Hoop houses (also known as “high tunnels”) are plastic-covered hooped frames tall enough to walk under that are used to extend the growing seasons of vegetables, flowers, and fruit. They’re also called “unheated greenhouses,” “polytunnels,” and “cold frames.” They can be used for year-round growing of seasonal crops, and although temperatures inside aren’t much higher than outdoors when the sun isn’t shining, hoop houses manage to facilitate impressive rates of growth and quality crops. Generally, hoop houses have no supplemental heating, and crops are grown directly in the ground. Hoop houses are often double-skinned, meaning they have two layers of plastic film, with a small blower keeping the space between the layers inflated. This provides increased insulation and improved strength against winds and snow or ice loads, and lengthens the life of the plastic by preventing flapping and abrasion. Provided winds aren’t strong, overnight winter temperatures in a double-layered hoop house can be about 8 degrees Fahrenheit higher than outside. Hoop house soil temperature rarely falls below 50 degrees in Zone 7a.
If you want to establish a hoop house of your own, here are some factors to consider before you invest in the time and effort, and ultimate reward, of having one.
Sun exposure is the first important factor to consider when siting a hoop house. Walk around your farm in midwinter, pacing out and flagging several potential sites. The sun will be at its lowest angle during this time of year, and shadows from obstructions will be at their longest. Hoop houses are solar-powered growing zones, so your site needs as much sunlight as possible in winter. Make sure you aren’t choosing a site in the shade. We looked at three or four possible sites at Twin Oaks Intentional Community, where I live and work, and assessed them against all the criteria described here. We chose the best site for winter sun, but then we had a last-minute idea to shift the structure 100 feet to the east for better drainage. It worked, but we underestimated the shade that would come from a group of nine loblolly pines about 100 feet or so to the southeast. In winter, a third of our hoop house was in shade after about 2:30 p.m. Over the course of two winters, we cut down those trees.
Hoop houses are usually oriented with the ends pointing east and west, and the longer sides facing south and north to get the most light. Some books recommend that greenhouses in the South be oriented the opposite way, with long sides facing east and west and ends pointing north and south. (See illustration, below.) I don’t think this is a good idea for hoop houses located where winter crops are most important, and sunlight is at a premium. If tall, vining crops will be your focus, having rows running north-south will provide the most even lighting.
Good soil with good fertility will be your goal. You can, of course, improve your soil as you use it. Just don’t start out with a boulder field, a gravel pit, a clay pan — you get the idea.
Your site will ideally have a 1 percent slope lengthwise from east to west or west to east. Warm-season, single-layered temporary structures, such as Haygrove tunnels, can accommodate steeper slopes, but permanent hoop houses won’t. You can adjust the slope of your site by a foot or two over 100 feet, but not much more.
The site needs to be fairly level from side to side. If you need to move a lot of soil to meet the ideal, be sure to preserve the topsoil. Search for “Growing All Seasons: High Tunnels” on the Natural Resources Conservation Service website. This federal agency also awards grants for some types of hoop houses through its High Tunnel System Initiative.
Your site will need to be well-drained, and you’ll also need to accommodate rainwater runoff from the roof. Our hoop house has a horseshoe-shaped V-profile moat around it. Some growers leave the plastic roof covering extra-long and use it to lead water away from the hoop house; and some growers whose hoop houses have roll-up or drop-down sidewalls choose to install plastic guttering on the hipwall lumber that these structures need, and then collect the rainwater for irrigation. The water catchment barrel will be low, so the water will need pumping or dipping and hauling to be useful.
Additionally, look for sites protected from the full force of the wind (at a non-shading distance). Choose a site with good frost drainage, not one at the bottom of a slope.
You’ll also need to consider road access and proximity to water and electricity. If it’s relatively easy to hook up to the grid, do so. You can decide later if it’s worthwhile to provide an alternative source of power. The inflation blowers will use very little electricity, so the financial cost will be worthwhile. You’ll probably want your hoop house near your house, barn, or main work area, where you can keep an eye on it and pop over for the odd half-hour of work. For winter harvesting, make sure your packing shed is nearby, as you won’t want crops to freeze in transit. Most growers don’t heat their hoop houses, but if you want to heat with wood, you’ll need to plan for access to firewood, or for an underground pipe from a furnace elsewhere.
