How to Make a Hotbed for the Earliest Sowings

, written by Benedict Vanheems gb flag

Hotbed with homemade cold frame

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.

A hotbed can be used to get plants growing earlier

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.

Fresh manure heats up to create your hotbed

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.

Make a manure stack 60-90cm (2-3ft) high

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.

Grow your crops in potting mix piled on top of 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.

A hotbed can be used to grow two generations of crops

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!

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Show Comments


"I would love to try this! I do have a question, however. I was told that I should NOT use horse manure in the garden, because their digestive system doesn’t kill the seeds from hay, weeds, etc., that they eat. That means their manure contains those seeds, which will promote more weed growth among the crops. Any info on this? Thanks!"
Shari Sprong on Friday 7 February 2020
"Some weeds may survive passing through the gut of the horse, yes. But the intense heat of the decomposition process will certainly kill off all of the weed seeds towards the middle of the heap. Some at the edges may survive, but it really will be minimal and you weeding won't be an all-consuming process - just a question of hoiking out or hoeing off the odd weed here and there."
Ben Vanheems on Friday 7 February 2020
"OK, thanks! Looking forward to trying it this year!"
Shari Sprong on Friday 7 February 2020
"Hi. I have a really small greenhouse (4x6 feet). We are building wooden shelving for the greenhouse and we would like to incorporate a hotbed into the design. Space is limited. Do you think that a hotbed that is 30 inches tall (shelf height) and 24x24" would work, or would it be too small to generate enough heat for my seedlings?"
Pat Cook on Saturday 28 March 2020
"Hi Pat. It might have some effect, especially if you can insulate it at the side, but I would make it deeper if you can. "
Ben Vanheems on Monday 30 March 2020
"Me again. Just wondering if we put the hotbed in the greenhouse, do we line the inside bottom of it with anything? I'm concerned about liquid leaching out of the hotbed and onto the floor of the green house. Does horse manure and straw breakdown into a liquid form? "
Pat Cook on Thursday 2 April 2020
"Hi Pat. Whether or not you line the bottom is up to you. I would say there is a high likelihood of liquid leaching out and staining whatever it's sat on - sort of like tanins in tea. So if you don't want that then yes, I would line the base with a non-permeable membrane."
Ben Vanheems on Thursday 2 April 2020
"You should have given some credit to Green Books for publishing Hot Beds. There is still time to do so. Be fair."
Jack First on Monday 13 July 2020
"Hi Jack. Yes, I've read that book and agree it's an incredibly useful resource on this topic. Lots of ideas for different setups and very comprehensive."
Ben Vanheems on Monday 13 July 2020
"I live in Tasmania, a very cold place. I am staying in Finke for 6 Months. Finke is 20 Kms from the dead-centre of Australia, in the desert. A very Hot place. Saying that, does your Program and Website Tailor it's content for adverse climates ? "
Ross on Tuesday 22 September 2020
"Hi Ross. Yes, our Garden Planner uses the data from your nearest weather station to adjust its calendar accordingly, so you should be fine using it in either Tassie or central Australia. Incidentally, I spent two very happy months working at the botanical gardens in Hobart and then enjoying a tour of the island. I have a very soft spot for Tasmania and hope to go back at some point. Such a beautiful place and such incredible people."
Ben Vanheems on Tuesday 22 September 2020
"I would like any email about hot beds and gardening sent my way Thank You Mike"
Mike Balko on Monday 18 January 2021
"Hi Mike. If you've ticked the 'Keep me informed of further replies to this article' box, you should now receive updates on comments on this article, which may be of use. To receive emails when we release new articles or video guides, sign up for our free newsletter (from which you can unsubscribe at any time). To sign up, click on the 'Guides' tab above. "
Ben Vanheems on Monday 18 January 2021
"Hi Ben, I can I use and old water butt to use as a hot water bed? A"
Caroline on Saturday 8 May 2021
"Possibly, but I would think it's just a bit too slim for that. It really needs to be wider in order to retain the heat properly and generate that rise in temperature."
Ben Vanheems on Saturday 8 May 2021
"Hi Ben, sorry, about my previous comment. Predicted text strikes again! My question is: can I use a water butt as a hot bed? "
Caroline on Saturday 8 May 2021
"Hi Caroline. It's okay - I got what you were saying. Unfortunately I don't think it would suit as it is too slim to retain enough heat."
Ben Vanheems on Saturday 8 May 2021
"Hi We have a raised brick bed inside our greenhouse and created a hotbed, using horse poo. mixed with wood shavings -bedding and some leaves topped off with homemade manure. Bed is approx 2.5ft high, 2 ft wide and 6 ft long 1/4 leaves, 1/2 manure and 1/4 compost on top. How long would this take to start heating up? Want to place germinated salad crops on the hotbeds."
Maria on Thursday 20 January 2022
Phillip Griffiths on Thursday 3 February 2022
"Interesting. Got me thinking now ;-)"
Terry on Tuesday 5 April 2022
"I made a hotbed with fresh horse manure in my greenhouse but now have an infestation of little flies! Help"
Pearl on Saturday 22 April 2023
"Hi Pearl. Hopefully the flies will decline in prevalence down as the horse manure begins to break down. This should happen fairly quickly with it stacked in a hotbed. If it isn't very cold where you are, you might want to ventilate (if possible) to improve airflow."
Ben Vanheems on Tuesday 25 April 2023

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