Heat Mats for Seed Starting: When You Need One
Tomatoes, peppers, melons, and squash sprout much faster with bottom heat at 70 to 85 F. Lettuce and broccoli do not need a mat. Remove it once seedlings...
Published by ThriveThyme
seed starting heat mat germination
Heat Mats for Seed Starting
A seedling heat mat warms the soil from below. It helps some crops a lot and does nothing useful for others. The deciding number is the soil temperature a crop likes for germination.
Try it with your own crops on the Seed Starting Setup Desk.
The Rule
The University of Idaho says crops with an ideal germination temperature of 70 F or higher sprout much faster with bottom heat from a seedling heat mat or other source. Remove the heat once plants emerge.
Temperatures here are for the seed starting mix, not the room air. Minnesota notes that the mix can be up to 5 F cooler than room air.
Which Crops Want a Mat
| Crop | Mix temperature (F) | Days to sprout |
|---|---|---|
| Tomato | 75 to 80 | 7 to 14 |
| Pepper | 75 to 85 | 10 to 14 |
| Melon | 80 to 85 | 5 to 10 |
| Squash | 70 to 85 | 7 to 14 |
| Basil | 70 to 85 | 4 to 10 |
| Marigold | 70 to 85 | 5 to 8 |
| Zinnia | 70 to 85 | 7 to 14 |
Eggplant is not in the Idaho table. Oregon State lists 75 to 90 F as its optimum range for eggplant.
Which Crops Do Not Need One
| Crop | Mix temperature (F) | Days to sprout |
|---|---|---|
| Broccoli and cauliflower | 65 to 75 | 5 to 10 |
| Cabbage, kale, and collards | 65 to 75 | 5 to 10 |
| Lettuce | 65 to 70 | 7 to 10 |
| Onion, leek, and shallot | 65 to 70 | 10 to 14 |
| Parsley | 65 to 70 | 10 to 25 |
| Pansy | 60 to 65 | 14 to 21 |
Utah State says most seeds germinate at 65 to 75 F, and broccoli, cabbage, and spinach prefer 55 to 65 F. A mat is optional for these crops. Use one only if the room is cool.
Using a Mat
- Pair the mat with a thermostat so trays do not get too hot (Utah State).
- Keep room air above 60 F (Minnesota).
- Do not plug the mat into the timer that runs your lights (Minnesota).
- Check trays at least once a day. Watch for a mat that quit, or mix that dried out (Utah State).
- Bottom heat can help prevent damping off. Minnesota's damping off guide suggests a heating pad to warm the mix to 70 to 75 F.
After Seedlings Emerge
Take the heat away and move trays under the light. Maryland says seedlings can then go a little cooler, 55 to 60 F at night and 65 to 70 F by day. Seedlings left warm tend to stretch. See how to fix leggy seedlings.
No Mat? Other Warm Spots
Maryland suggests the top of a refrigerator, or a clear plastic tent draped over the grow light fixture. The tent should rest on the frame, not the fixture. It can hold the air at 70 to 75 F. Remove clear covers from the trays as soon as sprouts appear.
Next Steps
- Pick your crops on the Seed Starting Setup Desk.
- Get the right grow light height and hours.
- Find sow dates on the seed-starting calendar.
Sources: University of Idaho Extension, Start Seeds Indoors with Success. Utah State University Extension, Starting Vegetable Seeds Indoors: Germination. Oregon State University Extension, Soil temperature conditions for vegetable seed germination. University of Minnesota Extension, Starting seeds indoors, and How to prevent seedling damping off. University of Maryland Extension, Starting Vegetable Seeds Indoors.