Some links on this page are to Amazon UK. If you buy through them, Plant Light Guide may earn a commission. This does not change the price you pay. Affiliate disclosure.
Propagation lights and heat mats in the UK
A propagation station is a seed tray or a few pots of cuttings, a clear lid, sometimes a gentle heat mat, and a light that does not cook the lid. It is not a grow tent. Fruiting crops in a tent, with a high-output panel and an extractor, are covered by Hydroponics Guide.
Light above the tray
Seedlings stretch when the light is weak or far away, and they scorch when a hot lamp sits on the dome. You want even light across the compost, which is why a bar or a small panel beats one bulb in the corner of the tray.
The RHS artificial lighting guide gives a home example of two 60 cm T5 tubes about 60 cm above a small group of plants, on for around 16 hours. LED bars are often hung closer than old fluorescents because they run cooler. "Often" is not your measurement. Use the distance chart for the lamp you bought, and treat any band on this site as indicative. The reasoning is in PPFD and distance.
A practical schedule for seedlings and cuttings is 14–16 hours on, then a dark period. Do not leave the lamp on overnight. Hours are spelled out in hours and a dark period.
Raise the lamp as the seedlings gain height. Leaves against the LED are too close, whatever the chart said on day one.
Domes, mould and a cracked lid
A humidity dome, or a clear lid on a propagator, keeps cuttings from drying out. It also keeps still, wet air, which is where grey mould starts. Vent the lid every day. If the inside is dripping constantly, crack the vent or take the lid off for an hour.
If the room itself is damp, the dome is not the only wet object. Breezewold on reading humidity is the context for a meter that says 70%, and their ventilation notes apply when the windows are streaming.
Do not rest a lamp on the plastic lid. Hang or clip it above, with air between the lamp and the dome.
Heat mats
A heat mat warms the compost from below, which helps germinate seeds in a cold room. It does not replace light. A warm, dark tray grows long, weak seedlings.
Many mats are meant to be used with a thermostat so they are not on at full power all day and all night. Follow the temperature the seed packet asks for, and the instructions printed on the mat. Do not fold a mat, do not put it under bedding, and do not cover it with anything the maker forbids.
We haven't tested this kit. Specifications are as the maker states them (checked 13 November 2026).
One large example, the Spider Farmer seedling heat mat, is listed at 108 W, with an input of AC 110–240 V 50/60 Hz, a size of 48 × 20.75 inches, and a built-in digital controller. The maker says it heats the root zone by 10–25 °C above the room. That is a big mat for a whole bench, not the first buy for a single windowsill tray. A smaller mat should still be 230 V with a UK plug, about the size of your tray, and controlled so it can switch off.
If that 108 W mat ran flat out for 24 hours it would use 2.59 kWh, about 68 p at the October–December 2026 cap rate of 26.32 p/kWh. The controller exists so it should not do that. The cost page shows the same arithmetic for lamps, which are on for a known number of hours. A thermostat makes the mat's hours unknown, so treat any mat figure as an upper bound.
Seeds that will go outside
If the tray is the start of an outdoor sowing rather than a permanent houseplant, the calendar and the seed tin matter as much as the lamp. Tilthwick's sowing calendar and seed storage notes are the outdoor half. Harden plants off before they leave the light. A seedling that has only ever seen LEDs will scorch in April sun if you skip that week.
Shelf length, cables and timers for the lamp over the tray are in timers and shelf layout. The shelf tool will suggest a bar or a small panel if you tell it the tray is seedlings.