I have a very geekish reason for wanting to know this, but I will not bore you by telling you about it. Suffice to say that I recently became interested in the offensive and lethal possibilities of botany. I gots a question, I asks the smart peoples. So, what’s the really man-killer among the leafy set?
My best guess, and although not a single plant, would be falling lumber.
Tobacco.
Followed closely by opium.
So, druid or ranger?
I’m guessing you want something faster-acting, though…
There’s so many toxic plants to choose from, its probably quite difficult to say which is the most dangerous.
Two plant natural products that are extremely toxic are strychnine , from the seeds of strychnos plants, and fluoroacetic acid (quite widely distributed). That should get you started, on whatever it is you’re thinking of starting.
Why do you want to know this again?
alkaloids. That’s the term I was thinking of earlier…
Audrey II?
I’m a comic geek, so I was trying to think of a way to make a plant-controller actually scary and dangerous. It always seemed like a power with some potential and depth to it, really. That and CoV. 
Also, I just like to know odd information.
Ricinus communis (the castor bean) has to be somewhere up there near the top slot; the toxin Ricin extracted from this plant is very very deadly - a single molecule of the stuff is apparently enough to trigger a metabolic cascade in a living cell, killing it.
Thunder-stealer 
Oh. Duh. Triffids.
Search function is your freind.
And as I said in that thread, it’s like asking what the deadliest animal is. In what context? Under what circumstances? There are any number of plants that are crammed with deadly poisons that will kill with the ingestion of even moderate amounts. There are also numerous stinging plants around the world as well as plants that have poisonous sap. The you have to factoring that some poisons have antidotes and treatments and some don’t. Very subjective term ‘deadly’.
There are innumerable plants that produce nicotine, ricin, fluroacetate and other compounds that are poisonous in relatively small doses, But they are also present in very small doses. A character that can control plants simply won’t get any mileage out of these things because you need to eat them in failry large quantities to be poisonoed. Even the castor bean isn’t nomrlaly fatal if swallowed because the subject simply vomits.
In contrast a stinging tree will have some potential to cause pain, posisbly even incapaciating pain while a bausor tree could well kill if the sap entered the bloodstream. But personally I’d go for something like boxthorn that simply causes damage by having lots of spiky bits.
Now if you could hybridise a bausor with a boxthorn…
According to the U.S. Government:
Cannabis sativa
It would certainly be the Upas Tree (Antiaris toxicaria) - if it actually existed.
I have always been under the impression that Aconite (monkshood) was the most common of the *really * poisonous plants (at least in Europe).
I was intrigued by the description of the Giant Stinging Tree in Blake’s link. It reminded me of a ‘Headache Tree’ they have in the gardens at Heligan - this was a big country house garden that was sort of forgotten for about 80 years or so. During the process of restoration they ran across one tree that managed to render the tree surgeon working on it unconscious. Apparently even pruning it is enough to give you a splitting headache. Wouldn’t fancy having to work on one of those stinging numbers though… :eek:
I belong to a citizen’s militia group, and a few months ago we had a class on “edible and medicinal plants.” It was taught by an expert in the field. Great class. 
While we walked around our property, the instructor would point out various plants that could be used for food, medicine, making cordage, etc. She then found a Poison Hemlock. “This is the deadliest plant on your property,” she said. “It can be used as a deadly weapon… just chop it up and throw it in your enemy’s water supply.”
While Poison Hemlock may not be the deadliest of plants, it is fairly common in the U.S.
More info:
The image on that page needs to be captioned, “And he scores!”
- Tis just linked to the image, so safe to click.
Dunno; Yew would also have to be up there, as would some of the nightshades. There are also some serious contenders amongst the fungi (though they are not strictly plants), notably in the Genus Amanita.
That website also needs to get a grip. While they’re correct about the toxicity of hemlock, they also list St. John’s Wort as a highly toxic plant, and state that it no longer has any accepted medicinal use because of it’s photosentive properties. Which is bunk. You need to take a whole lot of SJW - far more than commonly recommended doses - to get any photosentive effects at all - and those are tingling and rash which goes away as soon as you reduce the dose. It’s recognized world 'round as being a useful medicinal plant.
They also have a short scare blurb on ergot claiming that it’s a “natural source of LSD”, which then links to a page on psychedelic mushrooms. None of these plants contain actual LSD, only some alkaloids which may produce some of the same effects (but are much more deadly.)
Back to the OP, while they’re not the deadliest in terms of speed, I’m partial to pokeberries and rhubarb leaves for their duplicitousness: both are deadly, but parts of the plants are edible and yummy at some stages of development. I’d think it’d be far easier to trick someone into eating late fall poke (poison) after you’d served them early spring poke (not poison), or rhubarb leaves after they’d safely eatern rhubarb stem. Unpleasant death, though. Your blood basically thickens and turns to sand.