Summer is the time of year when people want to enjoy the outdoors in whatever way suits them. Whether in the yard, in the woods, or on the river, a run-in with an itchy plant can put a damper on the day or even the week. Read on to learn about identification of common rash-causing plants in Montana, and some best practices for avoiding contact.
WESTERN POISON IVY (TOXICODENDRON RYDBERGII)
The classic rash-inducing plant, poison ivy, is widely distributed in Montana and can be found in habitats like stream banks, woodlands, and open forests (Figure 1). It is often noticed it when recreating along the river.
Identifying poison ivy is important to avoid contact in the first place. This species usually grows as a subshrub or shrub, meaning it has a woody stem that persists from one year to the next. It can be from 4 to 20 inches tall. The leaves are shiny on the upper surface and compound, with each leaf made up of three leaflets (Figure 2 - view in PDF). Flowers are not clearly visible, being small and white and often underneath the leaves. Fruits are greenish-yellow and occur in clusters (Figure 3 - view in PDF).
About 80 percent of people are sensitive to the toxin in poison ivy and will have an allergic reaction to the plant, and people differ in sensitivity. All parts of the plant contain an oily resin that contains a mixture of chemicals called urushiol. Exposure can be directly from the plant, or from the resin that has adhered to clothes, pets, or other surfaces.
Plants in the genus Toxicodendron are the most common culprits of allergic contact skin issues in the United States. While there is one Toxicodendron species in Montana, poison ivy, it is important to note two others that do not occur here. There are no records of poison sumac (T. vernix) or Pacific poison oak (T. pubescens) occurring in Montana.
STINGING NETTLE (URTICA DIOICA)
Another plant to avoid contact with is stinging nettle. Like poison ivy, this species is also widely distributed in Montana. It is commonly found in moist habitats in meadows and open forests, often in a disturbed setting. Along the side of a wooded trail is a good place to find stinging nettle (Figure 4).
Stinging nettle is a perennial plant with a rhizomatous root system. Plants may be from 20 to 60 inches tall. The stems are square, and deeply-toothed leaves are opposite from one another on the stem. Clusters of small, greenish flowers droop from axils where the leaves meet the stem (Figure 5 - view in PDF).
Stinging nettle gets its name from its stinging hairs, and these are also helpful for identification. The hairs have two parts, which can be seen with a microscope or a hand lens (Figure 6). The tip of the hair breaks off upon contact, and a chemical is injected into the skin, causing a stinging sensation that usually lasts for minutes but sometimes continues for hours.
OTHER IRRITATING PLANTS
Many different plants can cause a rash or irritation under the right circumstances. For example, some people who handle the Montana noxious weed leafy spurge (Euphorbia esula) develop skin reddening, swelling, and blistering after contacting its milky sap. Another Montana noxious weed that can be irritating to skin is spotted knapweed (Centaurea stoebe), and gloves and long sleeves are always recommended when hand pulling this species. Some people even get itchy after exposure to zucchini plants in the garden.
These types of issues with plants can largely be avoided by wearing long pants and closed-toed shoes when walking through vegetation. For pulling weeds (or picking zucchini), consider wearing long sleeves and gloves to avoid unwanted issues with irritating plants.
*click on the article PDF for more plant photos.