Short answer: a 72-cell insert usually sits in a standard 10 × 20 inch tray, so a mat of about 10 × 20 inches fits one tray. Choose a 20 × 20 inch mat for two trays, and add a thermostat if you can.
Fit chart
| Trays | Total 72-cell capacity | Common mat size |
|---|---|---|
| 1 | 72 cells | 10 × 20 in |
| 2 | 144 cells | 20 × 20 in |
| 4 | 288 cells | 48 × 20 in |
Measure the shelf too. A long mat needs a flat surface that fits it completely, with no overhang and no folds.
Do you need a thermostat?
Basic mats have no temperature control. They simply run warmer than the room, so the result depends on how warm the room is. A thermostat with a probe placed in the soil holds a set temperature. Cornell's factsheet points to a thermostatically controlled mat as an aid to getting soil warmth right.
| Setup | Good for | Watch out for |
|---|---|---|
| Mat with no thermostat | A cool room and a small batch of seeds | Trays drying out or overheating, particularly on a warm day |
| Mat with thermostat | Peppers, mixed trays, rooms that change temperature | A probe placed in the air will give false readings |
What temperature to aim for
Crops differ, and your seed packet is the best guide. Peppers and eggplant are the most demanding, and common guidance gives 80–85°F for best germination. For tomatoes, gardeners often aim for at least 70°F. Cool-season crops such as lettuce don't usually need a mat at all and may germinate poorly if it's too warm.
Using a mat well
- Set the mat on a flat, dry surface. Use a solid tray above it, as it should never sit in water.
- Put the thermostat probe into the soil of a cell near the middle.
- Check moisture daily, because warm trays dry faster.
- Move trays off the mat as soon as seedlings appear, which helps keep growth sturdy.
Use mats as the manufacturer directs, keep connections dry, and unplug them when they're not in use. Don't leave a mat running under a dry, empty tray.
What to look for
Two trays
Long shelf
Look for a listing that states the waterproof rating and exact dimensions. A separate thermostat can be shared across more than one mat if the total load stays within its stated rating.
