Coastal Gardener – March 6, 2003
COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
What is the best time to get rid of poison oak?
A:
Poison oak is native to most areas of California. It is an especially common
inhabitant in the oak woodlands on the Central Coast. As people move into these
areas, they can expect to encounter poison oak in their landscape and the
surrounding vegetation. The plant is usually a woody shrub but may also have a
vine form (climb on other plants). In full sunlight it may develop a dense shrub
canopy one to six feet high and be a single plant or a clump several feet in
diameter. Poison oak is deciduous (loses its leaves in winter).
The plant usually produces leaves consisting of three leaflets. But vigorous plants
may have leaves with five, seven or nine leaflets. The leaflets have smooth, lobed
margins. The surface of the leaflets is glossy or dull. In spring, poison oak
produces clusters of small, white-green flowers, which emerge from where leaves
attach to the stem. In late summer/early fall whitish green, round fruit (berries) are
produced (no acorns). In spring/summer the leaves are green or sometimes light
red. In late summer/fall the plant develops pretty fall colors of orange, yellow and
red.
All parts of the plant are toxic to those persons who are sensitive to the urushiol
present in oil-resin in the sap of the plant. Interestingly, goats, sheep, deer, and
horses can graze poison oak without ill effect. In fact, goats are sometimes used to
control it. Be careful with animals that have been in poison oak. Sap from their
fur can give you dermatitis. Poison oak pollen can cause severe respiratory
problems if inhaled. And contact with smoke from burning poison oak can also
cause skin and respiratory problems. Contact with sap, pollen or smoke may cause
a severe dermatitis characterized by large blisters and swelling for several days.
Severe gastric distress and even death may result from the ingestion of leaves or
fruit.
The danger of poisoning is greatest in spring and summer when plants are full of
sap and bleed easily when bruised or cut. Sap, dried sap or pollen on clothing can
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Coastal Gardener – March 6, 2003
remain toxic for years. Persons vary in their sensitivity to poison oak with some
having no reaction at all to the opposite extreme of severe dermatitis and
respiratory distress when exposed to the plant.
If you are one of the latter persons, you probably should not get involved in any
control operations. Elimination of poison oak can be done manually by digging it
out. This is best done in winter when the plant is fully dormant. Most persons can
handle poison oak with leather gloves and a long-sleeved shirt. However, you
should probably plan to discard both when the job is completed. Herbicides can
also be used with good effect. Glyphosate (Roundup™, Kleenup™, Stopper™,
etc.), triclopyr (Garlon™, Ortho Brush-B-Gon™, Turflon™), 2,4-D, and dicamba
(Banvel™, Vanquish™) will provide control. In all instances, the poison oak
should be sprayed when in full leaf and growing vigorously. Glyphosate at full
label rate has been suggested to me as the most effective. The poison oak should
be completely wetted with the spray. To eradicate a patch of poison oak, more
than one application may be needed. So, spray once, then wait until the plant
produces some regrowth to apply a second application. The same procedure
should be used if a third, fourth, etc., application is needed. Never burn poison
oak. Uprooted plants and brush should be placed in an out-of-the-way location to
decay or take to a disposal site.
Send your yard and garden problem questions to: The Coastal Gardener, 624-A
West Foster Road, Santa Maria, CA 93455.
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