How to stop greenhouse seedlings going leggy

Diagnose the stretch

Leggy seedlings have long, thin stems, often leaning towards the brightest side of the greenhouse. Leaves may be small and pale and the plants may fall when watered. The principal mismatch is too little usable light for the warmth driving growth. The RHS seed-sowing advice emphasises giving emerging plants light and appropriate growing conditions; a greenhouse roof does not guarantee strong light if the bench is shaded by shelves, opaque covers or nearby trees.

Look at the tray at the time of day when the problem occurs. A top shelf may receive useful daylight while a lower shelf remains dim. Condensation, dirt on glazing, tightly packed trays and an opaque propagator lid all reduce light. In early UK spring, even a clean greenhouse has short days; starting a heat-loving crop very early can produce weak plants if you cannot provide enough light. The practical correction may be to sow later, not to make the propagator hotter.

Change the environment after emergence

Warmth helps germinate some seeds, but seedlings need a different setup after they appear. Remove a dome or open its vents as instructed for the crop, then move trays into brighter, appropriate growing-on conditions. Avoid putting delicate seedlings straight into fierce sun; increase exposure gradually and monitor moisture. A max/min thermometer helps you find whether the greenhouse is excessively hot by day yet too cold overnight. See when to move seedlings before shifting indoor-raised trays into an unheated structure.

Thin seedlings according to packet spacing once they are ready. Crowded plants shade each other and make a humid, poorly ventilated canopy. Give trays enough room for air to circulate and rotate them if they lean consistently towards one direction. Water the compost rather than repeatedly wetting foliage, and let excess drain. These changes also reduce the conditions associated with damping off.

Decide whether to save or resow

If stems are only slightly elongated but still firm, potting on at the right stage and providing better light may allow strong new growth. Handle by a leaf rather than crushing the stem. Do not assume that burying every seedling deeply is safe. Tomatoes can root from buried stems, but other crops may rot if their crowns are buried; check the relevant crop instructions. If stems have collapsed or pinched at soil level, investigate disease rather than treating it as mere legginess.

Very weak seedlings are a poor foundation for a long-season greenhouse crop. If a tray has a high proportion of fallen, thin plants, resowing at a better light window can be more productive than nursing it for weeks. Clean any reusable tray before the next sowing, use a suitable fresh seed compost and give each crop the conditions on its packet. Our propagation guide separates germination, seedling growth and final planting so one heated location is not expected to do all three jobs.

Avoid the repeat next season

Plan backwards from the likely move date instead of sowing everything on the earliest possible day. Reserve bright bench space before filling propagators. Label sowings, check them daily and move them promptly when they emerge. If a cool spell forces you to keep plants inside longer, choose the brightest suitable location and give them more space as they grow. The UK growing calendar is a planning framework, but the weather and available light still decide the precise transition.