The following is an excerpt from The Winter Harvest Handbook: Year-Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses by Eliot Coleman.
We have added one new twist to a winter-harvest system by reviving an old European practice—the mobile greenhouse.
According to the best historical information I can find, the first mobile greenhouse was built in 1898 in England. Even though it was a large glasshouse, it could be moved safely because railroad wheels running on steel rails supported its iron framework. We have copied the mobile greenhouse concept, but on a far less expensive scale. I describe our mobile greenhouses in chapter 8.
The mobile greenhouse offers a number of advantages. First, it allows us to avoid the expense of having to cool the house when starting our winter crops in August. Instead, we sow winter crops out of doors in the field over which the greenhouse will move. Meanwhile the greenhouse continues to provide protection for heat-loving crops such as tomatoes, peppers, cucumbers, eggplants, melons, or sweet potatoes. We leave the greenhouse over the summer crops until the summercrop season is finished, sometime in mid to late October here in coastal Maine. Then, we move the greenhouse to cover the winter crops. The following October, the same process takes place in the reverse direction.
This idea, protecting hardy winter crops with a second layer inside an unheated plastic greenhouse, was pioneered in the 1950s by E. M. Emmert, a professor of horticulture at the University of Kentucky. For some reason no one picked up on Emmert’s innovation at the time: possibly because plastic greenhouses were so new; possibly because the concept seemed too good to be true. But it is more likely that growers were deterred by unfamiliarity with the day-length factor in winter gardening. Because of the slowdown in plant growth due to shorter days (coupled with cooler temperatures), winter crops have to be planted before winter. Timing the plantings is the key to success. The goal is to get plants almost to maturity before the day length becomes shorter than ten hours.
A second advantage of the mobile greenhouse is the avoidance of the buildup of pests, diseases, and excess soil nutrients, which can be problems in a permanent greenhouse. For one year out of every two, our growing beds are uncovered, exposing the soil to the cleansing powers of sun, rain, wind, and snow. As an additional advantage during the initial soil-building years, the uncovered year allows us to plant a long-term, deep-rooting, leguminous greenmanure crop on that section. This green manure can occupy the soil of the uncovered site for as long as thirteen months (June through the following July), if you forgo a summer vegetable crop, or ten months (September through July) if you sow the green manure toward the end of the summer-crop season. The benefits of green manures for protecting, enriching, and aerating the soil were an important part of our soil-fertility-building program in the first few years. All green manures should be turned under three to four weeks before the planting date of the crops following them.