Using Heat Mats

Heat mats are flat water proof heat sources that you put under your plants to keep the soil at the optimum temperature.  Although temperature requirements vary, most seedlings prefer 75 - 85˚F soil and will germinate rapidly under these conditions.

A successful setup is not as simple as just plopping the tray on the mat and plugging it in.  The center of the tray or flat of plants gets hotter than the edges and tends to dry out the soil very quickly.  Its easy to kill the center seedlings if the flat sits directly on the heat mat. 

The key to a successful setup is to add an air space between the mat and the plants.  Below are the setups that work for us.

Manual Setup: Uses a thermometer to monitor the temperature.

Automatic Setup: Uses a thermostat to automatically shut off and turn on the mat.

Materials

  1. baking racks.  We've had success with racks that are 3/4 to 1 inch high.  See picture at left.  Too low and you may still get hot spots.  Too high and too much heat gets lost to the room.
  2. seedling heat mat
  3. heat mat thermostat (optional but highly recommended)
  4. thermometer (if no thermostat)

Manual Setup

This folding rack works when it is folded flat.

Adjusting a Manual Setup

If plants aren't warm enough

We drape clear plastic bubble wrap over the whole setup to keep the heat in.  A clear sheet of plastic should also help although it has less insulating capacity.  Some of our plants are grown in the basement.  Its really cold and yet the bubble wrap works great.

If plants are too warm: 

You can increase the gap between the plants and the mat.  Just make sure whatever you use to raise the rack doesn't trap the air under the plants.

You can also lower the temperature of the room.  Mats produce a constant amount of heat (+10 to 20˚F) so a cooler room will mean cooler plants.

Learn More

You'll find more useful information on starting your own vegetables and when heat mats should be used.  Check out our pages on how to grow tomatoes, cucumbers and peppers.

  1. Lay the mat down with the cord at the back out of the way.

  2. Place the baking racks on top of the mat.  Use enough of them to completely support the plant flats or trays.

  3. Plug the mat into an outlet unless you are using a thermostat.  See below for thermostat setup.

  4. Set the flats on the racks.

  5. Sandwich a reliable thermometer between the flats so its edge is resting on the rack.

  6. Monitor the temperature for the first hour or so to verify its within the range you want.

    These mats typically raise the temperature from 10 to 20 degrees above room temperature but we have found a lot of variation from mat to mat.  So, you need to monitor the setup and adjust things if necessary.
Thermometer is on its side wedged between the flats

Automatic Setup

Sensor is that white dot next to the flat
Sensor secured above the mat
thermostat with digital temp readout.

Adjusting an Automatic Setup

If plants aren't warm enough

If the room is really cold, the thermostat could be constantly on and yet the plants will not be warm enough.  We drape clear plastic bubble wrap over the whole setup to keep the heat in.  A clear sheet of plastic should also help although it has less insulating capacity.  We grow some of our plants in our basement which is cold and found the bubble wrap works great.

If plants are too warm: 

This situation is unlikely since the thermostat would just shut off the heat mat.  If you think the plants are too warm, lower the thermostat's set-point.  If that doesn't work, the equipment may be defective.

  1. Set the baking racks on the heat mat same as above.

  2. Place the thermostat's sensor in the middle of the mat.

    Lay the cord between the mat and the rack.  Don't put it on top of the rack or your tray will not lay flat and plants will not have uniform access to the water in the bottom of the tray.

  3. Attach the sensor to the rack so it doesn't touch the mat.  The little suction cups that are sold with these don't seem to work very well.  Twist ties work well.

  4. Plug the thermostat into an outlet and plug the mat into the thermostat.  When the sensor reaches the upper temperature set-point that you set, it will shut off the mat, preventing overheating

  5. Place the plants on the racks and your all set.

    This is a "no worries" set up.  Watch it for a day or two to make sure it is working properly and then, as long as nothing breaks, your seedlings will be very happy.

Comments

We don't recommend using heating pads like you get at the drug store for a sore back.  Seedling heat mats will get wet when you water the plants and heating pads aren't designed to be used when wet..