How to Make a Hotbed for the Earliest Sowings

Hotbeds are horticultural alchemy. But what is a hotbed and what makes them so magical?

Hotbeds are warm growing environments created using the heat that’s given off during the decomposition of organic matter such as manure. The heat generated by all that microbial activity is best put to use right at the start of the growing season, or rather before it – jumpstarting spring when the weather is still distinctly chilly.

These cosseted environments may be set up within a protected structure such as a greenhouse, or outside with the addition of a cold frame or similar housing to trap the warmth and raise the air temperature. Hotbeds are great for growing very early crops such as salads, spinach and early varieties of carrot.

Best Manure for Making a Hotbed

Plunge your fork into the middle of a compost heap, wiggle it about a bit, and you’ll probably notice steam escaping. The middle of a well-stacked compost heap reaches impressive temperatures – and it’s this warmth that has been used for centuries by gardeners looking to cheat the seasons. From the Roman Emperor Tiberius to the Victorian gardeners of a few generations ago, hotbeds are nothing new.

Today’s gardeners could learn a lot from the gardeners of yesteryear. Hotbeds, with their simple structure and low-tech credentials are ready for a comeback. All you’ll need to make one is a ready supply of nitrogen-rich organic matter. Nature will take care of the rest.

The absolute best organic matter for heating up a hotbed is horse manure. In actual fact, what you are most likely to source is stable litter – a potent mix of manure, urine and straw from the animals’ bedding. This is a good thing, because urine is very high in nitrogen, which speeds decomposition, generating that all-important heat.

Stables and equestrian centers are usually only too happy to get rid of their manure. See if you can pick some up locally. Alternatively, a truckload of manure can often be delivered for a modest fee. However you source it, make sure that the horses have not been grazed on pastures sprayed with persistent herbicides.

Normally when using animal manure in the garden, it needs to be very well-rotted, but for a hotbed fresh is best - certainly no more than six weeks old.

Making a Hotbed

To make a hotbed, stack your manure high. Tall and skinny is preferable to wide and shallow, which loses heat quicker. The ideal hotbed would be around 1.8m (6ft) square, with a height of around 60-90cm (2-3ft), though you could go a little smaller, or eke out your manure with equal parts collected leaves. These generous dimensions ensure that the manure towards the edges of the pile insulates the muck in the middle. Later on in winter, with less time to wait till the return of spring, you could get away with a stack 30-60cm (1-2ft) in height.

In most instances a frame made from old pallets or other wood, lashed together with wire, is a welcome addition to keep the manure contained, insulated, and to stop it from sprawling. You could line the frame with a few layers of cardboard to offer further insulation.

Add the manure evenly. Whack it down with the back of your fork as you build up the bed so that each layer is firm and level. The pile will settle down and shrink as it rots, so working to achieve an even finish at the start is essential to avoid disturbance at the growing surface a few weeks down the line.

Adding Your Hotbed’s Cold Frame

If your hotbed is in a greenhouse, you're done. Outdoors, a couple of days later pop a cold frame on top of the hotbed. Position it centrally on the pile then add a 15cm (6in) layer of potting mix into the cold frame to begin warming up – you will be sowing or planting into this, not the manure.

You can use any cold frame or a series of cold frames, but to make the most of the heat that’s generated you ideally want a frame that leaves no more than a foot of hotbed uncovered around the perimeter. The lids, or lights, should be on a slant, facing the midday sun to maximize the amount of solar radiation they trap.

Sowings start once the temperature just beneath the potting mix has peaked. Use a soil thermometer to take daily readings about a foot deep, or 15cm (6in) beneath the potting mix. Once the temperature has been the same for a couple of days or is starting to decline, get on and sow – pronto!

Plants to Grow on a Hotbed

The temperature at the heart of the hotbed can soar to a toasty 65ºC or 150ºF and stay there for weeks. At the surface, the potting mix will remain at a similar temperature to that inside a heated propagator, with the air temperature above remaining several degrees higher than that outside.

This manure-generated heat has shifted your season forward by a couple of months, defying the chill to create a comfortable warmth ideal for germination. Take your pick from a multitude of cold-season vegetables – beets, carrots, lettuce and other salad leaves, onions, spinach, chard, radishes and turnips – and sow knowing that you’ll be picking and plucking within weeks.

The first harvests will be ready by mid-spring, at the same time as other gardeners are only just beginning to tear open the seed packets! With the earliest harvests cleared, the heat coming from the hotbed will have tailed off – but that doesn’t matter now that spring is here. Into the growing medium can now go a host of tender crops like tomatoes, squashes and zucchini, cucumber and bush beans whose roots will positively gorge on all the nutrition lurking beneath the potting mix.

Ventilate the cold frame on sunny days to keep the growing environment healthy. At the end of the season all that manure will have completely decomposed and can be spread out onto beds around the garden, ready to feed next year’s crops. Magic indeed!