Movable greenhouse trial

There are various uses for greenhouse-type structures on a farm like ours, from starting and hardening off seedlings to in-ground production within the protected confines of the structure. A wide variety of permanent structures can be purchased or built, depending on what type of growing and use the farm desires. As we don’t have much experience with growing under greenhouses or other types of hoop structures, we’ve been looking for a way to test such a system without spending a lot of money or erecting something too permanent. Enter the Washington State Extension service, with their Portable Field Hoophouse publication.

This design is not portable in the day-to-day sense, but is not permanent. It’s intended to be set up and taken down in a few hours, such that it can be moved once or twice a year, or taken down at the end of the year for storage. It’s intended specifically for small farms who need smaller and adaptable types of hoop structures, and seemed to fit our needs perfectly. The authors estimate the cost of construction at $350 for a 10’x42′ hoophouse, which is far below the cost of a more commercial structure.

So we went ahead and built the thing to our specifications, making some changes and substitutions along the way as needed. Above is a rooftop photo of the completed 10’x30′ structure in our developing orchard area; if you look closely you will see this year’s asparagus beds marked in yellow flagging and our first blueberry row marked in blue.

The basic design is a set of PVC pipes bent into hoops and supported on rebar driven into the ground. The ends are constructed of lumber and corrugated plastic, braced into the ground. Greenhouse plastic is stretched over the structure and secured at either end, and along the structure (read the publication for details). It took us about 1 1/2 days to set up, including laying out the site and constructing the ends. Our costs were lower than the authors’ estimates, partly because of the shorter length and partly because we used all our own cedar lumber rather than purchasing any (makes it look nicer, and eliminates the need for treated lumber which wouldn’t be allowed on an organic farm anyway). The photo below shows the home-built ends with the PVC hoops and plastic inside.

We made a few changes to the authors’ plan, based on our experiences.

1) The plan calls for 1″ PVC, but the 1″ pipe I found at Home Depot couldn’t be bent by me even when I jammed it up against a storage rack, so I went with a thinner version that I could bend. It’s a bit floppy, but works so far.

2) The plan calls for drilling bolt holes through several parallel PVC pipes at the ends, then inserting the plastic between them to hold it taut. We tried this, but it weakened the structural integrity of the pipe too much, and involved putting an awfully big hole through the plastic, asking for rips. We adapted the plan by placing duct tape on both sides of the plastic anywhere we needed a hole, poking a very small hole with an awl, then tying twine loops around the pipes and through the reinforced hole. It’s not perfect, and we need to monitor it and look into other ways of securing the ends. UPDATE: since I first wrote and queued this, we’re finding that the string is slowly working its way through the plastic in a few places. Some form of change is called for, including possibly some form of installable metal grommet like a tarp.

3) The plan calls for 1′ rebar, but that seemed awfully short to me for really holding this structure down in Midwestern winds, so I used 2′. That was a good choice, as several strong windstorms since then have pulled one of the 2-footers out of the ground with its associated hoop, so I shudder to think where our structure would be if I’d stuck with 1′. I don’t think the small-farm district of Washington State gets the same winds and weather extremes Missouri does, and we tried to take that into account in planning our version.

4) We found that 4 sheets of 2’x8′ corrugated plastic were not enough to cover the ends, so we used some cedar planks to build up one end as shown in the photo. I think this looks better and is structurally stronger anyway, though it does add noticeable weight. The far end is built to specification.

5) We wanted to help conserve heat overnight in the structure, so set up several of our large 50+ gallon water tanks inside to hold thermal mass overnight. We’ll see how that works.

6) We made our doors from plastic sheeting with velcro-type fasteners sewed and attached to the sheet and the frame. This lets us seal the door more effectively against wind.

So far, we like the structure. It’s held up well to reasonably strong wind, and is doing what a greenhouse ought to in terms of warming the soil and air inside. Joanna has already observed the joy of working inside this thing while it’s raining outdoors. Some parts will be direct-seeded with greens, others used to grow and harden off transplants, and we’re planning to transplant some early tomato and pepper plants into the ground there. It’s basically our research lab for learning more greenhouse techniques in advance of planning and erecting larger, more permanent structures. We feel pretty strongly about trying things out cheaply and simply before diving in head-first. The few hundred bucks for this setup are well worth the experience we’ll get before laying out a real farm-scale green/hoop house complex.

I could easily see this working for home gardeners as well, as a simple, inexpensive, and removable option for advancing your growing. I’d love to hear from others who have tried it.

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