This week we welcome the Sonoma County Master Gardeners to the Press Democrat’s Home and Garden section. These specially trained volunteers are dedicated to helping people in the community become better gardeners by using sustainable practices. Every other week they will dig deep to answer your gardening questions. Email them at the address below.
Question: What are those big green balls of leaves growing in the trees around Sebastopol?
Answer: The large globes of leaves you see, especially around Sebastopol, are European mistletoe (Viscum album). This plant parasite was introduced to the area by the horticulturalist Luther Burbank around the turn of the 20th century. Burbank grew mistletoe plants from seeds and attached them to apple trees at his Gold Ridge Experimental Farm in Sebastopol. Since then, European mistletoe has slowly spread and you can now find it as far north as Windsor, west to Occidental, to the east side of Santa Rosa and south to Petaluma. Sonoma County is the only place in the United States where European mistletoe has become permanently established.
Mistletoe grows through the bark of the host tree, then sends root-like structures called haustoria into the tree’s water-conducting tissue. Mature mistletoe plants produce sticky white berries that are attractive to fruit-eating birds such as cedar waxwings and robins. The birds digest the pulp and excrete living seeds that can stick to any branch they land on.
Forest pathologists report that European mistletoe infects 25 different hardwood trees in residential landscapes and orchards, including apple, plum, maple, poplar, willow, elm and black locust. It is also spreading in riparian areas and on native trees including ash, willow, cottonwood, alder and maple, particularly around Forestville, Graton and Bodega.
There are two other kinds of mistletoe that grow on Sonoma County’s native hardwoods. Big-leaf mistletoe (Phoradendron leucarpum ssp. macrophylum) appears in riparian areas and is widely distributed along the Russian river and Sonoma Creek watersheds. It is also found on nonnative hardwoods in the communities of Ukiah, Hopland, Cloverdale and Sonoma. Oak mistletoe (P. leucarpum ssp. tomen-tosum) occurs on native oaks as well as on some nonnative oaks such as pin oak.
Healthy trees tolerate a small amount of mistletoe without significant ill effects, but trees that are already stressed by drought or disease can be seriously infected or killed. To control the spread of mistletoe, it’s important to prune infected branches before they produce seeds. When only the visible portions of the mistletoe are removed, the mistletoe grows back. Make pruning cuts at least one foot below the point of mistletoe attachment to completely remove the embedded haustoria. Severely infected trees will not recover and should be removed.
Visit the University of California Integrated Pest Management Program website (ipm.ucanr.edu) for additional information on the biology and management of mistletoe.
The UC Master Gardener Program of Sonoma County (sonomamg.ucanr.edu) provides environmentally sustainable, science-based horticultural information to Sonoma County home gardeners. Send your gardening questions to scmgpd@gmail.com. The Master Gardeners will answer in the newspaper only questions selected for this column. Other questions may be directed to their Information Desk: 707-565-2608 or mgsonoma@ucdavis.edu
