By Susanne von Rosenberg, UC Master Gardener of Napa County
One thing that Master Gardeners talk about a lot is putting the right plant in the right place. That means choosing plants adapted to the climate, to the soil in your garden and to the amount of sunlight and water they will receive.
One other consideration is whether the plants you're considering are invasive. Invasive plants threaten our wildlands, outcompete native plants and change the habitat so it is no longer suitable for some native species.
As a result, invasive plants pose a substantial threat to endangered species. Invasive plants can also threaten agricultural lands by crowding out crops and rangeland forage. Some are toxic to livestock or wildlife.
According to the California Invasive Plant Council, invasive weeds in pastures and farmland cost an estimated $33 billion per year nationwide. California alone spends an average of $82 million per year to deal with invasive plants.
Even in the suburbs, invasive plants can cause problems. Pampas grass and Scotch broom increase fire fuel loads; other invasive plants can clog creeks and increase the risk of floods. Still others consume large amounts of water that would otherwise have gone to other plants, waterways or drinking water.
Until I became a Master Gardener, it never occurred to me that garden centers and nurseries might sell problematic plants. Unfortunately, many of the plants causing problems in California were introduced through the horticultural trade. We need to educate ourselves to make sure that we don't plant a future problem.
I've been surprised how often a plant that seems otherwise ideal for my garden turns out to be potentially invasive. Gardening catalogs are another source of invasive plants. Some of these plants may be perfectly harmless in other parts of the country but invasive here.
How can you know if a plant you're considering might be invasive? First, you can check the Calflora website, which provides information on a large number of plants grown in California, including whether a given plant is considered invasive or has invasive potential.
You can also consult the California Invasive Plant Council website, which has an alphabetic list of problematic plants. That list includes plants commonly sold through the horticultural trade and assigns a rating of high, medium or low risk of invasiveness. In addition, IPC maintains a watch list of plants that aren't currently invasive in California but risk becoming so.
As you evaluate a plant for invasive potential, consider these questions:
Is the plant known to be invasive in other locations with a Mediterranean climate?
How readily does the plant reproduce? On average, how many seeds does it produce each year, and how easily does it germinate from seed? How long will it take until the plant is mature enough to set seed?
Does the plant spread vegetatively, such as via underground running stems (like running bamboo)? Can a new plant sprout from small pieces of root, like bindweed does?
How tough is the plant? Does it need regular water or special growing conditions to germinate?
Could the plant hybridize with existing plants?
Are other species of the same genus, or a related genus, invasive in a similar climate?
Would the plant increase the risk of fire? Is it highly flammable or does it soak up a lot of water and leave surrounding vegetation dry?
How easily can animals, wind or water spread the plant's seeds or vegetative matter?
How hard is the plant to remove or control?
Unfortunately, many of the qualities that make a plant desirable may also predispose it to being invasive. If it is adapted to our climate, easy to grow and requires little water, it meets some of the criteria of a potentially invasive plant.
Compared to well-behaved plants, invasives reproduce more readily, crowd out desirable plants and may be difficult to remove. So if a plant's description says that it “reseeds readily” or “spreads easily,” think twice. If it “grows rapidly” or is “adapted to adverse growing conditions,” do some follow-up research.
Unless you know the plant, it it's a good idea to spend a few minutes online to reassure yourself that the plant is well mannered. Don't rely on old information. Some plants have been designated invasive only recently, and new plants are added to the list regularly.
Food Growing Forum: Napa CountyMaster Gardeners will present a discussion of “Cane Berries” on Sunday, November 14, from 3 p.m. to 4 p.m., via Zoom. Register here to receive the Zoom link.
Got Garden Questions? Contact our Help Desk. The team is working remotely so please submit your questions through our diagnosis form, sending any photos to email@example.com or leave a detailed message at 707- 253-4143. A Master Gardener will get back to you by phone or email. For more information visit http://napamg.ucanr.edu or find us on Facebook or Instagram, UC Master Gardeners of Napa County.
by Penny Pawl, U.C. Master Gardener of Napa County
Oh no, I've done it again. I have fallen in love with a plant and its beautiful flowers. I planted it; it grew well and then I discovered that it is on the invasive plant list. What to do? And why is this plant on the list?