After seeing the benefits a hoop house brings, it’ll be hard to resist building another! Alternatively, you can construct a wide hoop house that’s relatively short. Later, you can buy more hoops and bigger plastic and double the size of your original house. Allow at least 10 feet between side-by-side hoop houses; more if your area gets deep snow. For readers in the North, they must make a building two times the height of the building south of it.
What Size Is Best?
When determining the size of your hoop house, ask yourself these questions: How much food do I want to grow? Do I have markets lined up already? I’ve never met a grower who complained their hoop house was too big. Height at the peak varies from about 6 to 17 feet, width from 10 to 30 feet, and length up to 100 feet, which is the size of the most commonly found plastic film.
Consider the surface-to-volume ratio. You can get a lot more volume for not much increase in surface area. The cost of materials is related to the surface area. In other words, it pays to go as big as is sensible. But 20-foot-wide structures need less bracing than 30-foot ones, so the price of the frames for a narrower, longer hoop house may end up lower than for a wider, shorter one, even if they cover the same area. One hundred feet may be the maximum length that works without forced airflow. Having a good amount of fresh air will help your plants stay healthy. High windows in the end walls will help release stale, warmed air without chilling your plants at ground level.
Soil and air act as heat stores during sunny days in cold weather. At night, the heat is released, to the plants’ benefit. So, the more air and soil you have inside your house, the better, up to a point. Small structures don’t store much heat. Also, edge beds are usually colder, so the more center beds you have, the cozier the plants will be. Steve Moore, a high-tunnel pioneer, calculated that a 28- or 30-by-96-foot hoop house is the best size for heat efficiency.
If you have a preferred bed width, choose a hoop house width that works for that. We have a 30-foot-wide hoop house with five 4-foot-wide beds down the center, two 2-foot-wide edge beds, and 1-foot-wide paths throughout. Don’t make beds wider than you can cultivate comfortably. If you plan to drive a cultivating tractor through, fit the beds to the wheel axle space.
Is DIY worthwhile? Although some people advocate using PVC piping rather than galvanized steel, I don’t recommend it. PVC isn’t strong enough for large hoop houses; it degrades rapidly in sunlight; it reacts with the polyethylene sheeting and degrades both; and the manufacturing, burning, and ultimate dumping of PVC all release dioxins that cause a wide range of health problems.
Some people like to buy and bend their own steel tubing, but this is hard work on a large scale. For most of us, the price of pre-shaped galvanized steel tubing is worth paying. You might be looking at making 25 hoops from 25-foot-long tubes. If you’re a good grower and have the markets, your hoop house could pay for itself after the first year. Commercial frames have the advantage of having the same curve, fitting together, and being the right length.
Consider the snow loading and wind force you might experience. Hoops at 6-foot spacing are adequate for snow loads up to about 9 inches. I sleep easier with only 4 feet between the hoops. If you’re unlikely to get more than 9 inches of snow, use 6-foot spacing. If you’d rather be prepared for climate change and more extreme precipitation, go with 4-foot spacing. We researched past weather extremes at Twin Oaks before building and learned that there had been snow 32 inches deep and wind speeds with a maximum sustained velocity of 60 mph, with higher gusts. That was sobering, but we went ahead anyway. The hoop house is still standing 15 years later. Other growers tell me that winds in the 75-to-80-mph range will take out a hoop house.
Hoop House Shapes
Hoop house shape matters, especially when considering how it’ll function in your environment. The Gothic shape sheds snow more easily than the round Quonset shape. Gothic provides more headroom over more of the area, and also better sun angles. If you’re bending your own tubes, round is probably easier.
How much sidewall height do you need? You’ll probably want to be able to stand upright to work the edge beds, so determine the standing room you’ll need 2 feet in from the sidewalls.
Choosing a Supplier
Shipping costs can be considerable for heavy materials, so investigate local suppliers. Some brands are more strongly built than others. Check not just the cross-bracing and end-bracing provided, but also the gauge of steel tubing. The more snow, ice, and wind you get, the stronger the structure you’ll need. Gothic arch structures are stronger than Quonset shapes, and 4-foot hoop spacing is stronger than 6-foot spacing. Double-layered plastic is stronger than single-layered. Weigh all these factors, and also your budget and willingness to take risks. Think not only about the likelihood of collapse at your site, but also the other dimension of risk: the impact of collapse. Growers generally rebuild if their tunnel collapses. You risk losing your crops, but so long as you’re sensible in storms, you won’t lose life or limb.
