September 11, 2010
Weeds, weeds, weeds. I'm so sick of weeds - to paraphrase Eliza Doolittle's lament against words and the pressure on her to learn proper English in the musical "My Fair Lady." With the additional rain this year, weeds have flourished all over Marin County.
Our diverse flora allows gardeners to showcase a wide variety of ornamentals from around the world, native and nonnative plants. Nonnative plants from Mediterranean climates thrive in our comparable climate. Often these exotic plants "jump the garden fence" when seed, root or stem fragments spread beyond the garden into natural areas. Of concern are weeds from South Africa, especially erharta (Erharta erecta), yellow oxalis (Oxalis pes-caprae), capeweed (Arctotheca calendula), iceplant (Carpobrotus edulis) and Cape ivy (Delairea odorata), which are rapidly expanding their range in Marin.
Many nonnative plant species fulfill horticultural purposes, but some nonnative plants have harmful impacts. These include altering ecosystem functions (such as nutrient cycles, the availability of water), out-competing and excluding native plants and animals, and adding to maintenance costs of roads, parks and waterways. Noxious and invasive weeds infest more than 20 million acres in California and result in hundreds of millions of dollars in control costs and lost productivity.
- Panic or erect veldt grass (Erharta erecta): This nonnative and extremely invasive grass (family Poaceae) is increasing exponentially in population growth and range. With a predilection for fog and sandy soil, it is severely threatening the native plants and the natural biodiversity occurring along California's coastline. Erharta was cultivated in Berkeley and Davis as an experimental grass in the 1930s. Populations have been reported in a wide variety of habitats in both exposed and shady areas in the greater Bay Area. It grows best in sand but thrives equally well in heavy, boggy, and thin, rocky soils. It grows in cracks in sidewalks or on a rock face and from there moves into adjacent home gardens.
The grass is composed of a long 7- to 10-inch shaft bent over from the weight of the seeds, which are distributed evenly up the weeds stem. Erharta can create a continuous turf in moist areas, with plants spreading via an aggressive root system and by its copious seed which are spread by the wind, birds, bicycles, hiking boots, and horses' hoofs. The weed chokes out other plants, such as delicate native grasses and mosses. Unfortunately, it is very difficult to control and eradicate.
- Buttercup oxalis, yellow oxalis (Oxalis pes-caprae): This low-growing perennial (family Oxalidaceae) is spreading explosively along California's coastal dunes. It also spreads in scrub, oak woodlands, gardens, turf, urban areas, orchards and agricultural fields, easily out-competing much larger native plants.
Buttercup oxalis was introduced here as a South African ornamental landscape plant. Although buttercup oxalis does not produce seeds, it is difficult to control because of its ability to form many persistent bulbs and lateral runners.
- Capeweed (Arctotheca calendula): This rosette-forming perennial (family Asteraceae) can be found in coastal prairies of the Bay Area and California's north coast. The infertile type of capeweed is very competitive and can escape home gardens via creeping rooting stems. The fertile capeweed spreads even faster, typically colonizing open or disturbed sites with exposed soil. Capeweed was introduced to the United States in 1963, propagated by the Los Angeles Arboretum and made available to the nursery trade in 1965. Unfortunately, it is still available in nurseries and has been widely used in landscaping.
- Iceplant (Carpobrotus edulis). This vigorous ground-hugging succulent forms impenetrable mats that compete directly with native vegetation, including rare and threatened plants. It was introduced to California in the early 1900s to stabilize soil along railroad tracks and until the 1970s was planted widely by Caltrans for this purpose.
For several decades it was widely promoted as an ornamental plant for home gardens and is still available at some nurseries. Iceplant spreads both by rooting stems and seed. Even small stem fragments can regenerate into a new plant, making control difficult. A noninvasive iceplant with smaller, succulent leaves and brilliant violet-pink flowers, hardy iceplant (Delosperma cooperi) is a recommended alternative.
- Cape ivy (Delairea odorata). Previously called German ivy, this South African species invades coastal and streamside plant communities. It is a fast growing vine that can resprout from any portion of the plant. It was introduced to California in the 1950s and by the 1960s had become naturalized in Golden Gate Park, San Francisco and Marin. In 2000 it occupied more than 500,000 acres in California; by 2010, that number has probably doubled.
Invasive ivies smother understory vegetation and trees by forming dense blankets and harbor nonnative rats and snails. The loss of native vegetation along invaded rivers and steams degrades California's most sensitive songbird and salmon habitats. Don't plant ivy near natural areas, never dispose of ivy cuttings in natural areas, and maintain ivy so it never goes to fruit.
To avoid purchasing or planting an aggressively invasive, nonnative in your garden, check out the brochures "Don't Plant a Pest" and "Invasive Weeds of Marin and Sonoma Counties." To view photos and obtain information on escaped nonnative plants, these websites are very helpful:www.cal-ipc.org and www.calflora.org.
By choosing a suitable replacement for the few problem plants, we can save ourselves and our neighbors trouble and expense and help to protect California's natural landscape.
The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.