In No Nonsense Vegetable Gardening we talk about using tunnels or unheated greenhouses to extend the vegetable gardening season.
Here’s what we say in the book No Nonsense Vegetable Gardening:
An unheated greenhouse or tunnel can take vegetable garden climate manipulation to the next level. They’re not warmer than a cold frame (unless heat is added) but can be made bigger. When the sun is shining, they trap heat from sunlight and become warmer than neighbouring parts of the garden. At night, as the air temperature drops, these covered areas remain slightly warmer as they hold in heat given off by the ground.
Tunnels are useful for three to four weeks and can be removed as the plants grow and outdoor temperatures increase. Like cold frames, unheated greenhouses and tunnels can benefit from an overnight cover when spring temperatures are cool.