Pesky plants
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James Campbell
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Poison oak is the most likely cause of plant dermatitis in Marin County. Photo: James Campbell
Contact dermatitis happens when the skin becomes inflamed or irritated after coming into contact with something that causes a reaction. Plants cause dermatitis either by chemical or mechanical irritants.
A mechanical irritant is usually sharp and can scratch, impale, or lacerate the skin. Some mechanical plant irritants are pretty obvious such as thorns, spines, and glochids, those tiny hair-like prickles on cacti. Less obvious but no less painful can be sharp-edged or pointed leaves, like those found on agave, yucca, or pampas grass.
Even harder to see are the trichomes found on some stems and leaves. Trichomes come from the Greek word for hair and are epidermal growths that can take the form of hairs, scales, or papillae, which are fleshy projections from a plant. Trichomes function to protect plants.
Called stinging nettle for a good reason, beware the tiny hairs called trichomes on this plant. Photo: John TannOther trichomes are less pleasant. When brushed against, the bulbous tips of trichomes on stinging or burning nettle will break off, allowing needlelike tubes to pierce the skin. They inject a mix of acetylcholine and histamine, which will cause an itchy burning rash. Non-glandular trichomes do not have a glandular head, just a thin apex that provides plant protection. They do not secrete biologically active chemicals.
Cow parsnip can be found along hiking trails throughout Marin and is best avoided. Photo: Courney CelleyA chemical compound in the Asteraceae family of plants responsible for allergic contact dermatitis and is found in chrysanthemums, sunflowers, and the leaves of the ginkgo tree.
Be careful when you handle tulip, daffodil, and hyacinth bulbs due to the presence of calcium oxalate, which can produce dermatitis localized on the fingertips, also known as daffodil itch, or tulip fingers.
In Marin, your chance of getting dermatitis from plants is highest with poison oak. Despite the name, poison oak is not in the oak family but is a member of the family that includes cashews, mangos, and smoke bush. Between one-half and three-fourths of people are sensitive to urushiol, the mixture of compounds that causes dermatitis from both poison oak and poison ivy. Poison oak can be difficult to identify in the winter, when it has no leaves. Casual contact with the bare stems can still lead to dermatitis for sensitive individuals, so take care when hiking in wildland areas. To learn more about the control of poison oak in your garden and landscape, please visit http://ipm.ucanr.edu/PMG/PESTNOTES/pn7431.html.