Your dog probably has an allergy to the plants. My dog was allergic to EVERYTHING including human dander. So much so that I spent a couple grand on a specialist to get him pretty much cured. He would get rashes, lick, scratch, bite, you name it. I would take your buddy to the vet during an outbreak. Tell them what's going on and just omit the fact that you think your plants are causing it. I would bet a lot that the vet will prescribe you a low dose of prednisone which is a steroid. It will clear him up really fast. It isn't recommended to use it long term though because with long term use it can cause health issues. I found when using prednisone that I could use even less than the vet prescribed and it would keep the allergies at bay. It's sort of like a pain medicine...it's used as needed so it's ok to skip days, use less, etc...
Your vet will also probably suggest trying over the counter Benadryl which is completely safe for dogs. Do some research about dosage before you give your dog some because obviously the weight of your dog is less than a human so he will need an adjusted dose. Benadryl didn't work for my dog probably because his allergies were so severe but I've definitely heard of it working for dogs before. Good luck brother!!