Not for every crop. Tomatoes and peppers germinate best in warm soil, around 85 °F, so a heat mat helps them; lettuce and onions start fine at room temperature. Put mats on a thermostat, not the light timer, and remove them once seeds sprout.
Open this example in the Grow Light Planner
Which crops need one
A heat mat warms the potting mix, which speeds up germination for crops that like warm soil. Oregon State lists the best germination soil temperature for each crop (Soil Temperature Conditions for Vegetable Seed Germination). Tomatoes, peppers, and eggplant do best around 85 °F, warmer than most homes. Minnesota recommends potting mix at 65–75 °F for starting seeds in general (Starting Seeds Indoors).
| Crop | Best germination soil temperature | Heat mat in the planner by default |
|---|---|---|
| Tomatoes | about 85 °F | Yes |
| Peppers and eggplant | about 85 °F | Yes |
| Broccoli, cabbage, and kale | about 85 °F | No |
| Lettuce and greens | about 75 °F | No |
| Onions and leeks | about 75 °F | No |
Broccoli, cabbage, and kale also have an 85 °F optimum, but they're cool-season crops usually started at room temperature, so the planner leaves mats off for them by default. Choose heat mats in the planner if your room runs cool.
How to use a heat mat
- Use a thermostat. A thermostat with a probe in the mix keeps the soil at the target instead of letting it overheat.
- Keep it off the light timer. Minnesota says not to plug heat mats into the same timer as the lights (Starting Seeds Indoors).
- Remove it once seeds sprout. The planner assumes mats run around the clock for about 7 days.
Worked example: 6 trays of peppers
Six 1020 trays of peppers on 48 × 24 in shelves, under 2-ft LED bars. Four trays fit per shelf, so they take two shelves.
The mats draw 108 W around the clock: 2.59 kWh a day, or $3.30 for 7 days at 18.2 cents/kWh. The lights run 10 weeks and cost $32.61. So the mats are only about 9% of the season's $35.92 electricity.
Run your own numbers
The free BedMath Grow Light Planner works this out for your trays, shelves, crop, and lights, and shows every formula and source. This link opens it with the example above filled in.
Sources
Published 2026-11-03. Worked examples come straight from the planner's current math.