I have a idea to attempt to grow tropicals outdoors year round in US zone 7 , at least the more hardy ones
My plan is to dig a hole about 4' deep, amend the bottom 2' of soil as if it were the top layer in preparation for something like caapi or whatever.
Plant a well rooted caapi 2' down in the soil as if it were on the surface, Then put down a layer of permeable fabric cloth, and place a section of 12" diameter tubeing about 26" long directly over the center of the caapi
Backfill the hole with coarse sand up to about 6" from the surface, lay down a permeable fabric cloth over the sand and fill the rest of the hole with topsoil.
It should grow all spring,summer without issue, Then come the first cold weather the plant could be cut away about 2' down and the hole could be filled with a insulation material, like preformed expanding foam. A pile of leaves/ compost over it could not hurt either.
come early spring the insulation would be removed and the plant should grow back from the roots (in theory atleast)
A drip water system 2' down would likely help too
theory being at the depth the earth will be warm enouth to keep it alive, but cool enough for it to remain dormant . . .