Garden innovations - soil warming cables

We all know that plants love heat. When they are warm they grow and if they're cold they don't - but too much heat causes them to shrivel and die. So what is the perfect heat? Recent research reveals that root growth begins when the soil temperature reaches or exceeds 7 degrees C. The important thing to note here is that for some seeds and many cuttings, roots can be growing underground, and increasing the plants chances of survival, if the soil is adequately warm - even if nothing is showing on the surface.

Professional growers long ago learned that if you can heat the soil without raising the air temperature around the tops of the plants, root development can be induced and or speeded. Heating the tops of the plants runs the risk of shrivelling or scorching tender growth. This technique is now available to the keen home gardener too.

Soil warming cables can be used to propagate seeds and cuttings on a greenhouse bench or in a cold frame, allowing hobbyists to plant seeds earlier and raise better specimens from cuttings.

A simple wooden frame forms the propagating area. It is lined with polythene and the cables are laid onto a two inch bed of sharp sand and covered with a further two inches of sand. If cables are laid in runs around three inches apart, the gardener can raise the soil temperature to 11-13 degrees C, assuming a greenhouse temperature of 7 degrees C. Higher temperatures can be achieved by laying the cable runs as close together as two inches but if you opt for the closer runs, you must use a thermostat and a soil thermometer to control the soil temperature carefully, to fulfil the requirements of seedlings, as they germinate at different heats.

Garden innovation soil warming photograph by aleske, used under a creative commons attribution licence