Hotbeds have been used for many years to bring on delicate plants in cold weather.
They where most commonly used by Victorian gardeners to force crops like Melons, Cucumbers, Strawberries, and Radishes. In fact any crop that was needed in the kitchen by the cook out of season, could be grown in a Hotbed.
Hotbeds, over the years have seen advances in design and heating methods. The original method of heating hotbeds was to use Horse manure and straw, later on wealthy households used hot water pipes from boilers to provide the heat, and towards the latter end of the 20th Century, electric heating elements where used to provide the warmth. In this article we will concentrate on the manure solution for heating. Although it would appear initially to be the easiest to employ, in fact, to make it work well there are several techniques known to gardeners years ago, which have been lost to us modern, ‘advanced’ gardeners, and by using these techniques we can vastly improve the effectiveness of manure as a heating solution.
The Traditional Hotbed Frame
The basic frame for the Hotbed is constructed from four pieces of wood. Traditionally the measurements are twelve feet long by four feet wide. But adjust the size to fit your requirements. The height at the front of the frame should be 9 inches, and at the back it should be 18 inches (Fig 1.). There should be a bracing strut fitted from the front to the back at the top of the frame to provide additional strength and also as a mounting for glazed sashes to rest on, which are called ‘Lights’. Place a strut every 3 feet along the length of the frame.
The ‘Lights’ should be constructed as a basic frame 4 foot long, by 3 foot wide, and glazed, traditionally with glass panes, but today it is easier, and safer to use multilayed Polycarbonate roofing sections (Fig 2.). The Lights are not fixed to the Hotbed frame, but they are free to slide up and down, or to be raised at the top or bottom to control the temperature of the hotbed. A wedge can be used to keep the Lights open. Make as many Lights as required to cover the whole Hotbed frame.
You now have the completed Hotbed frame, The same frame can be used on the free standing version and as the permanent version. The only difference is the permanent version sits on to of a brick wall; the height of which can vary depending on the design you require, but 2 feet was a usual measurement. The ground you build your Hotbed on must be flat and level. Don’t take any short cuts when leveling the land as it will cause all sorts of problems later.
The Manure – the ECO-friendly heating solution
Now for the most important bit of the Hotbed. You can use any sort of manure and straw mixture, but if you want it to work to at it’s best you must use horse manure. Don’t get well rotted manure, it is best to get it straight from the horses bottom if possible.
Take your mixture and pile it into a rounded pyramid using a fork. Don’t just dump the lot in a pile you must take each fork full and shake it into a loose pile. Leave the pile for 3 days, during which time it should have generated enough heat to produce steam. On the third day fork the pile shaking as you go into a pyramid by the side of the original pile, making sure the top now becomes the bottom and visa versa. Repeat this process on the sixth day (Fig 3). If the weather is dry then water will have to be added after the turning process. The old method was to add a gallon of water for every foot in height. If the mixture is not started in this way you can end up with hotspots in the beds, or the fermentation may be erratic.
After nine days of turning the manure mixture will have started to ‘ferment’ well and the process should continue nicely. It is time to build the Hotbed. For the free standing pile carefully fork the mixture into layers of four to five inches in depth and an inch or two larger than the frame. Tap down using the fork. repeat the process until you have two to three feet of manure. Now place your free standing frame on to the pile of manure and put the lights in place, but keep them open for ventilation. The process for a brick built bed is exactly the same, just fill the bed in four to five inch layers tapping down as you go.
It should take twelve hours for the heat to start to rise, place a stick into the bed and leave it in place, check each day to see how warm the bed is. It should take 3 days to reach maximum temperature, check the bed several times a day with the stick, or finger if you prefer, as the temperature will ‘plateau’ and gradually reduce. Once the Plateaux is reached it is now time for the soil to be put on top. If you put it on too soon, the heat build up can be too great, and the soil can ‘burn’, that is to say all the nutrients in the soil can be damaged, not that it will burst into flames. The soil should be moist but not wet, and should be put in a six inch layer over the manure. Your bed is now ready to sow seeds or plants in.
Controlling the Temperature
Once built the bed temperature is controlled by opening and closing the ‘Lights’ much in the same way a greenhouse temperature is controlled. If however, you are wanting to grow delicate root crops such as Radish, they can be inedible if grown in a bed too hot, you will need to mix more straw in with the manure at the beginning of the whole process. This will result in the maximum temperature of the bed being lower. Beds with a reduced temperature are known for obvious reasons as ‘Warm beds’.
How long does the Hotbed last?
It depends on the amount and quality of the manure used. But generally the bed will provide warmth until the spring sunshine takes over. You can continue to sow and plant in the bed throughout the summer months and remove the rotted manure and dig into the ground in late Autumn, early winter. There are variations on the design that include piling earth up around the bricks or the manure bed that can be made, and you should experiment to see if you can improve the effectiveness of your Hotbeds.