Heat mat and LED grow light for seedlings: the exact settings

Bottom heat and artificial light: the two reflexes of successful indoor sowing — but the details matter. The heating mat is useful yet optional, the lamp is near-essential from February on, and 3 mistakes cause 90 % of leggy seedlings. Here are the exact settings.

The heating mat: what it really does

First the principle: a growing medium is always cooler than the air above it because of evaporative cooling — at 20 °C room temperature, the medium can drop to 18 °C, too cold for some species. The mat compensates: placed under the tray, it can warm the medium 8 to 11 °C above ambient temperature.

• Target: most seeds germinate well between 20 and 26 °C, for soil and air alike; broadly, 21 to 26.5 °C for the medium.
• By species: tomato 21-32 °C (optimum 29 °C), aubergine at least 27 °C until emergence then 21 °C, pepper warmer than people like it.
• Careful: continuous bottom heat of 25-30 °C can slow root growth — the rule is "warm bottoms, cold tops".

When to remove the mat — the point everyone misses

Three thresholds coexist: at full emergence, at 50 % emergence, or "keep it if your space is a bit cool". The universal reflex: humidity dome AND mat removed at emergence, tray under the lights.

And honestly: the mat is optional — you don't have to use one, it is often unnecessary. It becomes valuable in a cool room (garage, conservatory), for warm species (tomato, pepper, aubergine) and to catch up on a late sowing. Never a domestic heating pad: fire risk — and a thermostat stops your seedlings frying.

Light: the real limiting factor

On the lamp side, fluorescent tubes remain one of the best choices for a small indoor garden — T5 tubes are designed to deliver triple the output of standard tubes, with ~20,000 h of service; high-intensity LEDs deliver more lumens but must be placed further from the plants.

Duration: 14 to 16 hours on, 8 to 10 off — and at the very start, leaving them on for 24 to 48 h straight after germination locks the cotyledons.

And the window? Sufficient until cuttings have rooted or for herbs in summer — but in the heart of winter, a south-facing window delivers a fraction of needs: it is the n°1 cause of leggy seedlings.

Legginess: diagnose and rescue

Leggy seedlings — overly tall, stretched, weaker and prone to toppling: the n°1 sign of insufficient light. The signals in order: seedlings lean toward the source, internodes lengthen (triggered by blue-light deficiency, the shade-avoidance stretch), the habit turns thin and lanky instead of thick and stocky.

Aggravating factors: excess heat (high temperature + low light = guaranteed stretch) and density (more than one seedling per cell = shading and competition). Watch the opposite symptom too: too much light bleaches leaves yellow/white with marginal burn.

Rescue, 5 levers: 1) bring the lamp closer or raise trays on crates; 2) pot on deeper — tomatoes readily form new roots on buried stem: plant them up to the top 4-6 leaves (two-thirds buried); 3) brush the tops of seedlings to thicken stems; 4) lower the night temperature; 5) thin out (1 seedling/cell).

The Germie protocol

1. Seed tray on the heating mat + dome — medium target 20-26 °C (27-29 °C for tomato/aubergine/pepper).
2. At emergence (or 50 %): dome and mat off, tray under the LED grow light at the maker's distance, 14-16 h/day.
3. 1 seedling per cell, night temperature 16-21 °C, bottom watering.
4. Deep potting for tomatoes (buried stem), then 7-10 days hardening before moving out — the tomato sowing guide details the rest.

The complete Germie kit (indoor greenhouse + LED + mat) covers both reflexes at once — and the winter sowing calendar tells you what to sow each month.