Poison works inside the body when consumed, toxic works outside the body. It should be toxic in this case, technically...
Well, if we are going with definition, it's not wrong to say the plan is "poisonous". As a poison is any chemical subtance that create an impact on bodily functions in organisms either sythesized or accumulated. While toxins are chemical subtances being produced through biological means, as in synthezised by organism. Which means, "toxin" is "poison" with a narrower definition.
However, when talk about "toxic", it's more likely than not refering to "toxicants", which are chemicals produced by human activity and can get into your system without you invoking an action on it other than what you are already doing every where else, outside of the zone it has the chemicals. Here though, you have to touch the plant so, it's definitely not "toxicant" and can be said as "poisonous".
As a bonus, a creature is "venomous" when it bites/sprays/stings or in other words, attack you, but it's "poisonous" if it's touched/biten/consumed or in other words, being the object of your actions. So yeah, this is definitely a "poisonous" case.
Weeds & PoisonAh, those are highly poisonous. Just touching them would land you in dire peril.Fufu! This area is just chock-full of edible plants♪Okay then! What about the stuff growing over here...?That's a weed... It doesn't hurt to eat it or anything, but...DECHI!?So, can you eat this one here?