I don't have a lot of faith in the durability of the soft, tent type greenhouses. My plan this year is to buy a few of these rigid, twin-wall polycarbonate panels. I'm going to frame them with 2x4 (or lighter 2x3) then build four walls and a roof over the tree with a few potted trees in there....
I figure once built I'll be able to bolt the whole thing together in minutes.
The twin wall will provide more R value and allow you to start earlier and keep it out later.
EDIT : Triple wall is out there if you want more R value... May be pricey though...Haven't seen it at my plastics place...