Making hot beds

One of the biggest problems with our garden is that it is cold. Being at the bottom of a slope means that cold air pours down and gets trapped by the dry stone walls. The result is a minimum temperature of at least -10C for the last ten years. The growing season is short with usually a maximum of three frost free months but last year it was just one.

For many years I have tried different ways to protect crops from the cold. The usual horticultural fleece does nothing more than prevent ice crystals forming on foliage which can prevent some frost damage but it does not insulate beds enough to keep frost at bay.

Having a small polytunnel helps but at just 4.5m x 3m it is hardly big enough. In 2005 I made some solar pods which are covered raised beds. The lid is glazed with twin wall polycarbonate. This heLps but again will not keep hard frosts out.

Last year the UK TV programme Gardeners World included a piece on hot beds in one of their early season programmes. The item covered the work of a gardener in Yorkshire Called Jack First and showed his numerous hot beds on his allotment. He was sowing salad crops in January an

d had very early new potatoes all from covered raised beds. The secret was the layer of fresh manure just under the soil.

As manure composts it produces heat and if you add enough you will get a significant amount of heat for a few weeks. It is nothing new as there was craze for growing pineapples in big country houses in the nineteenth century country houses.

So, this year I decided to convert a couple of solar pods to hot beds to see just what is possible. The process is quite simple:

  1. Dig out about 60cm of soil.
  2. Add a layer of fresh manure to about 45cms for cow and 30cm for horse.
  3. Replace a layer of soil on top.

(Click to enlarge)

A very big thank you to Jim for doing the hard work and supplying the manure!

The key property of any manure used is that it MUST be fresh and have not been stacked outside. It is crucial to check manure is from a source that does not use herbicides containing Clopyralid on their pasture. See http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=477

There is some debate as to whether it is best to compress the manure by walking on it. We did two beds, one compressed the other not. It will be interesting to see if there is any difference. Bed 2 is cooler than bed 1 but there was less manure in it, maybe 30cms instead of 40cms so that will make a difference.

Temp  C 1 day 2 days 3 days 4 days
Bed 1 soil 15 16.2 8 8
Bed 1 air 12 10 6 5
Bed 2 soil 11 15 6 7
Bed 2 air 9 9.5 2 5
Ambient 8 8.6 0.7 3

It is early days yet but there is certainly some heat in the beds.  More to follow later.

Update – 31 Jan 2013

The first seeds germinated a few days ago. With heavy snow and temperatures below zero for many days it is remarkable that the seeds have germinated.Spurred on by this success I sowed seeds in the second hotbed yesterday; baby leaf spinach, beetroot and another variety of cut-n-come-gain lettuce. This time there is a layer of compost on top of the manure instead of soil and clear polythene sheet covers the bed under the solar pod. That should help keep the soil and air temp higher.

HB_germination270113

Winter salad mix

Update – 01 Feb 2013
The temperatures in bed two today were; soil-9C air-10C. That is looking very promising for some rapid germination of the seeds sown on 30 Jan 2013.