Backyard Biodiversity July 2021 - Poison Ivy

Eastern poison ivy with its white berries, found as a vine climbing a tree.

 Poison Ivy

I do not remember ever getting a poison ivy rash when I was younger. Maybe I never was exposed, maybe I never really got into the woods, I certainly never learned to identify it ….but the past few years I have developed an allergic reaction to posion ivy AND have quickly learned to identify it.

Eastern poison ivy (Toxicodendron radicans) and western poison ivy (Toxicodendron rydbergii)  are native to North America.  Other familiar species that belong to the same genus include: poison oak (not really an oak) and poison sumac. These species all belong to the family Anacardiaceae (the cashew or sumac family). Many members of this family contain the oil urushiol which is the cause for the itchy, blistery rash when you come into contact with poison ivy. Other important members of this family include mango, cashew, pistachio and the Marula tree of South Africa. Mangos and cashews can also contain urushiol oil and can cause contact dermatitis if the skin of the fruit is touched. The cashew nut’s shell contains urushiol oil as well, which is why cashews are sold unshelled. 
​
Urushiol oil from poison ivy can get on clothing, gloves, and gardening tools. If those do not get washed off the oil can then transfer to you, leading to an allergic reaction. The best advice is to immediately wash with warm water and soap any clothing or equipment and skin that may have come into contact with the plant. Do not burn poison ivy, as the oil will be carried in the air on smoke particulates and if inhaled can cause a severe allergic reaction. Many other animals do not seem to have the same reaction to the oil, and in fact the leaves and berries of poison ivy are food for many.

I got a pretty, nasty poison ivy rash on both arms last year that I attribute to my dog having picked up the oil and me hugging her. I make this assumption because I am pretty sure I was not hugging a pile of poison ivy and could not figure out how else I would have gotten that pattern of a rash.

So how to identify poison ivy in order to avoid it? The old saying goes: Leaves of three let it be. This is perhaps the safest way to avoid touching it, but then a lot of plants have 3 leaves.  Phenologically poison ivy is highly variable. Eastern poison ivy is found in most of the eastern half of the US, ranging a bit into the southwest. It is a vine forming plant, often recognized by its hairy roots that are used to help it attach to and climb up objects ( see pictures below). Western poison ivy is found in the western Uas well as portions of the Midwest and eastern US. It grows more as a bush, rather than a vine. As a bush it can grow up to 6 feet in length and as a vine it can reach 150 feet.
 
Poison ivy I think can be best identified by looking at the pattern of the 3 leaflets. The middle leaflet has a long stem from the main branch and the other 2 leaflets look more like they come straight out of the stem. Also the 1st and 3rd leaflets are directly opposite of one another.
 
The sites below have some helpful diagrams and more information on how to ID poison ivy.
Eastern Poison Ivy (poison-ivy.org)
Western Poison Ivy (poison-ivy.org)
Western Poison-ivy (fs.fed.us)

Poison Ivy

---

Poison Ivy vine climbing a tree

Close up of the vine, showing the 3 leaflet structure of poison ivy.

---

Hairy roots of the poison ivy vine.

---

Close up of a smaller version of poison ivy in my backyard.

Plants that LOOK like poison ivy,,,,but are not!

This is Virginia Creeper, a vine native to Missouri. It usually has leaves made of 5 leaflets but you can see that some of the young ones only have 3 leaflets. The vining stem can also be tinged red which may make you think poison ivy, but it is not.

This is fragrant sumac (Rhus aromatica). It belongs in the same family as poison ivy and as you can see a similar leaf structure. But the leaflets all seem to come out of the main stem in the same location, or at least the 2nd leaflet does not have a very long stem. It does not cause an allergic reaction if touched.