Sonoma County’s dreaded poison oak can be removed with caution

Question: I live on property burned in a wildfire that was quickly reclaimed by poison oak. I am allergic to this plant and experience terrible rashes when I come in contact with it. Is there a way to safely remove poison oak without using chemical herbicides?

Answer: Controlling poison oak (Toxicodendron diversilobum) is one of the more difficult garden problems we face in parts of Sonoma County. Also known as western poison oak, it is a California native that can be a shrub or vine. Poison oak causes a dreaded allergic skin reaction in an estimated 50 to 85% of people who come into contact with it. However, with adequate precautions, you can remove poison oak safely, without chemicals.

You can identify poison oak by its leaves, typically three leaflets with the stalk of the central leaflet being the longest. The adage ‘‘leaves of three, let them be’’ refers to the three leaflets, although occasionally its leaves may have five to nine leaflets. Each leaflet is 1 to 4 inches long with toothed or somewhat lobed edges. The leaflets grow alternately along the stem, one to the left, then one to the right. The surface of the leaves can vary from glossy to dull and thin to leathery in texture.

Despite its name, poison oak is not related to any of our native oak species, although it may superficially resemble some of the shrubby oaks. Leaves of true oaks grow singly, not in groups. The common native shrub, skunkbush sumac (Rhus trilobata), resembles poison oak but its leaves lack a stalk on the central leaflet and it has hairy fruit.

In early spring, new poison oak leaves are green or sometimes light red. Later on, the plant produces small white-green flowers, and in late summer it bears small, whitish-green round and hairless fruit that can last into fall and winter. In late spring and summer, the foliage is often glossy green and turns attractive shades of orange and red in the fall. Poison oak is deciduous, making it very difficult to identify and remove it in the winter when the leaves are gone.

Poison oak seeds have a hard coat and can remain viable in the soil for many years. Once seeds germinate and the plants become established, they can spread from underground stems. Vines that contact the ground often form roots, creating new plants that contribute to lateral expansion. A single plant can cover a very large area over time.

How to remove poison oak

You can manually control poison oak by hand-pulling or mechanical grubbing — severing the roots below ground with a shovel or pick. You must remove the entire plant, including its roots. This is best done in early spring or late fall, when the soil is moist and it’s easier to dislodge roots. To discourage a reinvasion, replant the area with more desirable plants.

Grubbing when the soil is dry and hard usually breaks off the stems, but the rootstocks left in the ground vigorously resprout. Lopping mature plants near the base can also lead to resprouting. However, if you do it repeatedly throughout the season, this method may result in some control. In some cases, removal in late summer, when plants are stressed by lack of moisture, can slow their ability to recover.

Using a rototiller repeatedly throughout the year can be an effective technique for controlling poison oak in a garden area. If you rototill or cultivate an area only once, the rhizomes are fragmented, which encourages the spread of poison oak.

Many control methods are not effective, including anything that removes the aboveground parts of the plant but not its roots. Using large equipment to clear land creates a perfect environment for new seedlings to establish. Mowing is ineffective in controlling poison oak and can release harmful oil particles in the air. Goats, sheep and other animals will browse poison oak, but unless the roots are removed, it will come back.

Be cautious

After removing it, carefully dispose of poison oak. The detached and dried plants can still cause a skin reaction, so carefully bury or move the plant material to an out-of-the-way location. Local waste haulers prohibit disposal of poison oak in the green compost bins, but you can throw it in the trash bins.

Never handle these plants without protection. The oily resin called urushiol in poison oak is what causes the allergic skin reaction, most commonly a weeping, itchy rash. People who have never contracted a rash from poison oak may become susceptible over time.

Urushiol is found in the stems, roots, leaves and fruits of poison oak. Usually, plants must be damaged to release the oil. Vigorous activities such as clearing brush, weed eating, pruning or trampling can injure the plant enough to transfer the oil onto skin, clothes and tools. Pets can also carry the compound on their fur and transfer it to humans. Don’t burn poison oak — it volatilizes urushiol, releasing compounds into the air. If inhaled, the compounds can cause allergic reactions in sensitive people.

Ideally, anyone working in poison oak should have a high degree of immunity to the allergen and should wear protective clothing including washable cotton gloves over plastic gloves. After exposure, thoroughly wash all equipment, tools, clothing and shoes.

If you do come into contact with the plant or items that have picked up urushiol, quickly remove the oil. You can purchase liquid skin cleansers that, when used with cool water and applied soon after exposure, break the urushiol bond with the skin. They can prevent the rash or stop the spread and longevity of an ongoing infection. These products can also be used to decontaminate tools and fabrics.

If there is one good thing to be said about poison oak, it’s that the leaves and stems provide a valuable food source for many animals. Both birds and mammals forage on its berries without adverse effects. However, you don’t need to leave it in place just to attract wildlife; it’s already prevalent in our natural landscapes. Remove it from your landscape and your family and friends will thank you.

For more information on controlling poison oak, go to bit.ly/3SKvp7Q

Contributors to this week’s column were Janet Bair, Karen Felker and Patricia Rosales. 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. You will receive answers to your questions either in this newspaper or from our Information Desk. You can contact the Information Desk directly at 707-565-2608 or mgsonoma@ucanr.edu.