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With Winter not far away, I've put my outdoor heated bed back together ready for the cold weather. Though not essential, a heatbed has many benefits for enthusiasts in different climates.
Here in the UK (zone 9) there is ordinarily little need to protect my bonsai from the cold, however weak trees benefit from being kept slightly warmed at the roots. As well as preventing the roots of the tree being exposed to frost and freezing, the extra warmth extends root activity further into Autumn and early Spring.
I predominantly use my heated bed for recently collected yamadori and other trees that have weak roots or are under stress for one reason or another. In colder climates, heatbed's can be used to protect trees that are more susceptible to frozen roots such as Trident Maples and also mame-sized bonsai that are grown in small pots.
A heatbed is intended for frost hardy trees only; whilst the soil will be kept a little warmer than the ambient temperature outside, the surrounding air temperature is not altered. Tropical's and other frost tender species cannot be kept outside using this system alone.
Here is some of the soil warming cable laid underneath one of my benches in my garage. The edges of this area are shuttered off with timber.
The soil warming cable plugs into the mains and the cable itself heats up. It is kept switched on 24 hours a day all winter; electricity costs are minimal as the cable is low wattage. Soil warming cable can be purchased at garden centre's and nurseries; it can also be occasionally found second hand.
After the cable is laid on the ground, sand is spread over the top of it. The heated cable warms the sand sufficiently to keep any pots placed on top of it from freezing. I keep a thermometer handy to make sure that the sand never climbs above 10°C; warm enough to force many plants into premature Spring growth. Adding additional sand reduces the temperature of the bed; removing sand has the opposite effect, necessary if the sand isn't quite warm enough in the coldest periods of the Winter.
Pictured below is the completed heatbed; I have placed a weakly rooted Hawthorn air-layer that I had to separate a month ago. This should keep it rooting for few weeks yet.