As one academic expert explained, "plant resin is defined operationally as primarily a lipid-soluble mixture of volatile and non-volatile terpenoid and/or phenolic secondary compounds that are (1) usually secreted in specialized structures located either internally or on the surface of the plant and (2) of potential significance in ecological interactions" (Langenheim, 2003, p. 24).

Answer: The subject of our interest –poison ivy- uses its resin to protect its surfaces from fungal attacks. We humans get caught in the crossfire between the war between plant defense and fungal attack. As the old saying goes, "when the elephants fight, the grass suffers." In our case however, it seems that when the plants fight, it is the people that suffer.
Essentially, the 'protective armor' of varnish-like resin has nothing personal against humans, but rather is a defense mechanism that has been developed over 80 million years of inter-organism (plants vs. fungus) struggle.

This cross-sectional photograph of a 2 inch diameter cut poison ivy stem gives us a good look at resin (the white ring in this case) that protects the plant surface and causes the skin rash. Note that resin appears to be white in cool spring weather only. In more temperate weather, it would appear to be clear, like vegetable oil. The Resin turned shiny black in all cases once exposed to air.

On this leaf the shiny black spots are the plant’s ‘armor’. The leaf has been nicked, and the resin has oozed out in protective response.

Normal gardening activities that put us in contact with poison ivy expose us to contact with rash causing plant resin (for example, the black spots on these gloves).

Close up of resin strikes on a yellow vinyl rain suit surface. The amount of resin that fits on the head of this pin is potent enough to cause a skin reaction in fifty resin-hypersensitive people. (Armstrong & Epstein, 1995).

Answer: Across North America there are two kinds of poison ivy; climbing poison ivy and non-climbing poison ivy. This simple fact is very surprising to many people, who –depending where they live- are only familiar with 'their' kind of poison ivy, climbing or non-climbing as may be the case.

This is an example of poison ivy climbing a cherry tree. 

A more mature wintertime view of a leafless climbing poison ivy plant ascending an oak tree.

Non-climbing poison ivy as a ground cover will not ascend the white picket fence or climb the shrub pictured.

In its red and orange fall color, non-climbing poison ivy is at home cascading over a boulder field, along the Connecticut River in new Hampshire.

Answer: The same way you tell the difference between Lions, Tigers and Bears (oh my!)

Let me explain: A client who lives in the Philadelphia area recently wrote me to say that she had been to the doctor for a rash caused by poison oak, ivy or sumac, however she didn't know which. In fact, many people confuse these three plants (Poison Ivy, Poison Oak and Poison Sumac). I informed her that it was poison ivy, and she seemed surprised at my certainty, and questioned it. I explained that only poison ivy grows in the Philadelphia area, a fact that her doctor may have been unaware of.

It's really an ironic situation when confusion about which plant grows in a yard in any region of this country is a result of an original intention by botanists to categorize the nature of these plants on a continental scale.

To further explain: The botanists originally categorized the three plants together due to their common features and characteristics, which was a correct thing to do. The medical profession followed in this line of study and through their education, when patients would come to them exhibiting signs of rash, they would again group these three plants together as possible culprits, which was also correct.

The issue of confusion begins when professionals and the general public alike do not realize that these three plants, though similar in nature, actually inhabit very different environmental niches. Just like in characters in The Wizard of Oz were anxious about running into Lions, Tigers or Bears, the chances of you or I encountering poison oak, sumac and ivy together in the same place would be as probably as winning the lottery (or meeting the Wizard).

So where would we encounter these different plants?

For a moment imagine yourself leaving the front door of your home or apartment. It's a beautiful, sunny day. The temperature is mild, and you look around. You see other houses, streets, car activity, or a train in the distance... You are in Poison Ivy country. Poison ivy shares and lives in the same environment as people.

Now pause for a moment, again picture you are leaving your home, but now when you look around, you see wetlands and marshes. A swampy channel surrounds your home. Welcome to Poison Sumac land.

Once again, imagine yourself stepping out of your front door. This time, you look out to see an expanse of "fairly open, dry, savannah woodland of oak and pine." (Gillis, 1971, p488). In this case definitely keep an eye out for Eastern Poison Oak. (Just to clarify; I'm speaking of Eastern poison oak, not of Western poison oak.)

So remember; relate the plants to the correct environment, and you'll know you what you are encountering (Oh my!). 

Answer: Birds and people like similar things; they like color, they like things that shine, and they are curious about finding whatever is 'new' in our environment. We attract them to our homes to feed them and watch them because they are beautiful and they give us so much joy. They also drop poison ivy seeds in our yards. Well, we aren't going to get rid of the birds –and nor would we want to- so what option does that leave us? Well, that leaves us with identifying the poison ivy plants as they grow, and getting rid of them while they are small and manageable.

