Greenhouses can arguably be considered an essential component for market gardens. Ten years ago we started off with a simple 10’x12’ homemade greenhouse for starting our transplants. We now have six commercially built, steel framed houses (12,500 sq.ft. in total) with another 30’x96’ poly covered steel structure planned for early next spring.

At present, we heat only half of one 20’x96’ greenhouse in the spring for transplant production. The rest of the greenhouses are used as “cold frames” for summer heat-loving crops (tomatoes, peppers, long english cucumbers) and extending the season into early spring and late fall for a whole host of other vegetables – everything from radishes and spinach to carrots, beets and even potatoes.

It seemed that we never had enough space to grow the variety of crops we wanted, particularly for the early spring market. And yet several of our cold frames were empty for a good part of the growing season. The problem was that if we planted a house with an early spring crop of lettuce, carrots and beets for instance, by the time those crops were harvested it would be too late to plant the peppers. Conversely, as the tomatoes were being harvested until mid-October, that cold frame could not be utilized for extending the season into winter with cold tolerant greens.

So we started thinking about how to make better use of our greenhouses. The idea for moveable greenhouses came from Eliot Coleman’s The New Organic Grower, the reference book for many market gardeners. After contemplating his rather complicated looking designs for a few years, I decided to attach the frame of our next greenhouse to simple angle iron skids. We were so satisfied with the benefits of being able to move the house that we’ve put two more on skids and will do so for the next one as well.

Each cold frame now starts off the season with a wide variety of early vegetables giving us at least a 4-week jump on those same crops grown outside. By around mid-April, those early vegetables are approaching maturity and no longer need protection so we drag the cold frame (one tractor on either side) ahead one length and immediately plant the tomatoes, peppers, etc. into that protective plastic-covered tunnel.

By mid-August we begin seeding our late fall and cold tolerant winter crops into the original location of the cold frame. Around the middle of October, the house is then dragged back, over the already established crops, to its original location allowing us to harvest fresh greens until late December and, in the case of corn salad, starting again in early March.

One added benefit is worth mentioning here. Being able to move our greenhouses has eliminated our constant battle with couch grass creeping in from the edges. The couch grass can never really get established before being exposed to cultivation while the house is on its alternate location. I also seed aggressive green manures on those strips which eventually choke out the offending weed.

The end view diagram shows most of our design details. We’ve found that 3 foot long rebar anchor pins pounded (on a slight angle) through loops that are welded to the skids at 10 foot intervals are sufficient to prevent our houses from lifting off our heavy soils during windstorms. If you have a light sandy soil or are located in a wind-prone area, you may want to double the number of pins. Another option, being implemented by a friend who lost three houses to a ferocious windstorm this past year, is to use “grape anchors” to anchor the four corners. “Grape anchors” consist of a 3 foot long, 3/4” rod with a loop at the top and auger-like fluting welded on the bottom. These are easily screwed into the ground and could be attached to the gable end arches with cables.

One final point to consider is the design of the end walls. Obviously if one wants to move a greenhouse over existing crops the end wall must be able to clear the tallest crop. Our end wall consists of two large poly “doors” (same material as the top sheet) attached to two vertical 2”x4”s that hinge on bolts at the top of the arch. The bottom of each vertical is held closed by a pin extending through to a wooden 2”x6” stake driven into the ground on the inside. Each door opens like a large curtain (see end view diagram) allowing maximum ventilation and easy access. Despite the simplicity of this design, the end walls have withstood all windstorms.

Moving day begins with prying up all the anchor pins with a steel bar and tying ropes from one side wall to the other on every 6th arch (24’ intervals), high enough to clear any crops. These ropes prevent the side walls from drifting outward. The cold frame is then dragged ahead to its new location with a tractor on each skid and the anchor pins are pounded back in again. If all goes well, two people can move one house per hour.

Hermann & Louise Bruns operate Wild Flight Farm, a 20-acre market garden near Salmon Arm in south central British Columbia, Canada. You can contact Hermann by email at wildflight@jetstream.net for more details. You can also view their farm web page on the BC Certified Organic farners website: www.certifiedorganic.bc.ca