Propagation Heat Mats

Why bottom heat matters for propagation

Seeds and cuttings respond to root-zone temperature far more than to air temperature. A heat mat warms from underneath, so the medium the roots are actually in gets the heat rather than the air above the dome. Pro Grow put the effect plainly on their own mat: bottom heat increases germination and cutting strike rate, and it is most useful in a cool room or a cold snap — which in most Australian sheds and garages is autumn through spring.

The lift is relative, not absolute. These are not thermostatic ovens: a mat adds a set number of degrees to whatever the room is doing. Cultiv8 rate their flexible mats at 5 to a maximum of 11°C above room temperature, Pro Grow quote 5–10°C above ambient, and Gardis note that in a room already above 18°C their mat will add another 5–10°C on top. Work from your room temperature, not from a target number.

Our range of heat mats

Flexible mats by size and wattage

The Cultiv8 flexible range is the size ladder: Small at 388 x 282mm and 18W, Medium at 550 x 280mm and 25W, and Large at 644 x 350mm and 35W. All three are double insulated, C-Tick certified and supplied with plugs. The Pro Grow mat is the long narrow one — 225 x 525mm at 18W, made to pair with the Mondi 3-piece propagation kit. Gardis Pro is 527 x 254mm at 20W, waterproof, and adds an adjustable temperature control at that size.

Mats and controllers that hold a temperature

Two products here do more than add degrees. The Growy mat has a thermostat built in: 25 x 52cm, an IP65 splash-proof surface, double insulation to prevent root scorching, and it is set to hold 20–30°C with a controller range of 0–50°C. The Cultiv8 Heat Mat Thermostat is the separate plug-in version — it will control mats you already own, reads in Celsius or Fahrenheit, is adjustable from 20°C to 42°C in 1° steps, carries a 1000W maximum load and has an auto safety shut-off.

Insulation underneath

The most-missed accessory on this page. Pro Grow's own instruction is to always place the mat on an insulated surface, because a mat sitting straight on a concrete floor or a metal bench sends much of its heat downwards. The ROOT!T insulated mat is built for exactly that — 350 x 250mm, sits directly under a heated mat and stops the loss going down instead of up.

How to choose a propagation heat mat

1. Measure the tray, not the bench. The mat should sit under the tray footprint. A 388 x 282mm mat suits a single small tray; 644 x 350mm covers a much larger one.
2. Decide whether you need control or just heat. A plain mat adds a fixed lift above ambient. If your room swings between day and night, a thermostat — built in or plug-in — is what stops the root zone swinging with it.
3. Start from your coldest room temperature. The mats add 5–11°C to whatever the room is. In a room that drops to 12°C overnight, that is the number the lift is applied to.
4. Insulate underneath. On concrete, tile or metal, heat that goes down is wasted. An insulated mat under the heat mat keeps it in the tray.
5. Check wattage against your power setup. These are small loads — 18W to 35W — but a thermostat has its own limit; the Cultiv8 controller handles up to 1000W, which is many mats on one controller.

Running a heat mat safely

Keep the mat flat and dry, indoors, and under a tray rather than in direct contact with wet medium. All the flexible mats here are double insulated and the Cultiv8 range is C-Tick certified. If you are running a thermostat, put its probe in the medium rather than in the air, since the medium is what you are actually trying to hold at temperature — and back it up with a thermometer so you can see what the tray is doing.

Heat mats pair with trays and domes and propagation mediums; for a complete setup in one box, see propagation kits.