[Bruno]

Poking through GURPS Magic: Plant Spells (the green kingdom always like doing horrible things to us meatsacks) there's actually not much in there in the way of poison-y things.

Bloodsucking Branches (p11) is fun so I'm mentioning it, but it's about taking things OUT of peoples bodies, not putting dangerous things in.

Green Death (p14) isn't poison per ce (plants grow in your body until you suffocate and leaves burst out of you) but it's mechanically treated as a poison. You could steal the mechanics and just describe it as an actual poison instead. They choke from partial paralysis, throat swelling, or lungs filling with fluid, not throat/lungs filling with leaves.

Toxic Plant (p19) turns a random plant into a toxic-gas-spewing animated attack-plant. It's kinda indirect, but there's that.

Pollen Cloud is on the list with Stench - not exactly subtle, but hey.