In the frigid winter region within which I reside, the first thought that may come to mind in response to "bottom heat" could very well be that of heated car seats. That, however, is not what this article is about. To a gardener with thoughts about the hopefully forthcoming spring, it refers to germinating the seeds for the new crop of flowers to be, with bottom heat, indispensable in my basement with its temperature of 55°F, about 13°C. Some seeds prefer cooler temperatures for germination, such as Iberis sempervierens, Candytuft. These babies like to germinate at 45-50°F at night with a 60°F daytime temperature. I will soon see what they do with the more constant middle range temperature that they will have. My basement wine cellar likes it.
For me, bottom heat, when required, is supplied with a heating cable underneath a metal shelf that is thermostatically maintained at the desired temperature. As the recommended germination temperature refers to temperature at the seed level, the thermostat, at shelf level, is set for 5°F above the necessary seed germination requirement. This differential was determined by measuring the temperature at both places. The seed containers are covered with a plastic hood in order to maintain the warmer temperature at the seeds and to keep the surface from drying out. Three varieties of Sorbet Violas have already popped out, nourished by their warm underpinnings. After germinating they have been moved to the cooler clime and placed under fluorescent lighting after their initial germination stint in the required darkness. When the snows melt and warm breezes return, they will border a flower bed.
For those seeds that welcome a warmer temperature in order to germinate, my heated shelf is a necessity. Some seeds are more suited to germination temperatures approximating those found within the house. These are readily accommodated on a shelf by a southern facing window. Considering the dearth of winter sunlight locally, these plants are moved under lights as soon as they show.
Here are three links to useful sources of seed germination data.
Some seeds are beyond my ambitions to attempt to bring to fruition. Cyclamen and some ornamental grasses are included. I remember many years ago being given seeds for what I laughingly called Bear Grass. After several months of no apparent progress, I discarded the lot. Later
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