Are you considering a movable hoop house? Remember that you’ll need to anchor the hoop house thoroughly. A movable hoop house can easily blow away in strong winds. Several growers I’ve heard from built hoop houses with the intention of moving them several times a year to cover a series of crops; however, because of its inconvenience, they now only move them once a year.
Do you intend to hang things from the rafters, or train plants up twine or wires anchored to the rafters? If so, check with manufacturers, and get a hoop house with strong rafters. Most frames aren’t designed to support the extra weight of crops. An alternative is to install a separate support framework in the ground.
Double- or Single-Layered?
Single-layered plastic doesn’t retain heat at night; indoor temperatures will be much the same as outdoors. Double-layered plastic gives an 8-to-10-degree difference. This really counts in Zones where the crops are growing during winter. Both spinach and kale grow whenever the temperature rises above 40 degrees. This will happen more often in a double-layered structure. The extra expense is covered by the extra income from the faster-growing crops.
Double-layered plastic is stronger against wind and the weight of accumulated snow or ice, as the whole shape pushes out against the load and spreads the weight over the whole area of the hoop house. In Iowa, a double-layered structure withstood 80-mph winds, because the “bubble” stopped any ripple effect on the plastic. Double-layered plastic also likely reduces the light that reaches the crops. Some growers may debate about single versus double plastic. A 1 percent reduction in light transmission results in about a 1 percent reduction in yield. This must be balanced against the yield loss that could come from a single-layered hoop house with colder nights.
Nick Calabro from Klerks Hyplast (a source of hoop house plastic film) reported on research into Photosynthetically Active Radiation (PAR) light transmission through various combinations of film. (See table, below.) At Twin Oaks, we use regular 6-mil outer plastic and inner infrared (IR) plastic, and we’re happy with our choice. The inner plastic also has condensate control, meaning condensation doesn’t gather and “water-bomb” us or our plants. Another option is Solarig from Robert Marvel, a woven poly film advertised as being 10 times stronger than regular polyethylene film. It won’t tear in windstorms, even when punctured. It comes with a 4- or 6-year limited warranty, depending on type. It transmits 88 percent of the light compared with standard poly, and about 45 percent of the light transmitted is diffused, which reduces plant burning and evapotranspiration, and claims to increase photosynthesis and increase yields. I haven’t yet seen this material in use. It has condensate control and IR additives and UV blockers. It can be used as a single layer with strapping over the top, or as a double layer, using the lighter version for the inner layer.
A newer possibility without inflation that still has the advantages of a double layer is SolaWrap, a strong material that looks like bubble wrap. It comes in 4-foot-wide, 5-foot-wide, and 6-foot-wide rolls, so it must be fastened to every bow using specialized channels. It provides 83 percent light transmission and 83 percent light diffusion.
Double plastic generally requires an inflation system. If the site is too remote for grid access and other options are too costly or complex, then a second layer isn’t workable. Three off-grid power options for running inflation blowers are solar with batteries, air-driven motor, and wind power. Another option is to use plastic foam blocks attached to the bows to preserve an air gap between the two layers without any inflation. This will give some of the insulating value of inflation, but not the increased structural strength.
Planning and constructing a hoop house comes with its share of research and decisions, but a properly planned hoop house will be a valuable asset to any farmstead business for years to come.
Pam Dawling has grown produce for 100 people on just 3-1/2 acres as the garden manager at Twin Oaks Intentional Community in Virginia. This is excerpted from her book The Year-Round Hoophouse: Polytunnels for All Seasons and All Climates (New Society Publishers), available below.
|Photosynthetically Active Radiation Light Transmission Through Polyethylene Films|
|Type of Film||% PAR Transmitted|
|Single-layered standard 4-year 6-mil (0.15 mm) polyethylene||92%|
|Double-layered standard 4-year 6-mil (0.15 mm) polyethylene||85%|
|Double-layered: outer, standard poly; inner, Infrared Heat Retention (HR) poly||81%|
|Single layer of Infrared HR poly||88%|
|Double layer of Infrared HR poly||77%|