Build a cold frame or hotbed: a three-week head start in February

In late February the garden is still asleep. The soil is cold, the beds are bare, and yet your fingers are itching to start. This is exactly the stretch a cold frame is built for: a shallow box with a see-through lid that catches the sun's warmth and holds on to a little of it overnight. Against an open bed, that buys you about three weeks. Build a hotbed instead, a cold frame with fresh horse manure as a heater underneath, and you add another two to three weeks and sow in the middle of winter.

Cold or manure-heated: which box suits you

Both boxes look the same from the outside. The difference is in the ground, and in how much warmth you are willing to bring yourself.

The cold frame lives on sunlight alone. During the day the glass heats the inside, the soil and the box walls store part of it, and at night the ground slowly gives that warmth back. On a clear March night it sits only two to four degrees above the outside air, on a sunny afternoon twenty degrees above it. That is enough to sow radishes, spinach and cut-and-come-again lettuce three weeks earlier. After that the box costs you nothing but airing and watering.

The hotbed brings its own heater. Under the soil goes a thick layer of fresh horse manure, mixed with straw and leaves. As the microbes break it down they release heat, just like a compost heap steaming in winter. A well-built pack holds 15 to 25 degrees in the root zone for four to six weeks. That lets you sow carrots as early as January and raise young plants that would otherwise face far too much cold. The price is work: about a cubic metre of fresh manure, layered in and trodden firm.

If you want to harvest earlier (salad, radishes, spinach), the cold frame is enough. If you want to raise seedlings earlier and start heat-loving crops like tomatoes or squash, the manure pays off. Many gardeners build cold first and upgrade later.

Building the box: size, material, lid

Keep the box narrow enough to reach the middle comfortably; anything deeper than 1.20 m turns into a gymnastics exercise. A box of 1.20 m by 0.80 m has proven itself, a good fit under an old window.

1. Cut the frame

   Use untreated larch or Douglas fir, at least 24 mm thick. North wall 40 cm high, south wall 25 cm; the slope between them gives the lid its tilt. Screw the four walls into a frame with no bottom, so the roots stay connected to the ground below.

2. Choose the lid

   An old sash window is the classic and stores warmth well. Lighter and shatterproof are twin-wall polycarbonate sheets, 6 to 10 mm thick. Hinge the lid to the high north edge, then it swings up for airing.

3. Prepare the site

   Cut off the turf and loosen the soil two spade-depths down. For the cold frame you fill in good garden soil; for the hotbed you dig the pit for the manure pack here.

4. Set and seal the box

   Set the frame into the prepared area and press soil firmly against the outer walls so no cold draught slips underneath. A wreath of leaves or straw around it holds extra warmth.

Layering the hotbed: heat from below

The heart of the heated frame is fresh, strawy horse manure from a riding stable. Well-rotted manure no longer heats up and stays cold.

1. Dig the pit

   Dig 40 to 60 cm deep under the box. The deeper the pack, the longer the warmth lasts, because it cools down more slowly.

2. Layer and tread the manure

   Fill in the fresh manure layer by layer and tread each layer down firmly. Well compacted, the pack rots more evenly. Aim for about two parts manure to one part leaves or straw.

3. Let it fire up

   Lid on and wait three to four days. The pack first heats to 50 to 60 degrees, too hot for sowing. A simple soil thermometer tells you when it is ready.

4. Add soil and wait

   Once the first heat fades, add 15 to 20 cm of good seed-starting soil on top. Wait until the thermometer in the soil layer drops below 25 degrees. Only then do you sow, otherwise the roots burn.

What to sow when: the timetable from CW 6 to CW 12

These weeks are guides for a mild wine-growing climate; in harsher regions push everything back by one to two weeks.

Radishes are the perfect first crop: they germinate at just 8 degrees soil temperature and are ready in four to six weeks. Sow them thinly, about 3 cm apart, or they run to leaf instead of forming a bulb.

Head lettuce you raise in small pots and plant out in mid April. Old, cold-hardy varieties like lettuce 'Maikönig' or lettuce 'Wunder von Stuttgart' were bred for exactly this early batch. Lettuce is a light-germinator, so just press the seeds down.

Airing, watering, getting through the frost

The most common beginner's mistake is not the cold but the heat. As soon as the March sun comes out, the temperature in a closed box climbs to 40 degrees and more within half an hour. So: on every sunny morning crack the lid open, on mild days open it fully; close it again in good time in the evening. A cheap min-max thermometer takes the guesswork out of it.

Water sparingly and in the morning, so the leaves dry off by evening. Standing wet in cool weather is an invitation to grey mould and damping-off. Pluck out withered seed leaves regularly.

When the warmth is spent

From May you barely need the lid. Take it off and the cold frame becomes an ordinary little raised bed, and the rotted manure pack, now ripe humus, carries cucumbers, courgettes or bush beans through the summer. The circle closes: what heated in February feeds in July. If you want to think the whole year through under cover, the greenhouse year plan has the layout for every month; for what else is due around the cold-frame start, see The garden in February.

Häufige Fragen

What is the difference between a cold frame and a hotbed?

Cold frame is the umbrella term for a shallow box covered with glass or twin-wall sheeting that catches the sun's warmth; a cold cold frame uses only that. A hotbed is a special form: under the soil lies a thick layer of fresh horse manure whose rotting heats the root zone from below. The cold frame buys about three weeks' head start, the hotbed four to six, but for that you need about a cubic metre of fresh manure and more work.

How warm does a hotbed get and how long does the heat last?

A well-layered, firmly trodden manure pack heats to 50 to 60 degrees in the first days. After that first heat it settles at 15 to 25 degrees in the root zone and holds that level for about four to six weeks. Then the box cools to ordinary cold-frame temperature. The thicker the pack and the firmer it is trodden, the longer the warmth lasts.

Which glass or material works for the lid?

Old sash windows with single glass store warmth well but are heavy and shatter. Twin-wall polycarbonate sheets, 6 to 10 mm thick, insulate better and are light and shatterproof; that is the best choice for new boxes. A stretched horticultural film does for one season but holds the warmth less well. More important than the material: the lid must close tightly and open easily for airing.

When do I need to air the cold frame?

Whenever the inside temperature rises above about 20 degrees, often already on sunny mornings from March. Crack the lid open, or on mild days open it fully. Without airing the temperature climbs quickly past 40 degrees and the young seedlings take damage. Close it again in the late afternoon so the warmth stays in overnight. A min-max thermometer makes airing a routine.

What can I sow in the cold frame from February?

Into the cold frame go, from late February (CW 8 to 9), radishes, spinach, rocket and lamb's lettuce sown direct, plus head lettuce and kohlrabi raised in pots. In the heated hotbed it starts as early as CW 6 with radishes, carrots and cut lettuce, and from CW 12 also with tomatoes and squash. In harsher regions push all dates back by one to two weeks.