Location, Location LOCATION is the big deal with overwintering. Species, species SPECIES is the other top overwintering issue. All this depends on what USDA Zone you're in and what kinds of trees you have. We can't help you without knowing what kinds of trees you're overwintering.
Also, one of the biggest mistakes beginners make is assuming they're trying to keep their trees "warm" during winter. That is the WORST thing you can do. You want to keep temperate trees AS COLD as they can stand. It's not about warmth. It's about inducing adequate dormancy and sustaining it.
A greenhouse is an extremely bad overwintering choice for most temperate zone trees. A small greenhouse is the worst variation of that. Greenhouses trap heat, which can spike root zone temps and delay or even prevent a tree from entering dormancy. A small greenhouse will be subject to the worst--spiking temperatures, followed by plummeting temps in cold weather.--the small the greenhouse, the less mass it has to sustain the heat it gathers.
Most folks in your area mulch temperate zone trees into beds in sheltered spots in their yards. A thick covering (-6-10 inches) of mulch over the pot on the ground, will maintain a more even temperature range for many trees.
This all depends on what kinds of trees you have, though.