Seedling Heat Mats to Start Seeds Indoors| Almanac.com

Using Seedling Heat Mats to Start Seeds Indoors


Using warming mats for faster germination and even growth

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Want your seeds to germinate faster? Seedling heat mats really work!  They’re a cheap, compact alternative to a greenhouse, too. Learn more about how these warming pads not only get you a quick start but also help your plants perform better.

If you are lucky enough to have a car with heated seats you understand the value of bottom heat and when trying to get seeds to germinate, bottom heat comes in handy as well. In the past, we were advised to put our seed flats on a warm surface like the water heater or on top of the TV but with on-demand water heaters and flat screen TVs those hot spots are long gone. 

Invest in a seed-starting heat mat made for plant propagation. 

What are Seedling Heat Mats?

Seedling heat mats are waterproof, rubberized pads that you place under your seed flats or pots. The mats are waterproof so there are no electrical worries when watering plant trays. Several layers of plastic are heat-bonded over strong heating wire.

They increase the temperature of the soil 5 to 10 degrees so your seeds germinate faster. What’s just as important is that seeds germinate better with less uneven rates, and plants end up being healthy with such consistent warmth. 

Heat mats especially benefit warm-weather vegetables including tomatoes, peppers, and eggplants, giving them a chance to grow past the seedling stage before being transplanted outside.

This 20 X 23 inch heat mat holds 2 standard trays.

Buying the Best Seedling Heat Mat

When shopping for a heat mat, make sure it is meant to be used in a wet location. The waterproof rating should listed on the product. IPX4 is water resistant but can’t be submerged in water. IP67 is fully waterproof and can be under water for 1/2 hour and still work. 

Heat mats use very little electricity and only get warm enough to encourage your seedlings to pop up. Some are thermostatically controlled so you can set the temperature you want, while others just warm the root area 10 to 20 degrees above the air temperature. You can always check the soil temperature with a soil thermometer if you think it is getting too hot or not getting warm enough. 

A 10 X 20 inch mat holds one standard tray perfectly and costs under $20. There are larger mats that hold 2 trays or more and 3 inch wide strips made for using on a windowsill. Most roll up easily for storage.

The optimum temperature for most seeds to germinate is between 65 to 85 degrees. 

  • A heat-lover such as okra likes soil in the 85 to 95 degree range but others like cooler soil. 
  • The germination rate for lettuce drops off sharply in soil over 70 degrees so lettuces don’t need to take up space on your heat mat. 

See dates for starting seeds by zip code.

Bottom heat speeds up the germination process.

Seedling Heat Mats for Safety

Seed mats are very safe. Don’t put your mat on anything sharp that could puncture it and don’t pile soil directly on it. Covering it with insulating materials could cause it to overheat.

So many DIY efforts are safety hazards. Never use an electric blanket. They get too hot, use too much electricity, are a fire hazard, and can give you a nasty shock if they get wet. Same is true for the heating pad. Save it for your sore muscles. No, you don’t have to drive the seed flats around in your car until they germinate.

How Long Do You Keep Heat Mat On?

For consistent warmth, leave the mat on 24 hours a day rather than shutting it off at night until the seeds sprout. Do not turn it off. Seeds beneath the soil do not need nighttime cooling; they need constant warmth to sprout. 

Flats not covered with a humidity dome or in a plastic bag will dry out fast on the mat so check them daily and water as needed to keep the soil moist. 

When to Remove Seedlings From Heat Mat

Leave your seed starting trays on the heat mat only until they germinate. As soon as your seeds pop up, take off the dome or any plastic coverings, remove the flat from the heat, and place it in a cool bright spot to continue growing. Make room for the next seed starting tray(s).

Ready for grow lights to grow your seedlings on? Learn more about the best grow lights for growing vegetables.

You may find your heat mat comes in handy for other projects like making yogurt, kombucha, bread or beer. I put mine to good use warming my old cat Guido’s bed during his final days. 

I’m not big on gadgets but our heat mat gets used every spring to speed up the germination process.

About The Author

Robin Sweetser

Robin has been a contributor to The Old Farmer’s Almanac and the All-Seasons Garden Guide for many years. Read More from Robin Sweetser

2023 Gardening Club