Heat Mats for Starting Seeds

My seedling heat mat, a seedling tray, seeds, and an Android figurine with air plants.
Getting ready to start some seeds!

text-box-multiplePart 2 in our Seed Starting Series link

All the info you need to start your seeds indoors!

I don’t know about you, but I’m dreaming of warmer spring weather and getting back out in the garden! To satisfy that “gardening itch” that comes every winter, I like to dig out my seed starting supplies and grow tomatoes, peppers, and a few other veggies. Our last post focused on grow lights, which are arguably the most important piece of equipment for starting seeds. However, some seeds may not even germinate to see that light if you don’t keep them at the right temperature. So today, we’re going all-in on heat mats. 🔥

Temperature requirements

Some veggies will have no problem germinating in a cold basement (where I start my seeds), while others need some warmth to sprout in a timely fashion.

Luckily, master gardeners have determined the optimal temperature for germinating most veggies. The following chart is pulled from this paper by the University of California:

Soil Temperature Conditions for Vegetable Seed Germination

Crops Minimum (ºF) Optimum range (ºF) Maximum (ºF)
Asparagus 50 75-85 95
Beans, Lima 60 75-85 85
Beans, Snap 60 75-85 95
Beets 40 65-85 95
Broccoli 40 60-85 95
Cabbage 40 60-85 95
Carrots 40 65-85 95
Cauliflower 40 65-85 95
Celery 40 * *
Chard, Swiss 40 65-85 95
Corn 50 65-95 105
Cucumbers 60 65-95 105
Eggplant 60 75-85 95
Garlic 32 65-85 95
Leeks 32 65-85 95
Lettuce 32 60-75 85
Cantaloupe 60 75-85 105
Okra 60 85-95 105
Onions 32 65-85 95
Parsley 40 65-85 95
Parsnips 32 65-75 85
Peas 40 65-75 85
Peppers 60 65-75 95
Pumpkins 60 85-95 105
Radishes 40 65-85 95
Spinach 32 65-75 75
Squash 60 85-95 105
Tomatoes 50 65-85 95
Turnips 40 60-95 105
Watermelons 60 75-95 105
Source: California Master Gardener Handbook, 2nd edition, Regents of the University of California, Division of Agriculture and Natural Resources, Publication 3382 (Table 5.2, page 114).

You may have noticed the missing temperatures for celery. Celery requires diffused, non-direct light and a cooler night temperature than the day temperature for good germination. Its optimal conditions are 85°F during the day and 70°F at night.

Keeping the seed tray at the right temperature

Most heating mats don’t have an adjustable thermostat, so it’s hard to get the soil to the right temperature. And even if the heating mat works for one gardener, it might not work for you. This is because soil temperature can vary depending on a variety of factors:

  • Wattage of the heating matt
  • Amount of soil
  • Amount of water in the soil
  • Surrounding air temperature
  • Air movement
  • Heat from grow light(s)

That’s why we recommend a thermostat-controlled heating mat such as the Spider Farmer Heat Mat Kit. It comes with a temperature probe that you stick in the soil, so you know your seedlings are at the optimum temperature. No more guessing! However, if you already have a heat mat that is working for you and getting good germination rates, there’s no need to fix what isn’t broken.

One important thing to keep in mind: Not all seeds need a heat mat. Many cold-hardy crops such as lettuce, spinach, and peas will germinate just fine at room temperature. In fact, they might not germinate at all if the soil is too warm! Just take a look at the table above, the max germination temperature for spinach is just 75°F.

Not all seeds need a heat mat

What next?

After your seeds germinate, unplug the heat mat. Besides wasting electricity, keeping the heat mat on can stress veggie roots and can cause some plants to bolt!

That’s about all there is to it!

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