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 Penny Pawl, U. C. Master Gardener of Napa County
I can feel spring in the air, so it is time to think about which unique, beautiful plants I can add to my garden. As I peruse all the new garden and seed catalogs, I need to remember not to choose any invasive plants.
Many plants in American gardens and natural landscapes have come from another country. Over time, they have really made themselves at home. If you plant them, some of these imports will take over and crowd out native plants.
In Napa Valley, volunteers have had to remove Spanish broom from a local park. Spanish broom has bright yellow flowers and can take over a landscape in a few years.
Many plants become invasive because nothing keeps them in check. The wild mustard in Napa Valley is a good example. It is beautiful but an opportunist, and here it has found perfect growing conditions.
My neighbor planted a beautiful grass with small seedpods. In a short time, the grass was coming up all over their yard. Then it moved to mine. It was easy to pull, but when a seed went up their dog's nose, they pulled the grass out.
A fellow Napa County Master Gardener had a beautiful wisteria. I love this plant and admired hers which was growing on both sides of her 100-year-old house. The vine had grown under one side of the house and come up on the other. No wonder it is on the invasive-plant list.
When my husband and I were new home owners, he planted a weeping willow. He sited it many feet from our well and our home, but its roots advanced quickly toward us. It had to go.
Another neighbor had a beautiful stand of giant bamboo in front of the home. It even bloomed one year and looked wonderful. But then it spread under the house's foundation. Bamboo has a life of it' own and is extremely invasive. In Hawaii, it is everywhere but it is not a native.
One of the most invasive plants is the wild oat (Avena fatua L.). I hand weeded an area of my yard overrun by this plant. It took time but I vanquished it. The following year it did not return, but in two years, there it was again.
I also fell in love with Santa Barbara daisy (Erigeron karvinskianus), also known as fleabane. It comes from Mexico and down the coast of South America. I planted a few small plants that very quickly took over the beds. It had to go. There are natives in the same family that are not such thugs.
The California Invasive Plant Council maintains a list of the most invasive plants in California (http://www.cal-ipc.org/ ). Although nurseries still sell them, these plants threaten natives by competing for water and nutrients. These plants include big periwinkle, English ivy, giant reed, iceplant, onion grass, pampas grass, red sesbania, Russian olive and tree of heaven. Scotch broom and French broom have pretty flowers but they cause changes in the soil and shade out natives. And they produce many seeds that birds move around.
Most of these plants were imported in the 1800s for landscape gardens. The plants decided they liked it here and have moved to many areas where they are not wanted.
If you don't know what to plant, pick a California native. Natives have evolved to thrive in our soil and climate without producing rampant growth. Because they are adapted to California, most do not need much water to survive.