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.

 

Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Client's Request: The client has bought a house with an extensive garden and is inventorying his garden. He had previously visited the UC MGCC office to get assistance in identifying an unknown plant growing in his landscape. It was identified as Cotoneaster lacteus. Several days later he sent an email to the Help Desk asking for some advice on the . Several days later he sent an email to the Help Desk asking for some advice on the now identified plant. From his email… “It looks like the species that you identified for me, Cotoneaster lacteus, is one of those plants listed as an invasive pest (plant) and that it resprouts aggressively from roots. The UC Davis guifdance on this Cotoneaster plant, says control requires (1) complete root removal or (2) application of triclopyr. Any idea of how extensive roots are likely to be, or how to remove without undue disruption of nearby manzanita and other plants? Any idea of how to get small quantities of triclopyr or when/how to remove roots after poisoning them, so that I can replant? Finally, could this be related to die-off of Manzanita ‘Emerald Carpet'? Nearby lemon and Ceanothus thyrsyflora are doing fine.  ...Thanks for any insights.

Response from the UC MGCC Help Desk:  Yes, the Cotoneaster lacteus is invasive. For root sprouting species, it is very difficult to find and mechanically remove all roots. Root systems generally spread beyond the width of the leaf area by 3 - 6 inches or more. So if your desirable plants are within 1 foot of the Cotoneaster, their root systems will likely overlap, making complete removal of roots difficult.

The UC Davis guidance mentions 2 herbicides that are effective for chemical control of Cotoneaster and available in California.

Glyphosate concentrate (e.g., Roundup™ for stumps) used as a cut stump treatment is a good option for management of woody weedy invaders. To use, you would leave some of the stems protruding above ground and carefully apply the product to the cut surfaces immediately after making the cuts. A small paint brush is often used for this typr of treatment. Fall is the best time for this type of treatment since the product will be carried down into the roots. If it is not 100% effective the first time, you can re-treat the few sprouts that might come up. Glyphosate degrades quickly, does not spread to other plants in the soil, and has low toxicity to humans and other animals. Glyphosate is usually available in smaller sized containers (eg. 8 ounces). There would not be concerns about replanting if you use this method.

The triclopyr herbicide, sold as Garlon 4 ultra™, has some serious downsides, but is another option. I found a 1 quart size through a Google search of "Garlon 4 ultra" under the "shopping" tab. This herbicide is also best applied as a cut stump treatment and fall is the best time of year to do this. The serious downsides to this chemical are that it can easily spread into water sources via runoff, has a 30 day half-life (i.e. is persistent), and is very toxic to fish and other aquatic species. This product is also volatile and should only be used during cool weather when there is no wind.

If you choose to use chemical control, we recommend that you try the glyphosate concentrate first, and only resort to triclopyr if the glyphosate is not effective after 2 or more applications.

Precautions should always be taken when using pesticides, as detailed here: http://ipm.ucanr.edu/PMG/PESTNOTES/warning.html.

It is not likely that your Emerald Carpet is being impacted by the Cotoneaster. However, Manzanita is susceptible to several different root and crown rot diseases that cause partial die back of the above ground parts, and which may be worsened in damp conditions. You may want to check and make sure that the soil is not too wet, and that mulch or soil are not mounded up near the stem, which should be kept dry (no puddling… drains well). If the soil is not well drained, this will also predispose the plant to root rot and shoot die back. With proper water management, this condition should be manageable. Additional information on this condition can be found here http://ipm.ucanr.edu/PMG/PESTNOTES/pn74133.html.

I hope that this information is helpful. Please do not hesitate to contact us again if you need further assistance.

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Note:  The Help Desk of the UC Master Gardener Program of Contra Costa County (JL)Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (http://ucanr.edu/blogs/CCMGBlog/).