Short answer: a cold frame is an unheated, glazed box that protects low plants and hardens off seedlings. A hotbed is a cold frame with heat under the soil, so you can start seeds weeks earlier. A greenhouse is a walk-in structure for growing tall plants and working inside. Start with a cold frame, add heat when you want earlier starts, and build a greenhouse when you run out of headroom.
At a glance
| Cold frame | Hotbed | Small greenhouse | |
|---|---|---|---|
| Heat | Sun only | Soil-heating cable (or manure, traditionally) | Sun; optional heater |
| Best for | Hardening off, salad greens, extending fall | Early seed starting | Tall crops, propagation benches, working inside |
| Height | Knee-high | Knee-high | Walk-in |
| Build | A weekend | A weekend | 1–2 weekends |
The convertible option
The 1962 USDA / University of Maryland Greenhouse-Coldframe is two buildings from one set of panels. It starts as a low, vented cold frame. When the plants need headroom, you unbolt it and stack it on matching wall panels to make a 7'-8" × 5'-3" walk-in greenhouse about 7'-8" tall. Every panel is 2×2-framed and joined with carriage bolts, so it stores flat in summer.
Starting seeds early: the hotbed
The Hotbed & Propagating Frame (1964, University of Maryland) is a 6 × 6 ft low gable of bent thin-wall conduit, wrapped in clear plastic over a buried soil-heating cable. Hinged mesh covers open on warm days to vent and harden off. Have the heating cable wired by a qualified electrician.
A real walk-in house
The Portable Plastic Greenhouse (1963, after Oregon Plan 73) is the classic gothic-arch film house: 8'-6" × 12' and 7' tall. Each arch is two thin plywood bands glued into a curve, so there's no steam bending and no forms, and two people can move it.
Whatever you build: vent it
Every one of these old plans repeats the same warning: a closed glazed or film structure in full sun overheats quickly, even in winter. Open vents and lids on sunny days and close them at dusk.
Frequently asked
Is a cold frame worth it?
For the cost of a few boards and an old window or a sheet of film, it adds several weeks of growing at each end of the season. For most gardens it's the best-value build there is.
Which way should a cold frame face?
South (in the northern hemisphere), on well-drained ground, with the lid sloping toward the sun.