Pesky plants
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James Campbell
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“Leaves of three, stay clear of me!” Kids learn the little rhyme to keep on the lookout for poison oak. On the east coast, you might hear “leaves of three, beware of me” to identify poison ivy. However, if you enjoy time outdoors or working in your garden, you should know poison oak is not the only plant that can be irritating.
Poison oak is the most likely cause of plant dermatitis in Marin County. Photo: James CampbellContact 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.
Trichomes can be either glandular or non-glandular. Glandular trichomes have a glandular head and produce, store, and release biologically active chemical compounds. Some glandular trichomes secrete compounds that we value. For example, mint, oregano, lavender, basil, and thyme, trichomes secrete coveted essential oils.
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.
Phytophotodermatitis is an example of chemical dermatitis that happens after contact with plants containing psoralens, followed by exposure to the UV rays in sunlight. The result is hyperpigmentation, but blistering can also occur. The rash can range from mild to severe and can last from days to months. Sometimes the discoloration can be permanent. The juices of some plants can cause skin bleaching, or leucoderma, if a person has sensitive skin. Plants associated with phytophotodermatitis come from many plant families. About half the plants that can cause this type of dermatitis come from the Apiaceae family, including celery, parsley, parsnip, carrots, wild carrots, wild celery, wild parsnip, Queen Ann’s lace, and cow parsnip. In other plant families, lime and fig can also induce dermatitis in some people.
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.