Many of the wonderful plants that I have planted over the years are on this list. How do authorities create this list? Some of those plants include pampas grass (Cortaderia jubata), mullein (Verbascum),butterfly bush (Buddleia davidii), Watsonia and privet (Ligustrum spp).
Most gardeners are aware that bamboo is invasive. Years ago, my husband and I planted bamboo in a wine barrel. Because we knew it had a spreading habit, we set the barrel on cement blocks. A few months later we noticed that a stem had come out the bottom of the barrel and was inching its way across the soil toward our house. Bye bye, bamboo.
Scotch broom (Sarothamnus scoparius) is probably the most notorious problem plant. It has taken over entire areas of our state and moved up the coast into Washington. It is also a problem in many other states. I have read about groups going out to remove this plant from parks and other wild areas.
Scotch broom spreads by the root system. The roots have nitrogen-fixing bacteria so the plant grows all year. It also produces large amounts of seed. If left alone, it will take over and crowd out native plants.
Pride of Madeira (Echium candicans) is native to the island of Madeira. I visited there a few years ago, saw the plant and fell in love with its tall purple spires. I planted one about two years ago. It has grown well and has not needed water or care. It bloomed this year, and the bees and other pollinators were in heaven.
Before I planted more, I decided to check its status. Alas, it is also on the California list of invasive plants. Each one of those little purple flowers produces lots of seeds which take root easily in our soils.
I do not plan to remove it, so I went out one evening as the flowers faded and before the seeds set, and cut it back. I did it at that time of day to avoid the bees and other insects. I will need to repeat this every year to keep pride of Madeira from becoming a pest in my garden and my neighbors' gardens.
A couple of years ago I snipped two pieces of honeysuckle (Lonicera) and rooted them. They are now in barrels growing up a big trellis. When I looked for more information on this plant, which is currently in full bloom, I discovered it, too, was considered invasive. Like pride of Madeira, it produces many seeds and pollinators love it. I have to trim it after flowering to keep the seeds from spreading. It also spreads when long runners touch the ground and take root.
The California Invasive Plant Council (Cal-IPC) list has been around for many years. If you are serious about preventing the spread of invasives, you should check the list before planting. You can find it on the Cal-IPC website along with photos to help with the identification.
Some of the plants in the weed category came in with New World settlers. The weeds were hitchhikers. Others such as Scotch broom were brought in because they grew so well in their native areas.
What do these plants all have in common? They are generally drought tolerant, and they produce massive amounts of seed. Birds love the seeds and spread them far and wide. Some invasive plants, such as wisteria, spread when their branches touch the ground and take root, or their root system moves around seeking water. When I trim the flowers of invasive plants, I don't put them in my compost. I put them in my brown yard-waste bin so they can be hot-composted by the city.
If, like me, you have a beautiful invasive plant growing in your garden, take care to keep it from taking over the whole garden. Otherwise, be safe and replace it with a native plant instead.
The UC Master Gardeners of Napa County are volunteers who provide University of California research-based information on home gardening. To find out more about home gardening or upcoming programs, visit the Master Gardener website (napamg.ucanr.edu). Our office is temporarily closed but we are answering questions remotely and by email. Send your gardening questions to firstname.lastname@example.org or leave a phone message at 707-253-4143 and a Master Gardener will respond shortly./span>
By Susanne von Rosenberg, UC Master Gardener of Napa County
Plants are a terrific way to create a privacy screen for your garden, or to create a shady space in summer. For privacy screens, it's typically best to select plants that are evergreen.
The first consideration, as always when selecting plants, is the planting site. What type of soil do you have, and how much sun does the area get? Next, figure out how high you want your privacy screen to be. Do you just need to create a screen between your neighbor's patio and your own? Or do your neighbors look down into your yard from their second (or third) story?
Also consider, especially for taller privacy screens, where the plants will cast shade. If you need a privacy screen on the south side of your property, it may cast shade for much of the year.
