9.16 Cold Frames and a Small Hoop House

Difficulty: Easy (cold frame) to Moderate (hoop house) · Time: about 2–4 hours (cold frame); about 1–2 days (8×12 feet hoop house) · Cost: about $0–$80 (cold frame from salvage) / about $150–$450 (hoop house)

A cold frame is a bottomless box with a clear lid. It traps the sun’s heat and adds weeks to each end of the growing season, which matters most in short-season, high-elevation country. A hoop house is the same idea at walk-in size. Neither one heats anything at night. They only hold in the day’s heat and keep frost and wind off the plants.

What you need

• Cold frame: an old window or a sheet of polycarbonate (about 3×4 feet), untreated lumber or cedar (2×10 or 2×12 boards), screws, two hinges, a stick or adjustable prop to hold the lid open
• Hoop house: about ten 20-foot lengths of 1/2-inch or 3/4-inch PVC or EMT conduit (or cattle panels), 2–3 feet rebar or pipe stakes, a 6-mil UV-rated greenhouse film (not hardware-store drop cloth, which breaks down in one season), wiggle-wire channel or furring strips, a ridge pole, lumber for end walls and a door
• Thermometer (min/max type is best)

Tools: cordless drill with bits, a handsaw or circular saw, tape measure, square, screwdriver; for the hoop house, a hacksaw or pipe cutter, a sledgehammer for stakes, a conduit bender if you use EMT, a utility knife, and a staple gun or wiggle-wire channel.

Steps: cold frame

1. Pick the spot: full sun, facing south, ideally against a wall that blocks north wind.
2. Build the box to fit the lid. Make the back about 6 inches taller than the front so the lid slopes toward the sun and sheds rain and snow.
3. Hinge the lid to the back board. Pre-drill the screw holes (smaller than the screw) so the frame doesn’t split; on polycarbonate, drill slightly oversize holes so it can expand. Add a prop so you can open it partway.
4. Set it on level ground and bank soil or straw around the outside to stop drafts.
5. Plant cold-hardy greens (spinach, lettuce, kale, chard, green onions) directly in the soil inside, or set containers in it.
6. Vent it every sunny day. Open the lid a few inches once the inside passes about 70 degrees Fahrenheit, and close it in mid-afternoon to trap heat for the night.

Steps: hoop house

1. Mark a level rectangle (8×12 feet is a manageable first size) with the long side running east to west.
2. Drive stakes every 4 feet along both long sides with a sledgehammer, leaving 12 inches above ground. Measure both diagonals; when they match, the base is square.
3. Bend hoops over and slip each end onto a stake. Tie a ridge pole along the top of every hoop.
4. Frame the end walls with lumber, including a door on one end and a vent on the other.
5. Pull the film tight over the hoops on a calm day. Fasten it to a base board or channel on each side, and bury or weight the extra edge.
6. Vent with the door, end vents, or roll-up sides. On a sunny day, even in winter, it can pass 100 degrees Fahrenheit within an hour.

Safety

• Overheating kills plants faster than cold. Vent on every sunny day.
• Anchor against wind. A loose hoop house becomes a sail. Use stakes at least 2 feet deep, and add ground anchors or sandbags in windy sites.
• Snow: knock snow off from the inside or brush it off before it piles up. Wet snow can flatten the hoops. Steeper, closer-spaced hoops shed it better.
• Old windows may have lead paint. Scrape and repaint, or use polycarbonate. Tempered or plastic glazing is safer where kids play.
• Use untreated or naturally rot-resistant wood where the box touches food soil.