Many gardeners use cold frames and quick hoops for season extension, but just beyond these is a simple and super-productive option for the home gardener: a small, low-cost, portable greenhouse. We’ve found that you can build a 10-by-12-foot greenhouse for less than you’d spend on a store-bought 4-by-4-foot cold frame. Our goals in designing this movable greenhouse were that it be simple to build with off-the-shelf parts, easy to move, easy to anchor and inexpensive.
Even gardeners in moderate or warm climates can benefit from a greenhouse, which gives you much more variety in your winter fare, and also makes the experience of growing it more pleasant. A greenhouse furnishes a warm and sheltered spot for plants, but because you can stand up inside of it, it also shelters you.
Similar to a cold frame, a simple greenhouse captures the sun’s heat and eliminates the drying, chilling effects of wind. You don’t even need to heat your greenhouse in winter if you plant hardy crops that are most content growing in cool weather. Come spring, you’ll get in those early crops even sooner than normal and you’ll transplant your warm-weather tomatoes earlier in the year. Then, sit back to wait for extra-early ripening.
Constructing Your Portable Greenhouse
The ability to move a greenhouse from one place to another will ease the seasonal transition from winter to summer and back to winter for all of the crops covered by the greenhouse. You can leave it over summer crops, such as tomatoes, peppers and basil, to safeguard them from fall frosts and keep them producing longer. Then you can move the greenhouse to protect cold-hardy crops that you’ve planted nearby so you can enjoy them well into winter.
With a portable greenhouse, you get the positives of greenhouse growing — namely cold protection — while eliminating the negatives, such as the pest and disease buildup that can occur in soil that’s continuously covered. In addition, you increase the number of crops that can be sheltered by one greenhouse by covering plants only when they need protection.
All that’s required to make a greenhouse mobile is a slight modification to its construction. (Four Season Farm provides step-by-step building instructions in Building the Modular Movable Greenhouse.) Normally, the standard pipe-frame, plastic-covered greenhouses stand on a foundation of pipes driven into the ground. Ours is firmly attached to the ground when it’s in place, but it can be detached for moving and then anchored again in a new location.
A greenhouse large enough to make a significant contribution to supplying your family with homegrown food year-round should be at least 10 by 12 feet. Our basic DIY greenhouse is just that size, and builders can double or even triple the length by adding on modules of the same size.
The frame consists of three half-circles of metal pipe attached to structural cross-pieces. A 10-foot length of pipe bends easily into a quarter-circle, and two of them form a half-circle hoop. We bend them the same way we bend our quick hoops, but we use a bender designed for high tunnels instead of low ones. For pipe, we prefer the 10-foot-long and 1-inch-diameter pipes used for electrical conduit (called “1-inch EMT,” which stands for electrical metallic tubing).
For the foundation of the greenhouse, instead of inserting the bottom end of the hoops into larger-diameter pipes driven into the ground, as with standard hoop houses, we attach the bottom of the hoops perpendicularly to a length of 1-inch EMT lying horizontally on the ground. With this setup, all parts of the 10-by-12-foot greenhouse module are connected as a single unit rather than having each rib individually attached to its own ground post. The greenhouse is thus like a metal-pipe, plastic-covered bird cage that can be picked up and transported to wherever you want it.
When the portable greenhouse is in place, we attach it to anchors to hold it there (they’re easily unattached for moving). The corner anchors consist of four lengths of top-rail pipe, each 2½ feet long. One is driven into the ground at each corner of the greenhouse, and each is attached to a U-bolt that secures to the base connectors.
We keep the plastic cover in place with form-fitting plastic clips that hold well even in wind. All of this works smoothly and keeps the price low. The weight of the pipes, the connecting parts and the plastic for this portable greenhouse add up to about 100 pounds. Thus the “pick up and move” part is doable for two reasonably fit and able-bodied gardeners. The two of us have moved this greenhouse many times with no problems.
More on Successful Greenhouse Growing
If you build your own greenhouse, you’ll love its benefits in winter. Achieve the greatest winter gardening success by growing crops suited to chilly weather. Read Best Winter Crops and Cold-Hardy Varieties for advice.
Check out the article Try Quick Hoops — Easy-to-Make Mini-Greenhouses for another season-extension option designed by the authors.
Barbara Damrosch and Eliot Coleman are two of the country’s foremost authorities on organic gardening and winter growing. This article was adapted from their latest book, The Four Season Farm Gardener’s Cookbook — a great resource brimming with growing advice and recipes for the vegetable enthusiast.