Gardening glossary
Bottom heat
Most seeds and most cuttings care more about root-zone temperature than air temperature. A pepper seed sitting in 16 °C soil germinates in 21 days or never; the same seed in 26 °C soil germinates in 7. A cutting in a cold windowsill takes 6 weeks to root or rots first; in a warm propagation tray it roots in 10 days. Bottom heat is the single most effective way to fix germination and rooting failures for cool-climate growers.
The principle. A thermostat-controlled electric heat mat sits under the seed tray, holds the substrate at a target temperature (usually 21–24 °C for most veg, 26–28 °C for chillis, peppers, aubergines, and tropicals), and leaves air temperature to drift naturally. The plant gets warm roots and cool tops, which mimics what happens in spring when sun warms soil faster than air.
What benefits most from bottom heat:
- **Slow- or warm-loving seeds.** Peppers, chillis, aubergines, basil, tomatoes (above 21 °C), melons, cucurbits, and most tropical houseplant seeds. - **Hardwood and semi-hardwood cuttings.** Rosemary, sage, lavender, citrus, fig, hydrangea, and most woody perennials root much faster on a heat mat. - **Tropical houseplant divisions and cuttings** — alocasia, anthurium, philodendron, and aroids in general respond strongly. - **Seedlings of cool-climate vegetables** sown in February for an early start in a cool greenhouse or shed.
What does *not* need bottom heat:
- **Cold-stratified seeds** mid-treatment — keep them cold. - **Cool-season crops** like lettuce, spinach, peas, broad beans, and brassicas, which germinate fine at 12–18 °C and can become leggy on heat. - **Established seedlings.** Once true leaves appear, move them off the heat and into a cooler, brighter spot — heat plus low light produces stretched, weak plants.
Practical setup:
1. Lay the heat mat flat on a shelf or bench. 2. Set the thermostat probe in the substrate of a representative tray, not in the air. 3. Place trays on the mat with no insulation underneath (trays sitting in saucers full of water actually need direct contact for the mat to read temperature properly). 4. Once 70–80% of seedlings have emerged, move trays off the heat to prevent etiolation.
A 23 × 50 cm heat mat with thermostat costs £30–50 and pays for itself the first year you successfully grow peppers from seed.