Make sure the plant you're considering is not on an invasive-species or noxious-weed list for your area. The California Invasive Plant Council maintains a list of plants that are considered invasive in the state. In general, if a non-native plant reseeds readily and is drought tolerant, it has a good chance of being invasive here.
How fire resistant is the plant you're considering? Are the roots likely to be invasive? Will they affect your neighbor's property or damage sidewalks?
How much maintenance are you willing to do? While fast-growing plants are appealing when you're eager to create privacy, they will also likely take more maintenance.
If you have animals, you'll want to avoid plants that are toxic to them.
If you choose wisely, your privacy screen can provide other benefits, such as fragrant flowers, edible fruit or wildlife habitat. When evaluating potential candidates, make sure you understand their maximum height and width. I was considering fern pine (Afrocarpus gracilior) for a privacy screen at my property (it grows 2 to3 feet per year) until I learned that it can reach 35 feet in height. I need a privacy screen that is 20 to 25 feet tall and do not want it any taller because it would shade too much of my yard. Fern pine is commonly grown as a tall hedge and maintaining it at 6 to 8 feet is certainly feasible. But pruning it to keep it at 20 or 25 feet is not a task I want to take on.
For a low screen of 6 to 8 feet, you have the most choices. You can plant evergreen vines along a trellis, a variety of shrubs, or small trees that you can prune into a shrub or hedge. Traditional options include boxwood and privet hedges, and camellias for a less formal hedge in shady or partially shady areas. Other options include Meyer lemons (they are relatively frost tolerant and have dense foliage); lilac vine (Hardenbergia violacea), a vine with sprays of small, dark purple flowers); and trumpet vine (Distictis sp., either the red-flowered variety or D. buccinatoria, which is hardier). Or consider a native plant such as California lilac (Ceanothus), which comes in a range of blue colors and heights.
If you are installing a trellis or fence near a property line to support your plantings, make sure you know the local zoning and permitting codes. Typically there are limits on fence height.
Taller privacy screens can be more challenging. Typically you will be considering trees with a dense canopy or growth habit. If you select a tree with a relatively wide canopy or spreading growth habit, you will need fewer of them for your hedge. Good options include cypress, arborvitae, redwoods, evergreen magnolias (not all are evergreen), loquats, full-size (not dwarf or semi-dwarf) citrus trees (but consider frost hardiness), and English laurel (Prunus laurocerasus).
English laurel (also known as cherry laurel) is in the same plant family as peaches and nectarines; it is not a true laurel. It received its common name because its leaves resemble laurel leaves.
Again, make sure that your selection suits the site. Magnolia trees range in height from 10 feet to more than 50 feet at maturity. Many a gardener has planted redwoods only to be overwhelmed by their size 10 or 20 years later.
Most people who are interested in creating a privacy screen have heard that bamboo is fast growing. Bamboo is no longer recommended within 30 feet of structures because it is considered fire prone. Nor should it be planted along roads and driveways because it can present a fire hazard to emergency responders. In addition, bamboo can easily get out of control and it requires a fair amount of water.
Choose wisely, and you will have a privacy screen to enjoy for years.
Workshop: “Sustainable Vegetable Growing” (Four-Part Series) on Sundays February 23, March 1, March 8 and March 15, from 2 p.m. to 4 p.m., at the University of California Cooperative Extension, 1710 Soscol Avenue, Napa. For more details & online registration go to Online registration (credit card only) or call 707-253-4221.
Workshop: “Step-by-Step Garden Design” on Saturday, February 29, from 9:30 a.m. to 11:30 a.m., at the University of California Cooperative Extension, 1710 Soscol Avenue, Napa. For more details & online registration go to On-line registration (credit card only) or
Mail-in/Walk-in registration (cash or check only) or call 707-253-4221.
The UC Master Gardeners of Napa County are volunteers who provide UC research-based information on home gardening and answer your questions. To find out more about upcoming programs or to ask a garden question, visit the Master Gardener website (http://napamg.ucanr.edu) or call (707) 253-4221 between 9 a.m. and noon on Mondays, Wednesdays or Fridays.