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California Invasive Weeds Awareness Week: July 18-24

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By Rebecca Miller Cripps- Master Gardener: Assembly Concurrent Resolution (ACR) No. 114 “Invasive Weeds Awareness Week” declared in 2003 “the week beginning with the 3rd Monday in July as the annual Invasive Weeds Awareness Week in California and urges all Californians, during that week, to participate in activities that raise awareness of both the scourge of harmful nonnative weeds and methods to prevent their pernicious spread.”

Tuolumne County Master Gardeners invite you to look around your lawn, garden, or property and take action against any of the following weeds.

The Central Sierra Partnership against Weeds (CSPAW) is concentrating on Skeletonweed, Spotted Knapweed, Perennial Pepperweed, Yellow Starthistle (YST), Canada Thistle, Oblong Spurge and Smooth Distaff Thistle. To look at identification pictures, go to Noxious Weed Photographic Gallery . Weeds are listed alphabetically; for example, “smooth distaff thistle” can be found by clicking “S.”

Three pervasive weeds in Tuolumne County have outstripped state, county and city eradication efforts. These weeds require individual initiative and group action. They are: Tree of Heaven (Ailanthus altissima), Broom (Cytisus and Genista spp.), and Yellow Starthistle (Centaurea solstitialis). All three weeds share common traits: extremely prolific; form impenetrable, flammable thickets; and, once established, require enormous efforts to eradicate.

Tree of Heaven was first imported from its native China in 1784 as a host for silkworms. It became readily available as a landscape tree—fast growing, tolerant of poor soil, and resistant to atmospheric pollution. During the California gold rush, Chinese immigrants brought it to the Mother Lode. Tree of Heaven quickly overtakes abandoned mine sites, roadsides and plowed fields. It´s threatening to create mono-culture stands along South Washington Street between the bypass and main Post Office, along Tuolumne and Stockton Roads, and along Highway 49.

Ailanthus is difficult to destroy—reproducing both by seed and by shoots. Studies have reported that a single “mother tree” can produce 325,000 wind-dispersed seeds in one season. You´ve probably noticed the dry, brown seed pods hanging on bare trees throughout the winter. To see identifying pictures, go to

Ailanthus out-competes native vegetation. A 1959 study found that Tree of Haven produces substances toxic to other plants, thus preventing competition from native species. To destroy it, young seedlings can be pulled up by the roots when soil is moist, taking care to remove the entire root. Older trees can be foliar sprayed or cut. If cutting a Tree of Heaven, the cut stump must be painted within 5-15 minutes with an herbicide like glyphosate (Roundup, Rodeo, Accord) or triclopyr (Garlon) to prevent new shoots emerging from the roots.

Broom, with its yellow, attractive flowers, blooms profusely along Stockton Road, Phoenix Lake Road and into Twain Harte. It´s an escaped ornamental, popular in gardens and used along roadsides to stabilize banks. It reproduces by “flinging” seed from a twisted seed pod that snaps open when ripe. Seeds have a hard coating allowing them to survive up to 80 years. Carried by automobile tires, animals, and rainwater, broom seeds colonize roadsides, banks, and drainages. It out-competes local flora and forms dense, flammable stands to six feet tall. The U.S. Forest Service organizes weed abatement volunteer days to prevent establishment on the Stanislaus Forest.

According to the National Park Service website, “remove broom by pulling out the entire plant, including roots. When the soil is moist, small plants can be pulled easily by hand. Winter and spring are good seasons to do this in California. Larger plants must be removed using tools. Be sure to remove the entire plant. Broken stems re-sprout and are much harder to remove.”

Yellow starthistle: According to the California Department of Food and Agriculture YST Mapping Project, “Yellow starthistle (Centaurea solstitialis) is estimated…to cover over 12 million acres in California. It is toxic to horses and is avoided by most grazers once it has bolted and become spiny. It increases fire danger and poses a major threat to biodiversity in native ecosystems.

Yellow starthistle (YST) is completely beyond total statewide eradication. Such a project would cost billions of dollars and engage tens of thousands of people for many years. Currently, the major activity devoted towards YST is focused on reducing infestation levels in areas where YST is very abundant. While its method of spread has not been documented conclusively, it is widely believed that YST moves down transportation corridors or is spread in jumps by movement of contaminated feed, equipment, fill, etc. In areas like the mid-elevation western Sierra Nevada slope, control efforts should focus on prevention of further spread and on local

The University of California Statewide Integrated Pest Management Program states that mowing can be used to control yellow starthistle. However, mowing must be done in the early flowering stage before the plant has set seed. Mowing too early will cause the plant to regrow in a shorter form, flowering too low to be mowed effectively. Starthistle can also be hand pulled. Goats, cattle, and sheep will graze starthistle—goats will even eat the spiny stage.

For more information, call Master Gardeners at 533-5696, the Agriculture Commissioner´s Office at 533-5691, or log on to Statewide Integrated Pest Management Program

See you in the garden.

Rebecca Miller-Cripps is the Master Gardener Coordinator for Tuolumne County.