As prime seed starting season draws nearer, it’s worth recalling that one thing that most seeds need for germination is warmth. There are exceptions (plants native to cold climates, not surprisingly, may prefer cool temperatures), but probably 95% of the plants you’re likely to want to grow from seed prefer warmth. The average home, usually kept at around 71˚C (21˚C), is pretty close to the ideal temperature, but still, you’ll find you get better and faster germination if you boost the temperature just a bit to about 74˚F (24˚C).
Also, temperatures should be fairly stable for germination. Sunny windowsills, where you’re often tempted to start seeds, are a bit like a yo-yo. Temperatures get really hot during the day and drop off precipitously (from a germinating seed’s point of view) at night. This is not so bad for seedlings once they’ve started to grow, but not the best for germination. So, where in the house can you put freshly sown seeds?
The old answer (still widely shared on the Internet) is on top of the fridge or the water heater. Nope! Modern refrigerators and water heaters are thoroughly insulated and, unless they are antiques, no longer give off any appreciable heat through their top.
You might instead try a room that is simply warmer than the others. Attics, laundry rooms, kitchens, and bathrooms often fit that category. Some people have heated floors in their bathroom: if you can find some way to keep your family and pets from stepping in your seed trays, they make good spots for seed starting.
I start almost all my seeds under fluorescent lights, about 6 to 12 inches (15 to 30 cm) from the lamps. That is, theoretically, mostly because some seeds need light to germinate and the others all like even light once they do germinate. But guess what? Fluorescent lamps give off a gentle heat that is ideal for germination.
Failing any of the above options, there are also heat mats. They are designed for seed starting: just plop your seed tray on the mat and plug it in. You may have to buy your heat mat online or from a hydroponics store, as not all garden centers carry them. They’re fairly inexpensive and last for years.
After germination, most plants prefer cooler temperatures and actually benefit from conditions where it is warmer during the day than at night. So you can then move your plants to that old standby, the sunny windowsill. Until, that is, they outgrow the space available… but that’s another story!
Larry Hodgson (the Laidback Gardener)