Know Your Vines - Television Tropes & Idioms

A character is in a very nature oriented setting, with all kinds of wild plants surrounding them. They're doing whatever it is they planned to do, and at some point pick up or walk into an ordinary looking plant. After a small amount of contact with it, someone who is more knowledgeable about nature will casually inform them "That's poison ivy" (or poison oak, the plants have the same toxin).

Note: These are generally played for laughs. If you're weak of stomach, do not Google the types of injuries these plants can create (Unless you'd really like to see an example of a blister the size of a football). Oh, and poison sumac, if burned, releases a gas that causes allergic reactions that can kill. It really pays to know your plants before you go hiking.

Examples:

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Comic Books

One Archie Comics story ended with the revelation that the corsage Archie gave Veronica to wear at the prom was poison ivy.
In an early issue of Archie Comics' Sonic the Hedgehog, Sonic was picking flowers to bring to Sally, only for a random background character to wander by and inform him that he's making his girlfriend a bouquet out of poison sumac. Then he starts scratching madly, which makes him swell up until he can't move. This, unfortunately, coincides with Robotnik making Pseudo Sonic, a robotic duplicate meant to infiltrate Knothole and discredit Sonic.

Fan Fic

The Legend of Spyro: A New Dawn: After going awhile into the forest temple, Sparx begins getting an unpleasent rash. Ember identifies the plants in the bush he passed through as poison ivy, which she knows because her mother taught her about it (not because Dragons get poison ivy, which they don't, but because eating it is really dangerous).
One Saved by the Bell fanfic, "The One Gigantic Fix", riffed on by Rob Bricken here, uses poison ivy as the reason why Zack has a permanent erection and thus needs some Deus Sex Machina to save him. Maybe.
Now,it might had been the fact that he had covered himself with something that might possibly had been poison ivy or that there was something in the water.I really don't know.

Literature

The Poisonwood Bible by Barbara Kingsolver: at one point, an American missionary in Congo ends up with tree sap on his arm and forehead, but pays it no heed. A local tells him "That tree, it bites", a warning he can't make sense of. The next day, he wakes up with a severe skin irritation where the sap was.

Live Action TV

After a huge fight and emotional breakdown with an unrequited love interest in a forest, the title character of Malcolm in the Middle dries his tears with some leaves he found. The girl he is with tells him he's using poison oak. We see a rather disturbing shot of his face a little later, which resembles the character No-Face in Twisted Metal Black.
In NCIS, while investigating a murder in a national park McGee has to search a patch of poison ivy for a murder weapon. Tony recognizes what the plant is, but decides not to tell him.
- To be fair, Tony had been telling McGee about searching in the outdoors, when McGee went off on him for treating him like a probie, and assures Tony that he learned plenty in Boy Scouts.
Stuart and Ann's camping misadventure on L.A. Law. Remember the TP next time.

Newspaper Comics

Calvin and Hobbes: after Calvin utterly fails a report on plants, he angrily asks what good it does to identify plants while holding a branch. Hobbes then replies "I believe that's poison sumac you're holding."
In FoxTrot, Jason and Marcus get sent on a night hike through poison ivy as part of an Escalating War of pranks with the girls. They compound the folly by deciding to use the leaves as camouflage.

Video Games

Unisys Icon game A Week in the Life of... had one of the characters in some hiking trip. The end of that character's storyline required the player to remember what poison ivy looked like, then required having him run through plants as a Timed Mission. Also, plants were placed more rapidly on faster systems, and were randomly chosen: slower computers are better.

Webcomics

This xkcd comic is the Trope Namer.

Western Animation

The early camping episode on Hey Arnold! ended with Big Bob Pataki rushing through a small bush blocking the path (after interrupting Arnold that they have to go around it) to get back to his campsite after a disastrous hike. This bush, of course, was poison ivy, causing him more misery.
Early in the movie Coraline, the title character brushes some leaves off a stick and uses it as a water rod. She meets Wybie, and has a fairly long conversation with him, and at the end he throws in that the stick she's holding is poison oak.
On The Simpsons, in a three story episode about history, Lisa as Sacajawea is giving Lewis and Clark's expedition party quick advice, and tells one soldier that he's holding poison oak.
Total Drama Island had Katie and Sadie both squat in poison ivy to pee. Hilarity Ensues.
On Kick Buttowski, Kick and Brad are lost in the woods. While Kick, who likes to rough it, sleeps on the bare ground, Brad brags about how Kick wished he had a comfy blanket of natural foliage like him. "You're right, Brad. I wished I had a blanket of poison ivy."
On Stoked!, Reef picks wildflowers for Finn and leaves them on her bed. Unfortunately, there is poison ivy mixed in with the flowers.
On My Little Pony Friendship Is Magic, during the episode "Bridle Gossip", the mane six walk through a patch of bright blue flowers to stop Apple Bloom following Zecora. The next morning they wake with embarrassing changes, such as Pinkie's swollen tongue, that they blame Zecora for. Eventually it is revealed that the flowers were 'poison joke' and the effects were pranks. Played by the plant.
On Animaniacs, in "Sound of the Warners", after using the bathroom in a bush, Dr. Scratchansniff gets an awful itch, because he was in a poison oak bush.
On The Wild Thornberrys, Nigel starts to get real itchy from what is believed to be poisonous plants, but he is immune to them because he rubbed some of them all over his body. It turns out that it was chicken pox, which he missed as a kid.
Camp Lazlo: "Raj? Why are you hiding in that poison ivy?"
In the Pac-Man episode "Backpackin' Packy", Pac-Man was giving the scouts a plant identification lesson when he picked up a leaf that he learned too late would give him a severe itch.
The Adventures of Sonic the Hedgehog used the same basic Pseudo-Sonic plot as the above-mentioned Archie Comics incidence (not sure which came first), although since in this incarnation Sonic has no girlfriend for whom he might be inclined to pick flowers, he just randomly blunders into an entire valley full of "Poison Flowers" while saving a character from a completely different threat.
In the second season of Avatar: The Last Airbender, Iroh has a run-in with some of these.
"Delicious tea?... Or deadly poison?"

Real Life

This is the main hazard posed by "Stalin's revenge", also known as the giant hogweed. It's a grass taller than most sapling trees. The poisonous sap/juice of this plant is really potent and causes burns comparable to mustard gas. It's also basically the Eastern European equivalent of kudzu, an imported plant that the authorities considered potentially useful but it spread uncontrollably all over the land. The worst part? It looks harmless, even beautiful, if you don't know better, as many city kids learn to their chagrin after a trip to the country.

Know-Nothing Know-It-All

Comedy Tropes

Kung-Fu Jesus