Seedling Heat Mats: How To Use A Heat Mat For Plants

By: Mary H. Dyer, Credentialed Garden Writer

What is a heat mat for plants, and exactly what does it do?Heat mats have one basic function which is to gently warm the soil, thuspromoting faster germinationand strong, healthy seedlings. They are useful for rootingcuttings. Heat mats are marketed as a propagation mat or seedling heat matsas well, but the function is the same. Read on for more information and learnhow to use a heat mat for seed starting.

What Does a Heat Mat Do?

Most seeds germinate best in temperatures between 70-90 F.(21-32 C.), although some, such as pumpkinsand other wintersquash, are more likely to germinate in soil temps between 85-95 F. (29-35C.). Many won’t germinate at all if soil temperatures fall below 50 F. (10 C.)or above 95 F. (35 C.).

In many climates, temperatures aren’t consistently warmenough to germinate seeds, especially in late winter or early spring, prime seedstarting times. Keep in mind that damp soil is cooler than the airtemperature, even in a warm room.

You may be advised to put seed trays in a sunny window, butwindows aren’t consistently warm in early spring and they may be very cold atnight. Heat mats, which use very little electricity, produce gentle, consistentheat. Some heat mats for plants even have thermostats to adjust the heat.

How to Use a Heat Mat

Put a heat mat under seed starting flats, celled trays, oreven individual pots. Be patient, as it may take a couple of days for the matto warm the soil, especially with deep or large pots.

Check the soil daily with a soilthermometer. Even heat mats with thermostats should be checked occasionallyto ensure the thermostats are accurate. If the soil is too warm, raise the trayor container slightly with a thin piece of wood or a potholder. Seedlings canbecome weak and leggy in too much heat.

In general, you should remove seedlings from heat and putthem under bright light soon after they germinate. However, if the room is cool,consider keeping the seedlings on the warm mats until the air temperaturewarms. You may want to raise the containers slightly to prevent overheating, assuggested above. Check the soilmoisture daily. Warm soil dries out faster than cool, damp soil.

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How to Use a Heat Mat to Start Seeds

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.

  • Starting your own plants from seed is less expensive than purchasing nursery seedlings.
  • 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.

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.

  • Set the heat mat to the proper germination temperature for the type of seeds you are growing.
  • 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.

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.

Choose Your Seeds Carefully

You may be growing seeds indoors to get a jump on the growing season, to compound your fruit and vegetable harvest or both. Without the benefit of a heating pad for germinating seeds or a plant light, you must be extra choosy about the types of fruit and vegetable seeds you start indoors. The truth is that some plants would give you a hard time anyway, meaning that they would grow better if the seeds were planted directly into the ground without the risk of disturbing the roots.

Root vegetables like beets and carrots are notoriously fussy, so The Old Farmer's Almanac recommends planting these other fruit and vegetable seeds indoors instead:

  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Cauliflower
  • Eggplant
  • Lettuce
  • Peppers
  • Pumpkin
  • Swiss chard

Even if you were going full tilt with a heat mat or light, it would be a good idea to anticipate a few losses. So, plant a few more seeds than you had originally planned.

Starting and growing your own transplants from seed is one of the most challenging yet rewarding parts of this gardening passion we all share.

While most veggies will germinate in ‘room temperature’ without issue, there are cases where some added warmth could benefit the process. First, plants such as peppers, eggplant and tomatoes germinate better in warmer soil (about 70 degrees F is ideal). Second, you may have a situation where the only location you can do your seed starting gets cold in the winter, i.e. a garage or basement. In that case, it’s more sensible to only heat the trays rather than the whole room.

The solution? A heat mat. While grow lights placed above the seedlings provide some heat, a heat mat fits the job of warming the soil quite nicely. The problem? Heat mats can be pricey.

While I already own two mats myself, I started to see the need for a third one this winter. Some research online led me to cases where folks had strung holiday lights around young trees or perennials to help protect them from frost damage.

Recycle your rope lighting into an inexpensive heat mat

Since the holidays are upon us, lights of all sizes, colors and shapes stock the shelves of the seasonal departments of most stores. What a bright idea (pun intended) it would be to construct a DIY heat mat using lights?

