When it gets 15' tall it shouldn't (in theory) need the extra care.. I might be wrong about your avocado but they get hardier with age and size (I have a Fantastic). One thing I do for my mangoes is (of course) put a tent around it.. if the temps are dropping below where I feel comfortable with just a tent, I'll string Christmas lights. Another thing I do is drape a sleeping bag on the ground around the base & trunk, and I throw a couple hand warmers in with the sleeping bag. You can get a pack of 10 from Walmart for like 5 bucks or something.. the kind that activate with air temps.. know the ones I'm talking about? They work great, last 8-10 hours and help keep the ground warm. Added bonus with the tent is that it contains some of the heat they give off. Some people use spaceheaters but I have too many trees to do that. Using the aforementioned measures has worked just fine :).

I should add, I don't use regular tents. I use the Nuvue frost tents they sell at home depot. They make them up to 52", and for bigger than that I use the Plankets with support setup around the trees so the material doesn't make contact with the tree.