Poison ivy leaf shape. Two mittens on the sides giving a thumbs down with bilateral symmetry on middle leaflet.

I’ve posted in the past about poison ivy (1, 2, 3), but thought I’d post again, this time with an eye to identification. My goal with kiddos at Crow’s Path programs is to get them to be able to subconsciously identify poison ivy while running around playing games. This definitely takes practice – lots of dirt time looking, drawing, studying – but once you create a search image and lock it into your brain you no longer have to think to activate that process. Awareness, to a certain degree, becomes a passive, embedded process.

My original post on poison ivy from so many years ago focused on the diversity of poison ivy, but also the general form. It’s amazing to me that we can identify a species from a thousand different angles under a thousand different lightings. No two poison ivy plants are the same size, shape, color, texture. But the patterns are distinct enough that our brain can readily generalize a specific form and lump it in with other similar forms and parse it out from dissimilar forms.

Poison ivy with damage from a leaf miner

Researchers recently found the same process happens with ducklings, which immediately after hatching are able to imprint onto a mother figure. Imprinting isn’t about recognizing a static form, like a circle or star, but rather a complex moving creature that will guide them to safety. The guide may be sneaking through tall grass, flapping their wings, or swimming across the surface of the water. Whatever their varied shape, a duckling’s survival depends on its ability to recognize their alpha. So to do humans share an ability to recognize and lump, though as the ultimate generalist omnivores fortunately we lack the lazer like focus on a specific entity and instead have a far less specific and infinitely more powerful ID skillset. Being able to recognize many different forms translates to us being able to recognize and consume some 600 different genera in any one location (Carol Yoon describes this odd upper limit of 600 forms in her book on taxonomy Naming Nature, referring to a person’s ability to name a max of about 600 different species, bands, products, etc from memory).

Forging a search image requires spending time looking closely (or I guess just subconsciously assimilating small bits of information over long periods of time works too as I don’t ever remember learning to identify dandelion or pigeons or Katy Perry, but here I am capable of recognizing her almost instantly on a poster at Staples). As much as this can work, looking closely is more fun, more transferable, and more interesting. So, looking closely at poison ivy might reveal:


  • A woody stem
  • Each aerial stem typically has only 1-2 leaves emerging from it
  • Alternate branched leaves
  • Poison ivy that has taken to vining, at least in our region, is very uncommon
  • When it does vine it uses aerial/adventitious roots to attach to tree (not tendrils, spiraling, or hooks)
  • The 3 leaves are actually 3 leaflets that together form one leaf.
  • Smooth (not waxy or hairy)
  • Leaves often show signs of damage from leaf miners
  • Form a T-shape, with two on side having short petioles (leaf attachments) than the middle leaflet
  • Side leaflets are asymmetrical, middle one is bilaterally symmetrical
  • Two side leaflets often look like they’re given a thumbs down
  • The fruits are like little whitish yellow pumpkins and readily pop off the stem
  • They’re often hidden beneath the leaves so difficult to find
  • I’ve observed deer and catbirds eating them
  • Reproduce mostly via runners (I’ve hand pulled poison ivy and if you get a runner you can pull up a few feet of horizontal roots!!)
  • Roots are dark brown and around an 1/8″-1/4″ in diameter at most.

More on the topic

Categories: Quizzes

Digging all this natural history content?

Become a monthly supporter on Patreon.

Be sure to check the archives for back issues.
And shoot me an email if you have an idea for a future blog post, newsletter issue, or podcast episode!

Subscribe to the Newsletter