Husband-and-wife team Eliot Coleman and Barbara Damrosch are gardening experts known the world over for growing fresh produce all winter long in the harsh climate of rural Maine, without using any supplemental heat for their plants.
Their secret? Using cold frames, hoop houses, and row covers to create protected microclimates that are much warmer than conditions outside. We’ve talked about how Eliot Coleman employs these ingenious techniques at Four Season Farm, and in this article we’ll teach you how to build your own cold frames and hoop houses to harvest food all winter long in your backyard garden.
Caution: A Word on Wood
Before we jump into building techniques, let’s talk about materials. Many online building plans for cold frames and hoop houses call for rot-resistant or pressure-treated wood, which is treated with highly toxic chemicals such as arsenic that can leach into soil and contaminate plants. You’re better off using untreated wood and weatherproofing it yourself with non-toxic wood preservative, or using a rot-resistant wood like cedar.
Says Coleman, “We use standard pine or spruce for our cold frames. We purposely do not use treated wood, nor do we treat the frames with a preservative. Even the supposedly safe products should not be used in close proximity to food crops.
Wood rots where it is in contact with the earth, however, so we attach a strip of scrap wood about an inch thick to the bottom edge of the frame where it touches the soil. In a few years, when this strip begins to rot, we replace it with another. The rest of the untreated wood frame will last for many years.”
Eliot Coleman refers to the cold frame as “the magic box,” since it’s a ridiculously simple innovation that allows you to harvest vegetables all winter long. “It’s the simplest, most flexible…low-tech tool for modifying the garden climate,” says Coleman.
“It’s simple because it is basically a box with a glass top and no bottom that sits on the soil. It’s flexible because it can be made as long, as wide, or as tall as the gardener wishes.”
Cold frames slant diagonally, with the back board higher than the front, and face south to catch the winter sun. A good rule of thumb is to make your frame slope one inch for every 12 inches that it is deep; so if the frame is 4 feet deep from front to back, the back should be 4 inches higher than the front. Some gardeners build cold frames that slant at a steeper angle, like 45 degrees, but this isn’t necessary unless you live in a place with an extreme northern latitude, like Alaska; and Coleman says that plants are insulated better from the cold if the glass lid isn’t too far above the plants.
Here is the remarkably simple recipe that Coleman and Damrosch use for building a cold frame:
- Use three 8-foot boards (1- or 2-inches thick), two of them 12 inches wide and one 8 inches wide.
- The 8-inch board will form the front wall, and one of the 12-inch boards will form the back wall.
- Cut the other 12-inch board in half to form two identical 4-foot pieces; these will form the sides.
- Make a diagonal cut on each 4-foot side piece so that the height slopes from 12 inches down to 8 inches.
- On a flat surface, assemble all four boards into a box with the cut side of the diagonal boards facing up.
- With the boards assembled, you’ll see that the bottom edge of the frame is flat, while the upper edge hasslight discontinuities where the diagonal cut board meets the front and back walls. So that your glass pane can sit on the flattest surface, just flip over the box when you place it in your garden. Any discontinuity is then hidden by contact with the soil.
What Can You Use for Lights?
The glass part of the cold frame is referred to as the “light,” since that is where the light comes in. For lights, you can use old storm windows if you have any laying around; if you don’t have any, check garage sales, Craigslist, or your local Habitat for Humanity ReStore for old windows that people are getting rid of. (If the windows are very old, make sure you remove any paint that could be lead-based before you use them around your garden.)
Of course, storm windows were meant to be used vertically; when you use them horizontally the wooden crossbars holding the panes inhibit the flow of water and the trapped water can weaken and rot them until they start to drip onto the plants below. You can avoid this problem by skipping the storm windows and using completely flat panes of glass. You can buy pre-cut panes of glass, or if there’s a certain size you’re after you can get panes cut at a glass shop or a hardware store (Home Depot recently discontinued this service, but Lowe’s will cut a piece of glass to size for you if you bought it from them). You may want to get double-strength or even tempered glass to withstand the weight of snow if you live in an area with heavy wintertime snowfall.
Of course, your lights don’t have to be glass at all. You can use corrugated fiberglass. You can also use plexiglass, Lexan, Lucite, or another window-strength plastic. You can even stretch polyethylene plastic sheeting across a wooden frame and use that.
Cold Frame Plans and Videos on the Web
The designs and ideas for building cold frames that you can find online are endless. Some are as simple as Coleman’s basic setup; others are a little more complicated. Here are some of the best:
Martha Stewart invited Barbara Damrosch herself to The Martha Stewart Show to show viewers how to build the simple, straightforward setup used at Four Season Farm; you can watch the demo in this video, and on Martha’s website there are detailed instructions to building the cold frame.
This Old House
This Old House has an excellent, step-by-step tutorial for making a cold frame from a salvaged storm window. There are photos of every step or, if you prefer, a video to follow so that you can’t get confused. This model is a little more elaborate and complicated to build than Coleman’s, with reinforced corners and sturdy hinges holding on the storm window. This plan instructs builders to rest the cold frame on paver stones to create a sturdy foundation, but this isn’t really necessary; in fact, cold frames provide better insulation from the elements when they’re sunk into the dirt a few inches.
Organic Gardening’s article on making your own cold frame isn’t actually a step-by-step tutorial, but it details all the different materials you can use to construct the sides of your box (cinderblocks or concrete or hay bales, anyone?) and the best ways to insulate your winter crops if you’re gardening in a very harsh climate, including sinking your cold frame into the earth and piling soil, leaves, or wood chips around the outside of your frame to hold in heat.