The heat ‘mat’ I’ve built here easily accommodates two of the 1020-standard 11″ x 21″ seed trays or flats. Rope lights come in a variety of lengths and colors, so you can customize them to fit your specific needs. You’ll need the incandescent type of lights, not LEDs. The really cool part? I managed to built this one for about $16. In fact, you may already have the needed materials on hand.

In building the mat, the rope light weaves around long thin strips of wood, and both are attached to a ‘plank’ of plywood. The gaps between the wood strips serve to help with airflow. The rope light will only emit a certain amount of heat – not enough to burn the wood. Since it’s thinner than the wood strips, the rope light doesn’t come in contact with the seed trays. Since the rope light is insulated for outdoor use, it’s protected from water.

What you’ll need:

1. Plywood ‘plank’ (I used a 1-inch thick x 12-inch wide x 4-foot long piece)

2. Two 1″ x 2″ x 8-foot wood furring strips

3. Wood screws – two packs each of #6 x 1-1/2″ and #8 x 3/4″ sizes

4. Plastic cable clamps – two packs of 1/2″ size

6. Tape measure and/or square

9. Rope light – incandescent type (not LED). I used an 18-foot length.

Skill Level:

Easy to intermediate, depending on your wood-working/cutting skills.


1. To correctly wrap the 18-foot rope light as I wanted, I ended up having to cut the 4-foot plank of plywood to approximately 44 -1/2″ long, but you can certainly leave the excess on if you don’t want to cut it.

2. Cut the two 1″ x 2″ x 8′ furring strips to get four 40″ long pieces. Sand any rough edges or surfaces (Photo A).

3. Using the 1-1/2″ wood screws, attach the 4 furring strips to the plywood plank (Photo B).

4. Unwind and straighten the rope light (Photo C).

5. Place the closed end of the rope light at the bottom right corner of the plywood plank (if you have the long side facing you). Wrap the rope light around the 5 spaces between the furring strips (the two outside edges are two of them) (Photo D). This is only for rough placement – we’ll tighten it down in the next step.

6. Using the plastic cable clamps and the 3/4″ wood screws, attach the rope light to the plywood plank (Photo E). I ended up using five clamps along each run of the board, and put an extra one at the ends of the cable to secure it better (Photo F).

7. Light ’em up baby! I mean,… um… plug it in (Photo G).

Your lighted heater is ready for seed-starting duty (Photo H).

So, if you’re crafty and are out shopping, why not grab some lights to warm your seeds?

How well does it work?

The heat mat takes approximately 30-45 minutes to fully heat up, and using my hand, felt as warm as the commercial heat mat. Upon taking measurements with both regular and soil thermometers, the numbers were impressive.

Commercial heat mat
Surface temperature
(thermometer placed on top of soil): 73-75 degrees F
Soil temperature: 80-82
Bottom temperature (gap between heat mat and seed trays): 100-102

DIY light heat mat
Surface temperature: 72-74
Soil temperature: 78-80
Bottom temperature: 105-110

Note: To control heating times, use a thermostat or timer.

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How to Plant Seeds Indoors

I’m always in awe of a little tiny seed like this grows a big cauliflower head! For our seeds, we usually exclusively use Johnny’s Seeds which has given us good results with their seeds.

We fill our seedling tray up with a organic compost medium mix. This provides a great beginning start for the seedlings. There are many varieties of growing medium but we like the 512 Mix.

Then we use a fancy tool to make little holes where we put the seeds into each block. Just kidding, we just usually a pen to quickly make holes.

Then we insert one seed in each hole.

After the seeds are all placed, we cover the holes back up.

Here’s a helpful tip that you should do right away. Grab a piece of paper and chart out what you just planted. Grid the tray out and mark what seeds you planted where. In the very beginning, it’s very hard to tell the plants apart, so try to stay organized.

Then we just wait until the seeds germinate and push their way through the soil. Since the conditions we created are what the seeds like, it only took 3 days until we had a full tray of seedlings showing.

I hope this post helps you get ready for your garden season! With a heat mat and thermometer you can make the seedling process more efficient.

Make sure to follow me on Facebook and Instagram where I post daily updates of our plants!

And check out how we grow tomatoes from seed for Summer gardening!

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Watch the video: Does a heat pad really help?

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