Rash Doctors

Many of the botanists of yore were primarily medical doctors by profession. Examples include such greats as Asa Gray, George Engelmann, John Torrey and to some extent Carolus Linnaeus himself. If modern doctors had such affection for our wild flora, perhaps I wouldn’t cringe when I commonly hear of people in the Midwest diagnosed with Poison Oak or Poison Sumac rash. To them, I usually reply with some cynically derived question regarding the geographical location of their encounter with the offender. If they have been diagnosed with Poison Oak I ask, “have you been in the forests, woodlands or swamps of the southern states lately?”. My Poison Sumac inquiry is more specific. “Have you been in a high quality wetland community lately?”, I say.

Most folks eye me queerly and retort “no, I was doing yard work” or “whatever do you mean”. The bait being taken, I explain that Poison Ivy is a much more likely explanation for their dermal discomfort; especially for those living outside the range of anything but Poison Ivy, as most Midwesterners are. Some, in true argumentum ad verecundiam fashion, defend their doctor’s claim. Others are interested in the difference and enjoy learning a bit of natural history. For those that don't know the difference.....

...here is Poison Ivy (Toxicodendron radicans).

Notice the sharp pointed teeth on the leaflet margins, the pointed tips of the leaflets and the long petioles of the lateral leaflets. Its habitat ranges from anywhere to everywhere; xeric to mesic, acidic to calcareous, disturbed to pristine, the PI can live there. Poison Ivy usually grows like a true vine. It clambers and twines on structures and other vegetation. It is common throughout North America in every state and province east of the Rockies.

Here are two photos of Poison Oak (Toxicodendron pubescens).

Notice the rounded teeth of the leaflet margins, the blunt tips to the leaflets, the nearly sessile lateral leaflets and the relatively long petiole of the terminal leaflet. Its habitat tends to be quality dry to mesic forest and woodlands. Poison Oak does not vine or climb on other vegetation but rather grows as a small shrub. Its geographic range is basically the southern states of the Civil War with a few stragglers into border states.

Lastly, here is Poison Sumac (Toxicodendron vernix).
Notice that the leaves are entire and pinnate instead of trifoliate –so much for the “leaves of three” adage. The habitat of Poison Sumac consists of quality fens, bogs and wetlands. It grows into a shrub of ten feet or so. It occurs sporadically throughout most of eastern North America.

So the next time the doc says “you have poison oak”, Get a second opinion and get it from a botanist.


  1. The wait since your last post was worth it! I enjoyed the post. I, too, have people tell me things like, "I'm allergic to poison oak, but not to poison ivy," and these are people who haven't left the upper midwest. I always try to explain that we don't have poison oak here, but who wants to listen to a botanist?

  2. I have no experience with the southeastern poison oak (is this Atlantic poison oak, Toxicodendron pubescens), but when I lived in California I got into a mess of California poison oak (Toxicodendron diversilobum) collecting beetles off its flowers. I thought I was being safe, netting the beetles from the flowers and then reaching into the net to get the beetles. Apparently the oils got all over the net bag, as the next day my forearms began developing one of the most dog-awful rashes I've ever experienced.

  3. Yeah, Altantic Poison Oak would be the full common name. I had to google T. diversilobum. It looks like anywhere you go in the US could have a Toxicodendron waiting to jump out of the bushes. I once got the oils embedded in the arm rest of my truck. I had what I referred to as elbow ivy for about 10 months straight. When I participate in prescribed fires I am often fearful of inhaling the smoke of burning PI. I know people that have and it is not pretty.

  4. Belated but abundant thanks for this post, Justin. I will use the link to it in the future, I am quite sure.

  5. Great info and pictures. Thanks.

  6. Great post Justin. As a teacher of high school students, I had the same conversation several times over the years: A student would say he had poison sumac, and I would ask "How do you know?" It always came down to a doctor who purported to be able to identify it from the rash. I took a certain amount of pleasure in telling them (including a few of the doctors) that poison sumac was not found in Missouri.

  7. I used to occasionally get awful contact dermatitis. If it was bad, people said "poison ivy." If it was worse, they said, "poison oak." If it was really horrifying to see, they'd say, "You must have gotten into poison sumac!" (All this in Missouri.) Finally a dermatologist tell me that my worst skin condition ever was the result of contact with a plant in the parsley family, possibly cowbane or wood angelica.

  8. My thanks to everyone for your comments. It was a fun article to write. There certainly has been a "rash" of activity recently on this year old post.

    There is an endless list of botanically preposterous claims from purported experts. Just this afternoon I was having lunch with my sister who had just taken my niece to the doctor for a series of allergy scratch tests. One of several things identified as causing an allergic reaction for her was "goldenrod". While I don't doubt that she could be allergic to goldenrod, are enough people coming in contact with it to even test for it? It is more likely remnant of the whole false blame scenario involving ragweed (synchronous flowering). Also, during the allergy index report on a Springfield news station last month they listed Sagebrush pollen as being high. I guess it could be blowing in from way out west, but I am suspect. Have they consulted a palynologist?

    Anne- It looks like we have another case of doctors overstepping their expertise. While it is true that the sap of several members of the parsley family contain dermatitis inducing phototoxins (usually in the form of tall water blisters) Poison Hemlock and Wild Parsnip would be much more likely culprits than Cowbane or Angelica. The former two being common weeds and the latter being species of rather intact natural communities (one upland; one wetland).

  9. Great post!
    I think the leaves of poison sumac are pinnately compound, and the leaflets have entire margins.


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