Mother Earth News
Mother Earth News has an article on making an inexpensive cold frame out of polystyrene, as well as plans and instructions to build a budget cold frame out of just one sheet of plywood and some plexiglass. (We recommend swapping out the exterior plywood for interior plywood and then painting the outside with low-VOC paint to make it weatherproof.)
A hoop house, or high tunnel, is an inexpensive, quick-to-construct greenhouse. Its basic parts are: ribs, which provide the framework of the house; stakes or ground pipes to secure the ribs to the ground; and plastic sheeting that stretches over the ribs and provides shelter to the plants inside. Depending on how you decide to construct your hoop house, you may have to frame and build walls and doors for both ends, too.
Building Your Hoop House
Ribs can be made from PVC, rebar, electrical conduit, or any other strong but flexible piping. Sheeting can be Visqueen or any polyethylene plastic sheeting. Greenhouse plastic contains UV inhibitors that protect it from the sun, so it lasts the longest—about three years. It’s fairly expensive, however, so many builders opt for basic 60-mil plastic from the hardware store instead (it will last about a year).
When you’re deciding where to place your hoop house, choose a spot that can get at least four hours of sun per day. Ideally, your house will run east to west, maximizing its exposure (along the long side) to the south winter sun.
Whether you build your hoop house out of PVC, rebar, or something else, make sure that the arc of your “hoop” isn’t too wide—otherwise it could collapse under a heavy blanket of snow.
You’ll drive rebar stakes or PVC pipes into the ground at approximately 3 to 4 foot intervals, and then you’ll bend and secure the ribs and stretch the plastic over them.
Securing the Plastic
Once you stretch your plastic sheeting over the ribs of your hoop house, there are several methods to secure the bottom of the plastic. Eliot Coleman recommends digging a trench and burying the plastic in the dirt; once the ground freezes, the plastic won’t budge, and once warm spring days hit and you need to vent the hoop house you can just unbury the plastic.
Some plans for hoop houses involve securing wooden beams along the bottom edges and securing the plastic to those; another method is to simply tie the plastic down with rope, as seen in the Simple Gifts Farm video below. The important thing is that the plastic is snug and tight, because if it’s loose and flapping in the wind it will get tattered and worn much faster.
In any permanent greenhouse, there will be a build-up of pests, diseases, and excess nutrient salt levels in the soil. Some gardeners combat this by removing and changing out the top 18 inches of soil every few years. But there is a much easier solution: just move the greenhouse.
With a little planning, you can construct your hoop house on wooden skids so that it can be moved at least once a year to expose the soil underneath to the cleansing effects of the sun. See Coleman’s video below to learn how to do this.
Hoop House Plans and Videos on the Web
The Door Garden
One of the best tutorials on the web for hoop house building is David LaFerney’s 50 Dollar Hoop House on The Door Garden. His photos illustrate exactly what to do step-by-step, and he gives detailed instructions on the trickiest part of hoop house construction: framing the end pieces and doors. He has posted updates and improvements to his original design and updates on how the hoop house is faring. This is a must-read for anyone preparing to build a backyard hoop house.
Eliot Coleman on YouTube
In the video Eliot Coleman’s Tomato Tips & How to Build a Greenhouse, Coleman explains different options to use for hoop house ribs, including electrical conduit and rebar covered in plastic piping. He demonstrates his technique for burying the edges of the plastic to secure it to the ground, the use of rope to add stability to the structure, and the use of gardening clips to help secure plastic. He also shows ways you can vent your hoop house and make it mobile. The video is about 25 minutes long; the first part covers tomato-growing tips, so start the video at 11:30 for all the information on hoop houses.
Mother Earth News
Always at the forefront of innovations in sustainability, Mother Earth News has a great step-by-step guide for hoop house building that contains helpful insights and tips that are hard to find anywhere else. (For instance, did you know that there is a chemical interaction between PVC and polyethylene that will cause polyethylene to degrade? So if you’re using PVC ribs for your hoop house, you should line it with tape wherever it will touch your plastic sheeting so that the sheeting won’t fall apart prematurely.)
This guide includes an exceptionally helpful table of hoop pipe guidelines that tells you what diameter of rib piping you should use for the width of hoop house you’re building, and what diameter of pipe you should use for the ground stakes. For example, if your hoop house is going to be 10 to 12 feet wide, you’ll need ¾ inch pipe cut to lengths of 19 feet for the ribs, and 1 inch pipe for the ground stakes.
Simple Gifts Farm
In this basic YouTube how-to video, you can see, start-to-finish, the process of building a simple but effective hoop house, including tying rope onto the frame for stability and tying the plastic down with rope. In this video you can see how to side-ventilate your hoop house using spring clamps to hold plastic up off the ground.
This hoop house isn’t built onto a wooden frame; it’s staked directly into the ground, so it’s not portable. But the whole thing is so simple that it goes up in a couple hours, so it wouldn’t be hard to take it down and re-erect it if you needed to move it. Instead of framing end pieces and doors, this video shows you how to simply tie off the plastic on both ends so that it’s secured and out of the way.
Smaller Hoop Houses on Raised Garden Beds
If you already have raised garden beds, you can install little hoop house covers on your raised garden beds, as described in this DIY network article and this YouTube video/tutorial from Buddy Club Gardening.
Garden Fork also has a great step-by-step video showing you how to make smaller hoop houses that you can set on a raised garden bed or directly on the ground.
Coleman, Eliot. Four-Season Harvest, White River Junction, Vermont: Chelsea Green Publishing, 1999.