Cold Frames and Hoop Houses: Extend the Season

GardenDraft Team · August 9, 2026 · 5 min read

Part of: Planting Calendar & Frost Date Guides

A cold frame or a low hoop house is a simple, cheap way to cheat the calendar — to start earlier in spring, harvest later in fall, and in many regions keep hardy greens alive right through winter. They work on the same plain idea as a greenhouse, scaled down to a single bed: trap the sun's warmth, block the wind, and create a pocket of milder climate over your plants.

Cold frame vs. hoop house: what each one is

Both are unheated season extenders, differing mainly in size and build:

• A cold frame is a low, bottomless box with a clear, hinged lid (traditionally an old window over a wooden frame). It sits over a bed or holds flats of seedlings, capturing solar heat by day and holding it through the night. Compact, sturdy, and easy to build from scraps.
• A hoop house (low tunnel) is a row of hoops, PVC, metal, or wire, bent over a bed and covered with clear plastic or row-cover fabric. It's quick to put up, covers a whole row, and scales from a few feet to the length of a bed. A taller walk-in version is essentially a small greenhouse.

Both rely on the sun alone, with no heater, which is why they "extend" the season rather than defeat winter entirely. They build on the same principle as row covers and frost protection, just with more enclosed warmth.

What they let you do

The payoff is a longer growing year at both ends:

• Start earlier in spring. Warm the soil and air weeks ahead, so you can sow hardy crops and harden off seedlings while it's still cold out.
• Harvest later in fall. Carry crops like lettuce, spinach, and kale well past the first frost.
• Overwinter hardy greens. In many climates, cold-hardy greens under cover survive the winter and resume growing in late winter for an early-spring harvest — the heart of cold-season "winter gardening."
• Harden off seedlings. A cold frame is the perfect halfway house for acclimating indoor seedlings before transplanting.

The one rule: ventilate

Here's the mistake that cooks more plants in cold frames than the cold ever kills: forgetting to vent on sunny days. A closed frame in bright sun, even in cool weather, heats up startlingly fast — hot enough to wilt or kill the plants inside within hours. So on any sunny day, prop the lid open or roll back the plastic to let heat escape, and close it again in late afternoon to trap warmth for the night. An automatic vent opener (a heat-activated piston that lifts the lid on its own) is a worthwhile upgrade if you're away during the day. Mind the ventilation and a cold frame or hoop house quietly adds weeks, or a whole winter, to your growing season for very little money. Plan your seasons around it with the planting calendar.