Weed of the Month: Poison Ivy

Published: August 11, 2015

Poison ivy (Toxicodendron radicans) is a native perennial weed that can occur in landscapes, woods, fencerows, pastures and haylands across the U.S.  The plant is the major cause of allergenic dermatitis in the eastern U.S.  Yet it can be difficult to identify given the variability in the shapes of the leaflets and its growth habit; hence the commonly used phrase, “leaves of 3, let it be.” 

Figure 1: A young poison ivy plant may appear erect and almost shrub-like.

Figure 2: Poison ivy can commonly be seen growing on trees

Figure 3: Aerial roots allow poison ivy to climb.

Figure 4: Poison ivy can also creep along the ground.

Younger plants may appear erect and almost shrub-like as they get established (Figure 1).  However, as the plant grows the vine becomes woody and can climb on other objects and vegetation (Figure 2) by attaching to the object with aerial roots (Figure 3).  The plant may also trail along the ground (Figure 4).  The stems of poison ivy are capable of rooting whenever they come into contact with the soil.

Figure 5: The leaflets (A), berries (B), and newly emerging seedlings (C) of poison ivy.

While the individual poison ivy leaflets can vary in shape, the general leaf structure is consistent, which will prove helpful in identification.  Each poison ivy leaf is comprised of 3 leaflets.  The 2 lateral leaflets attach to the stem via short petioles (the appendage that attaches the leaf to the stem).  The middle leaflet tends to be attached by a much longer petiole (Figures 2 and 5a), and all 3 leaflets are glabrous, or lack hairs on the surface.  Leaflets can be toothed or without teeth, and most commonly has lobed edges.  Typically, the 2 lateral leaflets are distinctly lobed on one side of the leaflet but not the other, and each leaflet is from ¾ to 4” long and wide.  The leaves tend to be a bright, shiny green earlier in the summer but will turn red or reddish yellow as fall approaches.

Typically poison ivy reproduces by the creeping roots and stems; however, the plants can also produce seed.  Poison ivy flowers are small and yellowish green to green in color.  They occur in clusters of 2 to 6 and usually arise between the petioles and the stem.  Birds eat the berries, which turn from green to white as they mature (Figure 5b), and then disperse the seeds. When seeds germinate, the seedlings’ cotyledons are oval and the first true leaves are divided into three leaflets (Figure 5c).

Poison ivy can be mistaken with Virginia creeper (Parthenocissus quinquefolia) and poison oak (Toxicodendron toxicarium).  However, Virginia creeper leaves are divided into 5 leaflets as opposed to 3.  And Poison Oak leaflets are usually duller green in color and have hair on both sides of the leaflet surface.

The toxin present in both poison oak and poison ivy is a chemical called urushiol, which is yellowish in color and present in the leaves, flowers, stems and roots of the plant.  Once the plant becomes bruised or damaged, urushiol is emitted onto the leaf and the stems.  Approximately 85% of people are allergic to the compound1, and according to the Center for Disease Control, urushiol can remain active for 5 years following its emission.  Coming into direct contact with the plant is not necessary for getting the rash; the plant sap, which contains urushiol, can be picked up from clothing, pets, tools, etc., anything that has come into contact with the plant. 

Unfortunately, control of poison ivy can be quite challenging.  Regardless of whether mechanical, cultural, or chemical methods are used, it is important not to burn the plant’s debris as the toxin is volatile and can be inhaled.  This can lead to an allergic reaction in the lining of the lungs.  For chemical treatment, most commercially available herbicides that are specifically marketed for poison ivy control contain glyphosate (brand name Roundup) in combination with either 2,4-D or triclopyr.   High-volume applications of glyphosate have been shown to be effective for initial control of poison ivy.  
However, glyphosate is a non-selective herbicide and will kill any nearby desirable foliage that it contacts.  Additionally in recent years, research has shown that either 2,4-D or triclopyr alone are more cost effective options for controlling young poison ivy plants than glyphosate alone or glyphosate in a mixture with either of the 22.  Both 2,4-D and triclopyr are selective and will not kill grasses.  Recent research published by weed scientists at Auburn has shown that applications of 2,4-D at 0.5 lb/A on 2-year old poison ivy plants will result in 95% control up to one month following herbicide treatment.  However, when the researchers examined the same plants 4 months following the herbicide treatment, the group concluded that 2,4-D only provided 80% long-term control and a subset of the plants had begun to regrow3.  The authors of this study went on to find that 1.0 lb dicamba/A by itself was the most cost effective method for long-term control of established poison ivy.  Any combination of dicamba and 2,4-D together or 2,4-D alone was slightly less effective in controlling poison ivy regrowth long-term3.

For more information on poison ivy, visit our weed identification web site:  weedid.missouri.edu or download our free app IDWeeds.

For more information on tough-to-control pasture and non-cropland weeds, order a copy of our latest Integrated Pest Management publication, which has a comprehensive list of weeds, detailed images and control measures for each: http://extension.missouri.edu/p/ipm1031

1The American Academy of Dermatology: www.aad.org
2Wehtje G and CH Gilliam (2012) Cost-Effectiveness of Glyphosate, 2,4-D and Triclopyr, Alone and in Select Mixtures for Poison Ivy Control.  Weed Technology 26: 469-473.
3Wehtje G and CH Gilliam (2015) Poison Ivy (Toxicodendron radicans) Control with Dicamba and 2,4-D Applied Alone and in Tank Mixtures.  Weed Technology 29: 115-120.