- Author: Gayle Nelson
By Susanne von Rosenberg, UC Master Gardener of Napa County
If you live in an older home, you likely inherited some amount of old landscaping. Many of us have already tackled the grass, but what about the rest of the garden?
Many plants that were once landscape standbys have now proven to be undesirable for a variety of reasons. Unfortunately, many perennials became landscape standbys because they were hardy and vigorous. Some of the more common of these plants include juniper (highly flammable), Algerian and English ivy (invasive; can kill trees and destroy fences; provides habitat for rodents), broom (highly invasive and flammable) and Pampas grass (highly invasive and flammable).
Unfortunately, mulches will not kill these plants. Mulches can effectively control annual plants, but they do not work for perennial plants that can resprout from underground parts.
Three basic types of removal are appropriate for perennial plants: mechanical removal, biological control or chemical control. There are no biological controls for any of these plants because many people still consider them desirable.
Mechanical removal includes hand removal, mowing, burning and selective grazing. Use caution when mowing during the dry season because of fire danger, especially if your property is rural. Mowing later in the season could also spread seeds.
While mechanical removal is more environmentally friendly, using post-emergent herbicides can also be successful. Chemical control is most appropriate for controlling resprouts. Depending on what you intend to plant next, you may have to wait a year or two after using the chemicals. Always follow the label directions and wear proper safety gear.
As for digging up these unwelcome plants, do it now when the soil is moist and the roots are more easily removed. When the soil is dry and hard, digging often just breaks off the stems, leaving most of the plant material to resprout.
Burning is not recommended for resprouting plants as it increase the population of these plants. Never burn poison oak; the smoke is a serious health hazard.
Hand removal can be effective for juniper, Pampas grass and its close relative, jubatagrass. Cut down or pull out Pampas grass or jubatagrasss before it flowers to avoid spreading seeds. Be careful; Pampas grass has sharp edges. After you have removed the plant, dig out the rootball. Watch for reprouting from any root fragments. It may take several years to get rid of the Pampas grass completely.
To remove juniper, cut the bush at soil level. The main trunks are often thick enough that you need a chainsaw to cut them. You can watch to see If any sprouts emerge from the roots left in the soil, or you can remove the main part of the rootball to reduce the likelihood of resprouting.
If you are eager to remove the rootball, cut the main roots a foot or so from the trunk and then pry the stump out of the soil. Even if you remove the main stump, it's still a good idea to periodically check for resprouts. I have cut a juniper hedge back to the ground and had no problems with any resprouts, even though I immediately planted new plants between the stumps.
Of all the plants on this list, ivy will likely take the most effort to eradicate. It can spread by rooting from growing stems, seeds and even from cuttings left on the ground. Digging out the plants can be effective if you remove the roots and stems, including all the runners. Wear gloves when removing ivy. Many people react to dermatitis-causing chemicals in the plant. And be prepared to come back regularly to get rid of resprouts from roots that you missed.
If the unwanted plants aren't near any plants you want to keep, and the area you want to clear is large, goats can be effective. You will still need to look for resprouts in the grazed area.
Mowing the resprouts is not generally recommended as it can encourage regrowth. However, you may succeed if you mow during the dry season and follow up consistently to mow resprouts as well.
Broom is in a category by itself. Tackle it during the dry season. Lopping it back when the soil is moist can result in vigorous resprouting. Broom rapidly develops a large rootstock. Within a couple months of germination, plants typically have rootstocks large enough to recover from a single mowing. You will need to mow multiple times to eliminate broom.
You can also cut mature broom near the base. That will provide some control if you do it when plants are moisture stressed, such as in late summer or after a dry winter. Cutting it down when the soil is moist will cause vigorous resprouting.
Library Talk: Napa County Master Gardeners will present a talk on “Chasing Sun in Shaded Yards” on Thursday, December 2, from 7 p.m. to 8 p.m. via Zoom. Does your yard have a limited supply of sunny locations to grow vegetables? Learn about site selection and cultural requirements for the plants you want to grow. Register here to receive the Zoom link.
Workshop: Napa County Master Gardeners will lead a workshop on “The Art of Raising Succuluents” on Sunday, December 5, from 1 pm to 3 pm, at the Yountville Community Center, 6516 Washington Street, Yountville. Discover how to care for and design with these colorful, unthirsty plants in your garden or on your patio. Attendees will get plant starts to take home. Yountville residents: $16; Non-residents: $18. Free to Golden Ticket members.To register, visit Online registration or telephone the Parks & Recreation Department at 707-944-8712.
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 firstname.lastname@example.org 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 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>