Pat Hitchcock, U.C. Master Gardener of Napa County
Weeds aren't the only troublesome invaders in many Napa Valley gardens. Several other plants, not usually thought of as weeds, can be equally problematic. These thugs are the larger plants, shrubs and trees that, if left undisturbed, can spread into native habitat and crowd out native species.
Cotoneaster is a shrub in the Rosaceae family that grows enthusiastically in my neighborhood. The New Sunset Western Garden Book describes its growth habit as fountain-like with “graceful, arching branches.” Its summer sprays of small white flowers become red-orange berries in fall. Robins, cedar waxwings and other birds love them.
While cotoneaster is native to Eurasia, most of the cultivated varieties originated in China. They were introduced in California before 1900 but have only been reported as invasive in the last 20 years or so. With help from birds, they self-seed abundantly and can supplant native toyon (Heteromeles arbutifolia) in natural areas.
Another problem plant is the Himalayan blackberry or Armenian blackberry (Rubus armeniacus). Native to Armenia and Northern Iran, this plant was introduced to North America in 1885 as a fruit crop. It soon escaped into the wild and is considered invasive throughout temperate climates. While the berries are delicious, the plant is thorny and rambunctious. Canes can grow 30 feet long.
The plant propagates not only by seeds but also by “tip rooting” when the tip of the cane touches the ground and forms roots. If you ignore this plant for long, it creates a dense tangle of old brown growth and wicked, thorny green canes. It likes to grow among other shrubby plants, including the climbing roses and butterfly bushes (Buddleja) that screen my front bedroom windows from the street. Removing the blackberry vines that wander into that bed requires protective clothing as well as sharp pruners and a sturdy digging tool for prying out roots.
While the cotoneasters and blackberries probably invaded my property, another troublemaker—privet—was planted intentionally as a hedge. Three species of privet (Ligustrum spp.) are on the
California Invasive Plant Council's watch list, which means they have not yet been rated invasive in the state but are raising concerns. Those three are glossy privet, Japanese privet and California privet. Glossy privet (Ligustrum lucidum) is likely the one growing (and spreading) in my yard.
If kept trimmed, glossy privet makes a dense hedge. However, if left unclipped, it will reach tree height, bloom heavily in spring and produce a large crop of small black berries. The falling fruit can stain pavement and make a mess on parked cars. Birds attracted to the berries contribute to the mess and help spread the seeds, so new plants volunteer everywhere. After the fruit is gone, the fruiting clusters are unattractive. Sunset advises readers to consider the disadvantages of privet before planting it. I wish whoever planted it on my property had done so.
My property also has an abundant supply of ivy (Hedera helix, H.canariensis). Ivies are native to Eurasia and northern Africa but are now widespread. The California Invasive Plant Council lists ivy as being high in impact, invasiveness and distribution. I try not to let it climb into trees, and I pull it off the north side of the house, where it seems to like the micro climate. It infiltrated the overgrown hedges along the street and provided some screening where it grew densely on an old wire fence. I didn't think anything could kill ivy, although, after five years of drought, a small section in front of my house spontaneously died.
Once I was answering calls at the Master Gardener Help Desk when a client called to ask if we could help him figure out why his ivy had died. After doing some research, I was still stumped; this is a plant that does not succumb to pests or disease. I consulted a U.C. farm advisor specializing in weed science, and now we would both like to know why that ivy died. Finding an efficient way to control ivy in our forests and riparian areas would be a wonderful discovery. Meanwhile, in my yard, I whack it back from time to time in an effort to keep it from taking over.
All of these plants were brought to California intentionally because they had characteristics that seemed useful or attractive. Some of them are still sold in nurseries. But in our state's hospitable climate, the plants have escaped cultivation to become nuisances in wild areas as well as in some gardens.
For more information about invasive plants, check out the California Invasive Plant Council's website, CAL-IPC. The lesson here is to consider the unintended consequences of the things we do. After researching weeds for the past few weeks, I am vowing to be careful about the new plants I introduce into my garden.
Workshop: U. C. Master Gardeners of Napa County will conduct a workshop on “Rose Pruning and More” on Sunday, January 22, from 1 p.m. to 3 p.m., at the Yountville Community Center, 6516 Washington Street, Yountville. Learn 10 important facts about roses, rose pruning and rose care and maintenance. Registration is $12 per person, free to Yountville residents. Register with Yountville Parks & Recreation or call 707-944-8712.
Workshop: The Integrated Grape Team from the U. C. Master Gardeners of Napa County will host a workshop on “Home Vineyard 1” on Sunday, February 5, from 12:30 p.m. to 4:30 p.m., at White Rock Vineyards, 1115 Loma Vista Drive, Napa. Learn what to do, what to look for and what to plan for in the vineyard between February and August. This is an outdoor lecture so dress accordingly. Registration is $5 per person.Online registration (credit card only) Mail-in registration (check only or drop off cash payment)
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.edu/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.