Colorado Poison Ivy

In Colorado, the only form of poison ivy you’ll find is actually not even an ivy.  Known commonly as Western Poison Ivy, or Toxicodendrom rydbergii, it’s not quite as pernicious out here as eastern poison ivy can be.  In fact, you really have to go looking for it.  Places where it loves to grow include wide, sunny areas, along creek beds, and in abandoned open spaces.  Unlike the eastern poison ivy (Toxicodendron radicans), the western version is a shrub, usually growing as a single woody stem with leaflets of three almond shaped leaves that grows about knee high.  Larger, older stands may have more branches and grow as much as 3 feet high. Western poison ivy can form colonies that are as much as 20 feet across.  It spreads by rhizomes, and can be invasive in areas like utility easements, old fields, and along roadsides.   Key identifying features of this plant are short, woody stems and three leaflets, however, young plants may not yet have developed the woody stem, so it is important to recognize this plant in all stages of development.  The flowers bloom in May to June in the plains and foothills, with light green to yellow berries forming throughout the summer.  These berries will remain throughout the winter if they are not eaten by wildlife.  In the fall, foliage may turn an orange or crimson color before dropping the leaves.

Some key features that will always indicate poison ivy:

Poison Ivy Leaflet
Poison Ivy Leaflet

Leaflets of three.  The poison ivy plant will have three leaflets on the leaf stalk or petiole.  Sometimes a leaf or two will drop off, but there will never be more than three leaves on a petiole.  The center leaflet will be on a longer stem than the two side leaflets

Raspberry Leaflet with thorns
Raspberry Leaflet with thorns

No thorns.  Many of the bramble-type berries like blackberries, raspberries, and strawberries can have three leaflets.  However, these plants will almost always have thorns.  Any three-leaflet plant with thorns is NOT poison ivy.

Lastly, poison ivy leaves are always arranged in an alternate configuration.  That means that you won’t ever see the leaf petioles coming out of the stem.  The ashleaf maple and box elder often have three leaflets, especially in young plants, but these plants will have the leaves coming out of both sides of the stem.  In poison ivy, the leaves will alternate sides of the stem. See in the photo below how only one leaf originates at the same spot on the stem, and that they alternate sides.

Lisa AkersComment