Should you use a heat mat for plants?
It’s true that you can grow seeds on the windowsill in recycled yogurt cups — people do it all the time. But I’ve found that I’m more successful at growing indoors if I give seeds bottom heat with a germination mat.
Why is bottom heat important for seed-starting?
When starting seeds inside, bottom heat helps the germination mix you’ve planted in stay warmer than room temperature. You can plant seeds in cool soil without the extra warmth, but they may sit and languish in cold, damp conditions for days, which invites fungal diseases into your germination mix and can cause damping off. In contrast, if you warm the potting mix, seeds germinate significantly faster and you avoid those unhealthy conditions that cause seedlings to die. Plus, I’ve found it can cut germination time by as much as half.
Encourage sprouts with a germination heat mat
Most seeds germinate reliably at 60 to 70 degrees F, but some plants sprout best at higher temperatures. Strangely, plants considered “cold crops” often germinate more quickly at higher soil temps but prefer to grow at lower air temperatures. That makes them good candidates for starting indoors early in the season, then moving them outside while it is still chilly. Scroll down to see optimal germinating temperatures at which many common vegetables and flowers sprout best.
Why use a germination heat mat for plants?
If you plan to start seeds every year or do a bunch of them, it really is just easiest to get a commercial heat mat. It’s designed to do this job, after all, and is waterproof, safety inspected, easy to use, thin, stores easily, and the right size to fit a growing flat (or you can buy larger ones that hold several flats at once). There’s even a narrow one to fit on that windowsill. The heat is consistent at all times and keeps the soil about 10 degrees warmer than room temperature — perfect for growing in a cool garage or basement.
A germination mat may have a thermostat to fine-tune your temps for finicky seeds. You can get a basic germination mat for as little as $20. Not ready to buy a mat? Scroll down for more ways to warm your seeds below.
How long do seeds need bottom heat?
A germination mat should be on or warm all the time — no need to use a timer. Remove the seedlings from bottom heat as soon as most of the seeds sprout. Once leaves emerge, seedlings prefer slightly cooler temperatures to harden off and may actually get leggy and weak by remaining on the germination mat.
Other ways to create bottom heat for seed-starting
Any extra warmth can make a difference when starting seeds. If you don’t have a germination heat mat, here are a few hacks that will give your seed-starting tray bottom heat:
A warm appliance
Many household appliances, such as refrigerators, water heaters or freezers, give off just enough heat to warm a tray of potting mix. Just set it on top of the appliance.
Secure outdoor-use string lights under a shelf with s-hooks or a baker’s cooling rack with cable ties like you see above. Set your seed-starting tray right on top.
Radiator or heat vent
Set trays above the heater on a shelf, but make sure it doesn’t get too hot! Remember that it can be dry near a vent, so check the soil often to make sure it stays moist.
Foil tent around grow lights
Warm the germination mix by trapping heat given off by grow lights with aluminum foil tented around the lights and seed trays.
Pet bed warmers and reptile heating mats work just like a germination mat and won’t get too hot.
DO NOT USE electric blankets or heating pads as a germination mat. They get too hot for seedlings, should not be used in wet conditions, are not designed to be on constantly.
Optimal germinating temperatures for different seeds
All seeds have a range of soil temperatures at which they will germinate, but have an ideal temp at which more seeds sprout. Check out the lists below to see these plants’ preferred temperature.
65 degrees F soil temperature to germinate
- Bachelor’s button (Centaurea cyanus)
- Melampodium (Melampodium divaricatum)
- Nasturtium (Tropaeolum majus)
- Parsnip (Pastinaca sativa)
70 degrees F soil temperature to germinate
- Black-eyed Susan (Rudbeckia fulgida)
- Cosmos (Cosmos bipinnatus)
- Leek (Allium porrum)
- Snapdragon (Antirrhinum majus)
- Spinach (Spinacia oleracea)
- Sunflower (Helianthus annuus)
- Zinnia (Zinnia elegans)
75 degrees F soil temperature to germinate
- Coleus (Plectranthus scutellarioides)
- Impatiens (Impatiens walleriana)
- Lettuce (Lactuca sativa)
- Nicotiana (Nicotiana spp. and hybrids)
- Parsley (Petroselinum crispum)
- Pea (Pisum sativum)
- Petunia (Petunia hybrids)
- Wax begonia (Begonia semperflorens)
80 degrees F soil temperature to germinate
- Bean (Phaseolus vulgaris)
- Brussels sprouts (Brassica oleracea gemmifera)
- Cabbage (Brassica oleracea capitata)
- Carrot (Daucus carota sativus)
- Cauliflower (Brassica oleracea botrytis)
- Kohlrabi (Brassica oleracea gongylodes)