Poison-ivy is a
woody perennial. It may grow as dwarf, shrubby plants only a few centimetres
high and carpeting the ground (Figure
1), or as upright plants 60-90 cm (2-3 ft) high, or the vine-like
form may twine around trees, shrubs and posts, and reach a considerable
distance above the ground. These vines often develop root-like structures which
act as attachments, but apparently do not damage the living plants to which
Each leaf of poison-ivy consists of three leaflets so
the leaf is said to be "compound". (See bracket marked "b"
3A for one complete compound leaf and bracket and arrows marked
"c" for individual leaflets.) The stalk (e) of the middle leaflet is
longer than the stalks (f) of the 2 side leaflets. All 3 leaflet stalks (e and
f) are joined together at the tip of one much longer stalk (d) which is called
the petiole. The other end of the petiole is attached to the woody stem at a
node or "joint" (a). There is only one petiole at each node and, when
there are several leaves they usually alternate from one side of the stem to
the other at successive nodes.
Figure 1. Poison-ivy with bright green foliage during summer.
In spring and early summer the young unfolding leaflets are reddish or bronzy green (Figure 1, left side, below center) and droop or hang limply from the ends of their upraised petioles.
They gradually become firmer and stand out nearly level with the end of the petiole. Their color also changes to deep to bright green. Their upper surface is nearly smooth and sometimes has a glossy or "varnished" appearance. In bright, sunny areas the leaves turn a vivid orange-red to wine-red during autumn (Figure 2), but in shaded places they often lack the bright colors, just turning dull tan or light brown before dropping off.
Although the leaflets are somewhat oval, they vary greatly in shape and size. Their margins vary from perfectly smooth (Figure 3A) to finely or coarsely toothed (Figure 3D), to deeply and irregularly lobed (Figure 3E).
Undersides of the leaflets may be finely hairy all over, or just along the veins and veinlets, or may be virtually without hairs. Shortly after the leaves have formed in the spring, clusters of small, erect, greenish-white flowers develop on some plants in the angles where the leaf petioles join the stem (g). They are often hidden by the leaves (Figure 1, center). During the summer these flowers develop small round hard berries about 5 mm (1/5 in.) in diameter (h).
Figure 2. Poison-ivy may turn bright orange-red to wine-red in autumn.
After the leaves have dropped in the fall, poison-ivy can be identified easily by the clusters of grayish to white, waxy-looking berries (Figure 3B) on short, erect, bare stems which have alternately arranged buds and leaf-scars. The berries are distinctly longitudinally lined, and upon close examination may remind one of a miniature peeled orange. Some fruits vary from the typical, and may be quite hairy.
Poison-ivy is spread by seed, but it also has extensive underground stems from which new plants develop. For this reason, killing the aboveground parts does not mean that the plant has been destroyed, and new shoots may appear at any time from these underground structures.
Figure 3. Poison Ivy.
A. Low-growing form with short erect stems and a flower cluster from the axil of one compound leaf.
B. Cluster of dry, white, berry-like fruits produced from the flower cluster.