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How to Use a Heat Mat to Start Seeds

By Jenny Harrington ; Updated September 21, 2017
Most plants require warmth to germinate.

Starting your own plants from seed is less expensive than purchasing nursery seedlings. You also have a greater choice in plant varieties when you plant from seed, giving you a chance to try old heirloom plants or exotic flowers that are popular enough to be offered as seedlings. The proper temperature is vital to successful seed germination, and is often difficult to replicate indoors. Use a heat mat to start your seeds and ensure that as many seeds as possible are able to germinate.

Fill a seedling tray with a quality potting mix. Use a soil-less mix as it is sterile and made up of finer particles, which makes it easier for the seedlings to sprout through.

Sow the seeds to the depth indicated on the seed packet, usually at a depth twice that of the seed's width. Water the soil just until it is moist and cover the top of the seed tray with a layer of plastic wrap to help hold in the moisture.

Set the heat mat to the proper germination temperature for the type of seeds you are growing. Most plants require a soil temperature of 70 to 72 degrees Fahrenheit, but some plants, like peppers, require warmer temperatures of 80 to 85 Fahrenheit. Read the back of the seed packet to verify the temperature requirement of your seeds.

Set the seedling tray on top of the heat mat. Leave the heat mat on at all times until the seeds germinate. Germination usually occurs within seven to 21 days of planting, depending on the seed variety.

Remove the plastic wrap once sprouts appear. Move the seedling tray and the heat mat to a sunny windowsill. Alternately, place the seed tray beneath fluorescent grow lights, positioning the lights so they sit 3 to 6 inches above the top of the plants. Leave the lights on for 12 to 16 hours a day, and leave the heat mat on at all times until you are ready to transplant the seedlings outdoors.


Things You Will Need

  • Seedling tray
  • Potting mix
  • Seeds
  • Plastic wrap
  • Heat mat
  • Grow lights


  • Place fluorescent lights on a timer so you don't have to remember to turn them on. Just make sure the heat mat isn't plugged into the timer, as the seeds and seedlings require consistent warmth to thrive.
  • Most seeds don't require light until after they germinate, but check the seed packet to be sure, as a few plants require light to germinate.


  • More warmth is not better. Avoid turning up the heat mat beyond the recommended setting for your type of seeds. Too much warmth may cook the seeds and render them useless.

About the Author


Jenny Harrington has been a freelance writer since 2006. Her published articles have appeared in various print and online publications. Previously, she owned her own business, selling handmade items online, wholesale and at crafts fairs. Harrington's specialties include small business information, crafting, decorating and gardening.