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, thus
promoting faster germination
and strong, healthy seedlings. They are useful for rooting
cuttings. Heat mats are marketed as a propagation mat or seedling heat mats
as well, but the function is the same. Read on for more information and learn
how to use a heat mat for seed starting.
What Does a Heat Mat Do?
Most seeds germinate best in temperatures between 70 and 90 degrees F. (21-32 C.), although some, such as pumpkins and other winter squash, are more likely to germinate in soil temps between 85 and 95 degrees F. (29-35 C.). Many won’t germinate at all if soil temperatures fall below 50 degrees F. (10 C.) or above 95 degrees F. (35 C.).
In many climates, temperatures aren’t consistently warm
enough to germinate seeds, especially in late winter or early spring, prime seed
starting times. Keep in mind that damp soil is cooler than the air
temperature, even in a warm room.
You may be advised to put seed trays in a sunny window, but
windows aren’t consistently warm in early spring and they may be very cold at
night. Heat mats, which use very little electricity, produce gentle, consistent
heat. 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, or
even individual pots. Be patient, as it may take a couple of days for the mat
to warm the soil, especially with deep or large pots.
Check the soil daily with a soil
thermometer. Even heat mats with thermostats should be checked occasionally
to ensure the thermostats are accurate. If the soil is too warm, raise the tray
or container slightly with a thin piece of wood or a potholder. Seedlings can
become weak and leggy in too much heat.
In general, you should remove seedlings from heat and put
them under bright light soon after they germinate. However, if the room is cool,
consider keeping the seedlings on the warm mats until the air temperature
warms. You may want to raise the containers slightly to prevent overheating, as
suggested above. Check the soil
moisture daily. Warm soil dries out faster than cool, damp soil.