I am having a problem with these giant plants that spew noxious gas. They totally wipe out my party. Could someone tell me how to combat these things.

Yeah, worst enemy in game. Only way to kill them is to keep distance and shoot spells.

If you have to fight only one plant, you can kill it using this strategy:
-Keep your distance and wait for it to approach
-When the plant starts breathing run towards it and strafe trying to avoid the vapors
-This should get you close to the plant, and away from the vapors
-Beat the hell out of it (the plant rarely uses its breath when you get this close)
Hope this helps...Good Luck..

Yes a real pain as they always attacked me when I was in a battle with a mob. Make sure its daylight have lots of manna potions for your wizard I used dragon fire and found 3 to 4 hits killed them as soon as you see them fire off the spell and back off. If plant stays put move forward again and fire again if its following fire and back off again. I think there are only three but not sure.

Run by them (it may take a few attempts & you need room to escape), then fight & run, as I will assume you are not that powerful yet, using as the previous post said, your fire spells. Do NOT fight them hand-to-hand, you will lose every time. Also, please be aware that they will follow you no matter where you go albeit slowly. So deal with them as quickly as possible. Good luck.