Using a Seed Starting Heating Pad

Many seeds require a warmer soil temperature for germination than the seedling needs to grow after germination.  This ensures that the seeds germinate when the weather is warm enough for the plants to survive.

Frost on my lettuce seedlings.

For most seeds, directly planting the seeds in the garden is the preferred method, but many gardeners start some of their plants indoors with "bottom heat" to provoke germination and get an early start on the season.

• They want to harvest as soon as possible, whether for market stalls or just bragging rights.
• Their growing season is too short for this plant.
• By the time the soil is warm enough, the remaining growing season is too short to get any harvest.
• Their soil never warms up enough to germinate the seeds, but transplants will grow.
• Their area for starting seeds indoors is too cool for germinating.
• They need to get the plants germinated and seedlings established before hot weather arrives.
• They want to get their garden work done before hot weather arrives.

I'm gardening in several of those categories, especially the last two.  I want to get the chili peppers and tomatoes in the ground to avoid the coming hot weather, and my workshop is too cool.
Use a heating pad meant for seed starting, not a random heating pad you found in the linen closet.  You need a waterproof pad that won't overheat.  Over the mat, you place a tray to hold the pots or with soil and seeds.

Place some pots with moist soil, but no seeds, in the tray over the mat and turn it on. Leave it overnight to stabilize and check the temperature.  Is it right for the kind of seeds you want to germinate?

Most seed mats will keep the temperature of the pots or slats on the mat 10 to 20 degrees F above the ambient temperature of the room.  To control the temperature more precisely,  you will need a thermostat.

Tips for temperature control

• Insulate the bottom of the mat.  The blue material in the picture is a dense insulating mat.
• Use some sort of thermal mass to stabilize temperature swings. The sand-filled tray shown here will slowly heat and cool.
• Make sure the temperature sensor for the thermostat is at root level. Mine is buried in the sand near the middle of the tray.
• If the mat never gets to the temperature you set, cover the mat and pots with some sort of insulation to trap the heat . (not shown) A sheet of bubble wrap works.

Seed starting mat (under the tray of sand) with
chili and tomatillo seeds in soil blocks.

I'm using soil blocks in small food storage containers to germinate the seeds. They are inexpensive and the 20 tiny cubes from the smallest soil blocker fit perfectly. The lids are resting on the container, not sealed, to allow air circulation.

TIP:  Check the seeds daily, and don't keep the pots on the pad more than a day or two after germination. It encourages fungal growth and warm soil can slow down root development. Move them to a cooler place under grow lights.

Are mats and thermostats expensive?  

It depends on the size of the mat. The 9x18" mat and thermostat I am using cost under $50, and should last for several seasons. Being able to get the chili varieties I want, not what the two nurseries in town decide to order, and having seedlings ready to plant out when it's best for me is worth the cost.