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Seed-Starting Heat Mat: When to Turn It Off

A seed-starting heat mat is a great tool for getting indoor seeds to sprout evenly and quickly, but they also inspire two common questions: Do I leave the heat mat on all night? Do I keep it on after the seeds germinate? Here are the answers.


Question: After I’ve sown the seeds and turned on the heat mat, do I leave it on all night?

Answer: Yes. Leave the heat mat on and set to the same temperature 24 hours a day until the seeds sprout. The idea of turning it off at night usually springs from the observation that the earth cools at night and warms up again in the day, thanks to the sun. Seeds do not need that cyclical cooling to sprout, however, and in fact they will germinate more quickly with constant warmth. So leave the seed-starting heat mat on all the time.

Question: Do I keep it on after the seeds have sprouted?

Answer: No. Turn the seed-starting heat mat off and put it away once the seedlings come up. Leaving it on may spur rapid, lanky, weak growth or encourage fungal diseases at the soil level.