A cardinal sits on a poison ivy plant, contemplating which berry among the many he should eat first. The poison ivy berry is an important food source for over fifty species of overwintering birds. 

A penny for your thoughts? … Pull that sucker while he’s small!
To the left of the penny, a just-germinated poison ivy seedling.

We enjoy the effect of air conditioning inside, and birds are fascinated with it outside. For this reason, always check around your air conditioner for poison ivy seedlings. (You’ll often find them and so will your dog!)

Clients call us to clear a major poison ivy challenge from a hedge of Ginkgo trees. Held by two men is a 14 year old, 20 foot long, 80 pound poison ivy vine.

Answer: Because they are doing different jobs.

The baby leaves of a poison ivy seedling growing in its first year. The edges of the leaves are toothed and terminal leaf has short neck. Toothing of leaves facilitates good water evaporation which leads to quick growth. The short neck enables conservation of plant tissue and allows for complete and coordinated movement of the three leaflets together to better determine and note the sun's path. 

This leaf from a sexually mature vine is in its finest prom clothes, complete with sleek, untoothed leaves, which conserve water and give it a stylish, finished look for others to admire (from afar).

It’s back to the office with this post-mature, ‘casual dress’ look which combines the best of water use and conservation, tooth leaves and a loose, rugged leaf texture and style.

Another post-mature gaggle of competing runners climbs a large oak tree. Just beginning its red and orange fall color, these various leaves exhibit a wide range of shapes, types and styles, showcasing the plant’s wide range of leaf possibilities, but all still “leaves of three that look like me”.

Answer: “Leaves of three must look like me.”

Each of us carries around a fool-proof poison ivy identification tool: our upper body shape. Our head, neck and arms are all connected to our body.

Likewise,the poison ivy’s three leaves are connected to the stem of the plant in the same way a human’s head and arms are connected to the body, and in the same position as well. Using this simple physical reminder, we can always identify a poison ivy leaf; hence “Leaves of three must look like me.”

Remember: a person is a person whether they are right side-up or upside-down; the same holds true for poison ivy leaves.

In the poison ivy leaf pictured above, can you identify the 'head''neck' and arms' of the leaf? (look closely...) 

Identify the ‘head’ ‘neck’ ‘and ‘arms’ of this leaf. 
Good job! Now you’re getting’ it!

On the right, the lighter and smaller plant is a poison ivy seedling. Notice the ‘head’, a small ‘neck’ (like a baby) and two ‘arms’. To the left and top are mature poison ivy leaves. Please show your skills by identifying their head, neck and arms.

Congratulations for learning this invaluable identification tool! Now you are ready to avoid them, wherever they may be!

Answer: Climbing poison ivy and gingko trees share recognizable structures in their woody growth habits. Just like the diverse variety seen in the poison ivy seeds in the section 'seeds and seedlings', poison ivy plants mature into a variety of distinct recognizable forms. The plants in the photograph below will show how we have used the stable structure of a one hundred and thirty year old gingko tree to understand the very flexible but similar structure of the poison ivy stems and runners.

The gingko tree to the left of the page shares a feature in common with a twenty year old poison ivy vine to the right. Note that both raise the "V for victory" sign, signaling their status as mature, fruiting plants 

 

Both gingko and poison ivy node-clutches structurally exhibit up to eight 
arm-like branches radiating from a single node.

 

Answer: Look for the very hairy stem, the slightly hairy stem, multi-vine hairy stems, or the smooth stem.

This is where poison ivy identification gets a little tricky. Even though this plant does have a specific physiological shape and characteristics that denote it from other plants, it is important to realize that the potential for diversity within poison ivy forms can be surprising. For example, the four pictures of poison ivy below were all taken within a hundred yards of each other. Each photograph exhibits a slight variation of the plant stem and vine.

Very hairy stem 

A slightly hairy stem

Multi-vine hairy stems.

Smooth stem, not attached to tree trunk, but attached to upper branches of host tree.

Answer: Poison ivy goes through a few different stages throughout the course of its life: from seed to seedling, shoot to sprout, runner to immature vine, to mature seed producing plant. The leaves also go through a transformation as the plant matures.

After leaf drop in fall until bud break in spring, look for tiny hands-in-prayer-like buds. 

Buds open and unfurl ‘leaves of three that look like me’.

Honey bee collecting pollen from blooming male poison ivy flowers.

A stalk of ripe poison ivy berries produced by
a female plant and ready for consumption by hungry birds.