeds in the soil and are moved to new sites. Requring heavy equipment to be steamed cleaned before it arrives on a new site is a good idea. YST almost always starts new populationsalong a road. Any small colonies should be removed as soon as discovered. Once mature seed heads are present, it is best to pull up the plants and remove them for burning or disposal off site. Herbicides are effective before flowering, often best when only basal rosettes are present

.

Yellow starthistle starts as small leaves on the ground, then send up a stem later in the growing season. The stem has distinct flattened ridges running parallel to each stem.

Mature plant- Yellow Starthistle

Italian Thistle

Italian Thistle (Carduus pycnocephalus) is a widespread annual weed in Californialand. It arrived in California in the 1930s and can be a serious pest on soils that are disturbed. In general, many weeds like this can be controlled by reducing the amount of soil in the project area that is subject to constant mixing. This is why the sides of dirt roads are such avenues for weeds. Between the constant driving, and annual grading, abundant bare, newly disturbed soil is available. Italian thistle can get started in such places and spread. Effective controls including cutting down plants with immature flowers (mowing to reduce annual see crop), pulling them by hand, and using timed grazing. Biological control agents, particularly the weevil Rhinocyllus conicus and the rust Puccinia Carduii-pycnocephali, show considerable promise in controlling this thistle. Herbicides (Picloram and 2,4-D) are somewhat effective, but an integrated pest management program is suggested (Pitcher and Russo, 2001) Other annual weed thistles (eg. Bull Thistle or Cirsium vulgare) can be controlled with similar techniques.

Leaf of Italian Thistle (left) Flower of Italian Thistle (right).

Genista, French Broom

   French broom (Genista monspessulana) is an invasive weed in Mediterranean climates. In California, a great deal of information on the management and control is available (IMPPA, 2001). This legume shrub invades from nearby ornamental plantings. In California, it is still sold as an ornamental plant, even as it is listed there as a noxious weed !
    Some combination of treatment of mature plants with herbicide, with follow up removal, and herbicide treatment of re-sprouting or newly germinating plants seems most effective (Bossard et al., 1995). A licensed herbicide applicator can apply 30% triclopyr in 70% penevator oil on each French broom stem of .5 cm or more. When dead, the broom is cut and removed. It can then be piled and burned on site during the winter under a back yard burning permit. Glyphosate (Round Up) should then be applied to seedlings that germinated by late June. This treatment resulted in no surviving Genista and the lowest number of seeds in the soil after 3 years.
   Removal of these seed-producing shrubs is the only long-term method that will reduce the population. These shrubs produce thousands of seeds each year and the seeds can persist in the soil and continue to germinate for up to 20 years. Large shrubs can be removed with a special tool developed for this purpose (Weed Wrench- see Appendix). Grazing with goats during the early spring growth of Genista will suppress the small seedlings, but the goats cannot reduce the large shrubs that can grow to 20’ in height. Goats can remove young Genista plants before they produce seeds, but they will also reduce native flowering plants. Herbicides, such as Glyphosate (Round Up) that are contact poisons which only kill living, green vegetation, can be used on small Genista plants later in the season when they are the only live vegetation. Care should be taken to cover any native perennial grasses as Round Up will kill them as well.

Pampas Grass

Pampasgrass (Cortaderia selloana) is a native in Brazil, Argentina, and Uruguay, where it grows in soils that remain damp nearly year-round.

   Pampas Grass was first introduced to Europe in the early 1800s and from there, in 1848, it was introduced to Santa Barbara, California as an ornamental in the nursery trade. In 1946, the Soil Conservation Service started planting pampas grass across southern California’s coast to provide supplementary dryland forage and prevent erosion. It is now an invasive weed along much of the coast of California (DiTomaso et al, 1999). Pampas grass can be controlled by physical removal or by direct application of a post-emergent herbicide such as glyphosphate (Round Up).