In the life of every author, there comes a time when you need to kill someone off. You can do it off-page so nobody has to watch, you can do it quickly, you can do it slowly, you can do it with some beautiful exposition at the end or with said character cursing the gods all alone. You can do it with weapons or accidents or other people or old age.
Or you can do it with a tree.
People like to talk about poisonous plants and how you can use them to brutally or sneakily or creatively murder your characters, but they don’t usually mention the plants’ taller, stronger cousin, the tree. Which can be every bit as deadly, and adds the dubious distinction of being somewhat hard to get rid of once you figure out how dangerous it is – tap roots can go down a long, long way, you know. You may cut the tree down, but it will be back. (Yes, I also heard the tree say that with an Austrian accent.)
Right now you’re thinking, you don’t eat trees. You don’t eat fruit on trees you can’t identify – hell, you may never have picked fruit from anything but a grocery store display bin in your lifetime! So you’re thinking you’re safe from stupid poisonous trees, because it’s not like they can kill you just by standing there outside of cheesy musicals and B movies.
Sorry to tell you this, but some of them can. 
Allow me to introduce you to the manchineel tree, also called the beach apple tree, which is found in tropical places like Central America, the Caribbean, and of course Florida. Luckily the people who live where manchineels grow know that nobody ever expects a killer tree in real life, so they paint big red Xs on the trees and sometimes put up signs warning people to keep their distance. People eat the fruit anyway, and it’s apparently very sweet and nice before your mouth starts to blister and your throat begins to burn and swell because manchineel sap is nature’s toilet bowl cleaner – seriously, if you stand under a manchineel during a rainstorm the water falling through the leaves will burn you, and if you park a car under the tree it will damage the paint. Burn the wood and it releases toxic fumes which can make you go blind. Touch any part of it and you’ll get blisters or even burns. If you decide to take a selfie with the warning sign you are nominating yourself for a Darwin Award. Apparently there are people who make furniture out of the wood, but I’m just going to assume those people are making furniture exclusively for supervillain lairs.
The manchineel may be the most poisonous tree in America, possibly in the world, but at least all it wants to do is kill you; The gympie-gympie wants to torture you until you kill yourself. Covered with fine stinging hairs filled with potent toxin, the lightest, briefest contact with the gympie-gympie can get you stung and initiate agonizing pain which can last for months. The hairs are so fine that they’ll go through regular clothing, so fine that breathing them in is also a possibility…and the only way to stop the toxin and with it the pain and swelling the toxin those hairs are pumping into you is causing is to remove the practically-invisible hairs from your skin. Locals and researchers alike in the rainforest areas of Australia and Indonesia where the gympie-gympie grows recommend using waxing strips as an ‘antidote’. Gympie-Gympie hairs can remain hazardous even when apparently dried out, and although the toxin itself won’t kill you directly (although your reaction will get worse with repeated exposure), an oft-repeated anecdote tells of an Australian officer who shot himself after accidentally using some gympie-gympie leaves as toilet paper. Whether he was real or not, can you blame him? I can’t.
Trees that will poison you if you eat them are actually pretty common, so I’m not going to recount the whole list in detail here – that’s what Wikipedia is for. A few notables, though:
- English Yew, made famous by Robin Hood and once used to make longbows, is entirely poisonous, quick-acting, and has no antidote.
- Dumb Cane, i.e. one of those tropical-looking trees-in-a-pot you buy to put in a corner of your family room, can make your mouth and throat swell so much it will strangle you.
- Chokecherry leaves and seeds are full of cyanide – not necessarily enough to kill a human (we make jam out of the berries, after all) but a common cause of sudden death in ruminants like cows, sheep, goats and deer.
- Leaves and seeds of peach, plum, cherry, apricot and almond trees all contain cyanide as well. Apple seeds have it too, but you’d have to eat an awful lot of them – herbal medicine practitioners say the lethal amount is a full cup of seeds eaten all at once.
- Mango stems, leaves and sap contain the same irritant (urushiol) found in poison ivy. The poisonwood tree also contains urushiol, but lets the sap ooze out of its bark just to taunt you.
- The adorable hat-wearing acorns which fall off your local oak tree are full of tannins that will sicken a human and can kill a cow within a week.
- The Otholanga – also called the ‘suicide tree’ – has seeds so poisonous just one can kill you and is responsible for more deaths (murders, suicides and ‘trials by ordeal’) than any other plant.
- The wilted or dry leaves of a Red Maple can stay toxic to livestock for up to 4 weeks after falling off the tree – and vets aren’t entirely sure what the toxin is.
- And if you put Black Walnut wood shavings in your horse’s bedding you’ll give the horse laminitis (painful inflammation of the hooves) – and again, vets don’t know what exactly the toxin is.
I hope this was as much fun for you as it was for me and you might find it helpful if you end up needing a creative way to commit charactercide, and that you haven’t just vowed to never go outside again ever. Most trees aren’t actively trying to kill you, honest. 😉