Are You Planting a Pest?
By Penny Pawl, U. C. Master Gardener
Pampas Grass
While most of California's pest plants affect agricultural land and open space, some affect home gardens, spreading by seed or roots. The California Invasive Plant Council has a large list of weeds and grasses not native to California. Several organizations, including the University of California and the California Native Plant Society, publish guidelines on nursery plants to avoid.
Some of the problem plants include pampas grass (Cortaderia selloana), English and Algerian ivy (Hedera helix and H. caneriensis), French and Scotch broom (Cytisus), periwinkle (Vinca major) scarlet wisteria, (Sesbania tripetii), licorice plant (Helichrysum petiolare), and the one that really surprised me: cotoneaster (Cotoneaster lacteus and C. pannosus).
When plants reseed without human help, it is called naturalizing. Birds and the wind help spread the seeds. These pesky plants can produce many seeds that don't need a lot of resources to sprout, and they have few natural predators.
A recent discussion of this subject on the California Gardening forum of GardenWeb (www.gardenweb.com) added many plants to the list of "what I wish I had never planted." Some make trouble because of their invasive roots, others because of their seeds. Many are more aggressive in Southern California because they don't have cold and frost to help keep them in check. Here are a few of the garden plants that have given me headaches:
Mexican primrose (Oenothera) has a very aggressive root system. The flowers are pretty, but I found their little roots and seeds spreading everywhere. It took two years to get this plant under control.
Bermuda buttercup (Oxalis pes-caprae) spreads by seed and roots. It blew in and I thought it was nice until I found it growing everywhere. Other oxalis to avoid are those sold as shamrocks (Oxalis triangularis), both the red and green. They are difficult to contain and will take over an area.
Bamboo (Poa) spread by underground roots. One at my home even left a tub and started across a patio toward water. A neighbor who planted giant bamboo many years ago eventually found it coming up all over his lawn, under his walk and working its way toward his foundation. The battle went on for several years before my neighbor won.I have had a problem that I am still waging with mondo grass (Liriope spicata) that spread by runners. It worked its way under my fish pond and came up on the other side. Korean grass (Zoysia tenuifolia) also spreads by roots.
Asparagus (Liliaceae), that tasty vegetable, can go where it wasn't meant to go. Years ago, my neighbor planted an asparagus bed. After it went to seed, it managed to come up everywhere, including in my yard. I pulled it up for several years. Asparagus fern can also take over if not contained.
Redwood trees (Sequoia sempervirens) are wonderful, but they have a shallow, aggressive root system. I have found roots from these trees in my raised vegetable beds, 30 to 50 feet from the base of the tree. Never plant one close to your home, sidewalk or lawn. The redwood will win.
Coral iris (Iris foetidissima) turned up in my yard, and I thought it was a native, but I have learned it is native to Europe. The seed pods are beautiful, and the birds enjoy spreading them everywhere. After they got a toehold in my yard, it took a lot of work to get rid of them. Now I just cut off the seed pods I find.
Baby tears (Soleirolia) will take over a moist, shady area and grow over, around and up everything.
Sometimes, just leaving the plant in a pot and clipping off the flowers before they produce seed will control the problem. For more information, consult the California Invasive Plant Council (www.cal-ipc.org) or the University of California at Davis (http:tncweeds.ucdavis.edu). Or consult Sunset's Western Garden Book for information on a plant's growing habits.