Managing Poison Ivy

Maine has two species of poison ivy:  Toxicodendron (Rhus) radicans and Toxicodendron (Rhus) rydbergii.  We also have one species of poison sumac:  Toxicodendron (Rhus) vernix, which is not found north of southern Maine.  There is no poison oak in Maine.

The two species of poison ivy are superficially similar: each has three leaflets (that is, leaves are “three-parted”) and have long petioles (long “stalks”); the middle leaflet has a longer petiolule (“stem”) at its base than the two sides leaflets; leaves may be stiff, leathery or thin; they may be hairy or not; they may be shiny or dull; they may be toothed or wavy-edged; they may have a reddish tinge or not.  Poison ivy is our only common thornless woody plant with alternate three-parted leaves and visible buds.

These plants can be grubbed out of the ground when the soil is wet and when there are just a few plants to manage.  When the soil is dry, pieces of root generally break off during the digging process, and remain in the soil to sprout.  Such sprouting can be vigorous – it is common for roots to extend into lawn areas, and when they are severed by digging they produce many new shoots, causing the problem to worsen.  Tilling generally produces these same negative results. 

Repeated cutting back to ground level eventually stresses the plants, depleting the energy reserves in the root systems and resulting in plant death.  This method, however, requires multiple exposures to the plants’ toxic oils.

Burning these plants, either to control them in place or to burn plants that have been removed from a site, is NOT recommended, as burning produces particles that can carry the poisonous oils into the air, from which they can be inhaled, causing internal poisoning.

Several herbicides, when used appropriately, provide good control of poison ivy.  They are widely available either as concentrates or as premixed formulations.  Brand names vary, and concentrations of active ingredients vary.  Check labels; if concentrations of active ingredients are adequate to control these weeds, the weeds’ names will be stated on the product labels.  Not every product is suitable for every situation or time of year.  Of course, general pesticide rules should be followed:  dedicate sprayers to herbicide use; always use suitable Personal Protective Equipment; follow label guidelines for personal and environmental safety.

GLYPHOSATE, commonly sold under trade names Roundup, Kleenup and Rodeo, can be applied in two ways.  (1) Glyphosate can be sprayed on leaves and other green tissue, from which it is translocated throughout the plants.  This is most effective from late summer to mid-fall.  Thorough coverage is important; spraying just a few leaves is ineffective.  Glyphosate is inactivated quickly when it contacts soil, so root uptake by nontarget plants is not common and replanting of areas can be done promptly.  But, glyphosate is nonselective, having the ability to kill most plants whose foliage is sprayed, and making this an unsuitable choice in situations where target weeds are in a lawn or among other plants whose preservation is desired.  (2) Glyphosate can be painted onto stumps of just-cut woody plants in fall and early winter.  Of course, this method requires close handling of stems, and poison-sensitive individuals should avoid this method.

TRICLOPYR, sold under trade names Garlon, Redeem and Brush-B-Gon, controls many broadleaf herbaceous and woody plants.  Best results are achieved when it is applied early in the season (when the first flush of growth is just maturing) and when the foliage is actively growing.  Minimize drift to desirable broadleaf plants by using a coarse spray and covering them if needed.  Read the label carefully; there are limits on what can be planted for several months after triclopyr application in an area.

Source: email from Dr. Lois Berg Stack, Cooperative Extension Specialist, Ornamental Horticulture, and University of Maine, June 12, 2008

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