Cold frame and manure hotbed
A glazed box that starts seedlings weeks early, and the same box over a pit of rotting manure that warms the soil from below.
Build
Tier 3 · Medieval to early modern
Food
Easy to build, needs looking at every day
1–2 days, then several days for the manure's first heat to pass
Crew 1–2
Home: Protected growing, Level 3 · Mill, level 1 of 3 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
Most annual transplants need 30–40 days from seed (University of Nebraska), and a cold frame lets them start that long before the last frost. Over a pit of fresh manure it becomes a hotbed: the heap heats as it rots and warms the soil above it, as gardeners did in Gilbert White's day in the 18th century. Everything here is boards, glass and a thermometer; the numbers are the University of Nebraska's.
Manure hotbed, in section
Fig 1
Earth / subsoilTimberLoose fill / grainGlassFire / hot zone
Cold frame for two lights, in plan
Fig 2
TimberGlassVoid / air
Keyed parts
| 1 | Frame | Rot-resistant boards, 25–50 mm, never preservative-treated; 20 cm high at the front, 35 cm at the back, sunk 5–15 cm. |
| 2 | Lights | Two glazed sashes, 0.9 × 1.8 m, sloping 1 in 12 toward the sun. |
| 3 | Prop | A notched stick or an opener that holds each light open at a set gap. |
| 4 | Gravel | 10 cm of coarse gravel at the bottom of the pit, for drainage and air. |
| 5 | Manure and straw | 20–30 cm of fresh horse manure with 5–10 % straw, trodden firm and wetted. |
| 6 | Soil | 7.5–10 cm of good, weed-free soil on the manure. |
| 7 | Earth bank | Soil banked against the outside of the frame, sloping away for drainage. |
| 8 | Soil thermometer | Pushed into the soil; sow when it has fallen to 24 °C. |
| 9 | Corner posts | 5 × 10 cm, one in each corner, holding the boards. |
| 10 | Middle bar | A bar across the middle of the frame on which the two lights meet. |
Specifications
| Size | 1.8 × 1.8 m | 0.9 × 1.8 m at the smallest; lengths in multiples of 0.9 m. |
| Slope | 1 in 12 | 20 cm high at the front, 35 cm at the back, toward the sun. |
| Lights | 2 × 0.9 × 1.8 m | Glass, or twin-layer film, acrylic or fibreglass. |
| Pit | 60 cm deep | Gravel 10 cm, manure 20–30 cm, soil 7.5–10 cm. |
| Sow at | 24 °C | Soil temperature, falling, after the manure's first heat. |
| Seedlings | 16–18 °C / 18–24 °C | Air at plant height, cool-season / warm-season crops. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Boards, cedar or other rot-resistant wood, untreated | about 14 m | 25–50 × 200 mm. Front 30 cm tall (20 above ground), back 45 cm (35 above), the sides tapering between; each with 10 cm in the ground. |
| Corner posts, 5 × 10 cm | 4 × 60 cm | Plus one bar 1.8 m long across the middle. |
| Glazed lights, 0.9 × 1.8 m | 2 | Glass in a wooden frame, or a rigid clear sheet. |
| Coarse gravel | 0.3 m³ | 10 cm over the 3.2 m² pit. |
| Fresh horse manure, with 5–10 % straw | 0.6–1.0 m³ | 20–30 cm over 3.2 m²; free of weed seed. |
| Good soil, weed-free | 0.3 m³ | 7.5–10 cm. |
| Soil thermometer | 1 | Reading at least 0–60 °C. |
| Insect mesh, sacking or straw mats | 3.2 m² each | Mesh over the frame when the lights are open; mats over the lights on frosty nights. |
Tools: Spade · Saw · Hammer and nails, or screws and a driver · Spirit level and a straight board · Tape measure · Watering can with lukewarm water
Construction
Choose the place
Full sun all winter, no shade at noon on the shortest day, shelter from the wind, water nearby, and ground that drains. Face the front toward the equator. A wall or hedge behind the frame shelters it; keep it out from under eaves, where snow and icicles fall.
Dig the pit
Mark 1.8 × 1.8 m and dig 60 cm deep for a hotbed; for a cold frame alone, 15–40 cm is enough (University of Nebraska). Keep the topsoil aside, weed-free, for the top layer.
Build the frame
Nail the boards to the corner posts: 35 cm above ground at the back, 20 cm at the front, with 5–15 cm more in the ground, so the top slopes 1 in 12 toward the sun. Fit the middle bar across the top. Set the frame over the pit, check the top with a level along the slope, and bank soil against the outside.
Fill the hotbed
Spread 10 cm of coarse gravel. Mix fresh horse manure with 5–10 % straw, spread it 20–30 cm deep, tread it firm and wet it with lukewarm water. Cover it with 7.5–10 cm of soil. Put the lights on.
Wait for the first heat to pass
The manure heats far past what seed will stand. Push the thermometer into the soil and read it every day. Sow when it has fallen to 24 °C and is still falling slowly.
Sow and run it
Sow in the soil or in pots set on it. Seeds germinate best at 21–24 °C; then hold the air at plant height at 16–18 °C for cool-season crops and 18–24 °C for warm-season ones. Prop the lights open on every sunny day, close them in mid-afternoon, and cover them with mats on frosty nights. Water sparingly and feed little nitrogen.
Harden off
In the last week or two before planting out, take the lights off for more of each day, then leave them off at night too. Most annuals are ready 30–40 days after sowing.
Empty and store
When the bed has cooled and the crop is out, dig out the spent manure for the compost heap. Store the lights flat and dry; replace cracked glass before next season.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Slope of the lights | 1 in 12, toward the sun | Rain runs off, and the low winter sun meets the glass. |
| Gaps in the frame | none that show daylight | Every gap leaks heat and lets mice in. |
| Soil at sowing | 24 °C or below, falling | Seed dies in the manure's first heat. |
| Air at plant height | 16–18 °C cool-season, 18–24 °C warm-season | Warmer and wetter makes weak, leggy seedlings and disease. |
How this fails
Lights left shut on a sunny morning. The frame heats in minutes of full sun and the seedlings cook. Open them before you leave, every sunny day.
Sowing into the first heat. Seed dies in soil hotter than it can stand. Trust the thermometer, not the calendar.
Treated boards, which injure plants, or manure and straw full of weed seed, which fills the bed with weeds (University of Nebraska).
Too much water and nitrogen under warm glass: soft, leggy seedlings and disease.
A frame in shade or in a frost pocket, which never warms.
How you know it worked
VerificationSeed sown in the hotbed germinates within its usual time, the air in the frame stays in the crop's range through a sunny day and a frosty night, and the transplants go out on the date of the last frost.
Scaling and variants
More frames: a row in multiples of 0.9 m, lights side by side. An electric hotbed needs no manure: about 6.5 m of heating cable per m² of bed (2 ft per ft²), looped 15–20 cm apart on 10 cm of sand, on a thermostat, earthed and never spliced (University of Nebraska); that needs electricity. A glasshouse against a sunny wall is the next size up: cold frames, hotbeds and the glasshouse.