Poisonous and Aggressive Plants
Poison ivy to brambles, defending the earth
Plants with thorns or irritating oils may not endear themselves to humans, but they are nature's way of trying to reclaim disturbed areas and return them to wilderness.
This article does not address invasive species or those that are poisonous when ingested but plants that sting, scratch, cut, or cause painful rashes. Thorns, stinging hairs, and irritating oils are among the weapons by which these warrior plants defend their slowly recovering turf.
The best-known aggressor is poison ivy, which is so various in shape, color, and size that many people just avoid all plants with leaflets arranged in threes. Poison ivy can grow as a vine, in a bushlike shape, or as a small plant coming straight out of the ground. When young, the leaves are shiny and reddish, but they soon turn green and lose their shine. It is helpful to know what poison ivy does NOT have, such as thorns, as well as sharp teeth along the edges of the leaves. The leaves are smooth-edged or wavy, while the closely related poison oak has a few large, blunt teeth, never sharp, jagged edges. Both poison ivy and poison oak have a long-stemmed terminal leaflet (the middle of the three leaflets) that is symmetrical, i.e., the two sides of the leaflet mirror each other in shape. The two side leaflets are asymmetrical, with the lower edge of the leaflet bulging out more than the top edge.
Grindelia tincture, available at many health food stores, is an often effective external application for poison ivy rash. Grindelia grows in hot, dry climates of the western U.S. and contains a sticky sap. Jewelweed is a plant often used in the East as an application to prevent or soothe poison ivy rash.
Poison sumac is not very common and only grows in swamps. It is a shrub or small tree and has compound leaves with 7 to13 long, pointed leaflets, similar to the leaves of staghorn and smooth sumac, the more widespread species, which are not at all poisonous. Poison sumac, however, has white berries (not red, as in the other species), and the leaves have no teeth. It is even more toxic than poison ivy and should be strictly avoided.
Thorny plants include roses, hawthorns, raspberries, and blackberries–all rose family members that have medicinal properties and/or edible fruit and are therefore welcomed in our territory when they behave themselves. But anyone who has tried to walk through a tall, dense thicket of wild blackberry will appreciate the fiercely protective nature of brambles. Among the thorned non-roses are barberry, honey locust (a well-armed tree), and thistles.
Stinging nettle and its milder cousin, wood nettle, use a different strategy of attack. The stem and the midrib on the underside of each leaf are lined with tiny, hollow hairs containing formic acid (as in stinging ants). Brush against a hair, and it injects the irritating acid into the skin, much in the manner of a hypodermic needle. While this substance is stimulating and can be healing to some kinds of joint problems, it also hurts, as hikers in shorts can attest. Stinging nettle, which has coarsely toothed, roughly triangular leaves, grows in moist meadows and along streams. Its sting can persist for a day or two. Wood nettle, with larger, rounder leaves and a less painful sting that usually lasts under a hour, is common in open woods.
Arrow-leaved tearthumb has tiny, razorlike, downward-pointing scales along its stem. Any pressure upward against the stem results in a slash that can slice through skin and draw blood. It grows along the edges of streams and ponds. Like other members of the buckwheat family, tearthumb features papery sheaths around the leaf joints. The small white or pink flowers are oval, inconspicuous, clustered. The leaves are arrow-shaped, clasping the stem, while a related species, halberd-leaved tearthumb, has leaves shaped like medieval axeheads. Perhaps one should be warned away by the resemblance of the leaves to weapons.
