Propagation Heat Mats

Why bottom heat matters for propagation

It is the temperature of the medium, not the air, that seeds and cuttings respond to. A heat mat puts that warmth in from below, so the roots sit in heated medium instead of waiting for the air under the dome to catch up. Pro Grow put the result on their own listing: bottom heat increases germination and cutting strike rate, and it is most useful in a cool room or a cold snap. For most Australian sheds and garages that is autumn through spring.

The extra heat is always relative. These are not ovens with a set temperature: the mat adds a fixed lift to whatever the room is already doing. Cultiv8 publish 5 to a maximum of 11°C above room temperature for their flexible mats; Pro Grow quote 5–10°C above ambient; Gardis say that in a room already above 18°C their mat will add another 5–10°C. Use your room temperature as the starting point, not a number you wish the tray would sit at.

Our range of heat mats

Flexible mats by size and wattage

Cultiv8 make the flexible size ladder: Small at 388 x 282mm and 18W, Medium at 550 x 280mm and 25W, and Large at 644 x 350mm and 35W. Each is double insulated, C-Tick certified and supplied with a plug. The Pro Grow mat is cut long and narrow — 225 x 525mm at 18W — to sit with the Mondi 3-piece propagation kit. Gardis Pro is 527 x 254mm at 20W, waterproof, and brings adjustable temperature control at that size.

Mats and controllers that hold a temperature

Two items here hold a temperature rather than just adding one. The Growy mat has the thermostat on board: 25 x 52cm, IP65 splash-proof, double insulation to stop root scorching, set to hold 20–30°C with a 0–50°C controller range. The Cultiv8 Heat Mat Thermostat is the plug-in box for mats you already own — Celsius or Fahrenheit, 20°C to 42°C in 1° steps, 1000W maximum load and an auto safety shut-off.

Insulation underneath

Most growers skip this one. Pro Grow tell you to always put the mat on an insulated surface, because concrete, tile or a metal bench will take the heat downwards. The ROOT!T insulated mat is 350 x 250mm, sits directly under a heated mat, and keeps that warmth moving up into the tray.

How to choose a propagation heat mat

1. Size to the tray, not the bench. The mat should cover the tray footprint. 388 x 282mm is a single small tray; 644 x 350mm covers a much larger one.
2. Choose heat only, or heat plus control. A plain mat gives a fixed lift above ambient. If the room swings from day to night, a built-in or plug-in thermostat is what stops the root zone swinging with it.
3. Work from the coldest the room gets. The mats add 5–11°C to the air they sit in. A room that falls to 12°C overnight is the number that lift is applied to.
4. Put insulation underneath. On concrete, tile or metal, downward heat is wasted. An insulated mat under the heat mat keeps it in the tray.
5. Check the wattage against the controller. The mats themselves are light loads — 18W to 35W — but a thermostat has its own ceiling. The Cultiv8 controller takes up to 1000W, which is several mats on one box.

Running a heat mat safely

Leave the mat flat, dry and indoors, and keep it under a tray rather than against wet medium. All the flexible mats on this page are double insulated and the Cultiv8 range is C-Tick certified. Run a thermostat with its probe in the medium, not the air — that is the temperature you are actually holding — and keep a thermometer on the tray so you can see the number.

Pair heat mats with trays and domes and propagation mediums, or take a complete box from propagation kits.