Among the products commonly sold for starting seeds are heat mats. Just lay them underneath your tray, plug them in and they’ll keep your seed trays warm, helping speed up germination. And they do work, just as they promise to do so, keeping the soil up to 20˚ F/11˚ C warmer than the surrounding air. They’re not all that expensive either, especially when you consider they can easily last 20 years. But are they really necessary?
Given reasonable growing conditions, most seeds will germinate, perhaps more slowly or irregularly than when you use a heat mat, but you’ll still get germination from almost all seeds. So, strictly speaking, no, they’re aren’t necessary. But they can be useful.
Soil Is Cooler Than Air
Soil temperatures are almost always quite a bit cooler than air temperatures, so when you think you’ve reached a quite toasty 70 to 75˚ F (21 to 24˚ C) air temperature, the ideal temperature for germination for most seedlings (temperatures above 85˚F/30˚C will actually hinder the germination of some seeds), the soil temperature can easily still be less than 65˚ F (18˚ C). You can see that a heating mat would be quite useful under those circumstances.
Taking the Greenhouse Effect into Account
However, when you cover seed trays with a clear plastic dome, plastic bag or sheet of glass (i.e. a mini-greenhouse), the traditional situation when you’re starting seedlings, the well-known “greenhouse effect” plays a major role. Not only do temperatures inside the mini-greenhouse warm up more during the day (which is why you’re always warned not to place seed trays in full sunlight while they’re still covered: you don’t want heat to build up to a lethal degree!), but they also greatly reduce heat loss at night. Seedlings grown in a mini-greenhouse are far less needy of extra heat than seeds exposed to the ambient air.
Moderating Window and Greenhouse Temperatures
It’s when you start seeds in front of a window without using a mini-greenhouse that heat trays are often the most useful. Temperatures there rise precipitously on a sunny day and drop just as surprisingly at night. A heating mat can be very useful to help the seeds through the night.
Likewise, if you start seeds in a cool greenhouse (I’m talking about an actual greenhouse here, an outdoor structure, not the mini-one you use to start your seeds indoors), yo-yo temperatures are the norm, not the exception, with nights often being out-and-out cold even well into spring. Therefore a heating mat will be very useful if you start seeds there*.
*Personally, I have two temporary greenhouses I use for “growing plants on” once they’ve germinated. I never use them to actually start seeds, though. I start my seeds safely indoors, under lights.
When you start seeds under lights, temperatures are much less variable—at least inside a mini-greenhouse—and often stay essentially the same day and night. And soil temperatures tend to rise to ones very close to air temperatures. Fluorescent lights, notably, actually give off some heat, enough so that most people find they can reach soil temperatures of 70 to 75˚ C (21 to 24˚ C) with no need for a heating mat and that will be good enough for most seeds.
LED plant lights are said not to produce heat, but in fact, that isn’t true, especially at the density used for plant lights. They do. Usually not as much as fluorescent lights and certainly much, much less than incandescent lights (which are never used to start seeds!), but they do give off heat and if you start seeds under LED lights in a “warm” (warm to you) room, using a mini-greenhouse, you probably won’t need a heating mat.
I start my seeds under fluorescent lights in my basement. It’s cooler than the rest of the house, but nice and warm under the lights. I have a heat mat, but I haven’t used it in years, finding I get enough heat for fast and even germination under my lights. I keep my heating mat for seeds that are reputed to need a special heat boost to germinate well (85˚C/30˚C or greater), such as moonflowers (Ipomoea alba).
Once Seeds Have Sprouted
One of the odd things about seedlings is that, as much as almost all seeds need warmth and stable temperatures to germinate well, once they germinate (once leaves appear), they grow best in cooler conditions and actually prefer it when temperatures drop at night. So, with the exception perhaps of a terribly tropical plant being grown under exceptionally cool indoor temperatures, domes and other coverings should come off after germination and heat mats should be unplugged and put away for next year. They’re strictly used to start seedlings, not to grow them on.
Heat mats: they can be useful or not, depending on your growing situation!