It stuns them so that they can’t move as soon as they come into contact with it, and inflicts heavy damage on them round by round until they die. It works on every kind of triffid in the game as far as I can tell, including the heart itself. I found triffid hearts to be the most challenging areas in the game before I started using teargas; now they’re no problem at all as long as I have a safe way of breaking down the root walls.
I definitely can’t take credit for thinking of trying teargas against triffids on my own, I must have read it somewhere at some point. Possibly here on the forum, I’m not sure.
Teargas kills both the vines and creeper hubs. It kills the hubs, but not the vines. If you’re a good marksman you can shoot past the vines to the hub without too much difficulty, but it’s not ideal and the hub may be around a corner with vines flooding the only approach. I used to use guns against triffids. I found using melee against the vines tends to involve taking a lot of damage. I’d never go back to using bombs to clear out the heart, it’s too dangerous and imprecise.
Teargas is really the most situational weapon in the game that I can think of. Setting triffid queens on fire is also fun to watch, but fire is useful in a lot of situations.