When to move seedlings into a greenhouse

Read the greenhouse, not the calendar

An unheated greenhouse is useful protection from wind and rain, but it is not a heated room. On a clear spring day it can warm rapidly; after dark, the air can fall close to the outdoor temperature. A single warm afternoon is therefore a poor reason to move a tray of tender seedlings permanently. Record the minimum overnight temperature for several nights, including a colder forecast night, and compare it with the needs of the crop. Put the thermometer where the trays will stand rather than high at the ridge.

The RHS greenhouse vegetable guide distinguishes starting hardy vegetables early from moving tender summer crops into their final growing positions later. Lettuce and other hardy seedlings may be ready for cool protected growing while peppers, cucumbers and tomatoes still need warmth. A greenhouse in a sheltered southern garden and one exposed to a northern wind will not have the same usable start date.

Group seedlings by cold tolerance

Label every tray with crop and sowing date. Hardy salads and brassicas can often make the transition earlier, provided they are sturdy, rooted and not suddenly exposed to a hard freeze. Tender crops are different. The RHS cucumber guide, for example, advises planting greenhouse cucumbers when the inside temperature is consistently above 12°C. The RHS sweet-pepper guide notes that peppers grow better above 15°C. These are crop-specific guides, not a universal threshold for every seedling.

Plants from a heated propagator need light as soon as they emerge. A warm base does not compensate for a dark greenhouse or a cold night outside the covered unit. If a crop has germinated indoors but the greenhouse is not ready, pot it on into a bright, suitable protected position rather than rushing the move because roots have filled the tray. For sowing fundamentals, see starting seeds in a greenhouse.

Make a gradual transfer

Start with short daytime visits on mild, bright-but-not-scorching days. Increase time under glass, then leave plants overnight only when the measured minimum is suitable for that crop. Open vents as appropriate so plants become used to air movement, but avoid a strong draught over small trays. Check compost moisture: a tray that remained damp indoors may dry much faster in sun and wind. Conversely, a cool tray needs less water and should not sit saturated.

This move into an unheated greenhouse is a transition between protected environments. If the plants will eventually go outside, they still need hardening off to outdoor sun, wind and cooler air. The RHS describes a gradual two- to three-week outdoor acclimatisation for tender plants; a few greenhouse days alone do not make a cucumber frost-hardy.

Watch for evidence that the timing was wrong

Limp or scorched leaves after a bright day can indicate an abrupt light or water change. Darkened or collapsed tender leaves after a cold night point to temperature stress. Long, pale stems signal too little light relative to warmth; see the leggy seedling guide. If plants wilt despite moist compost, inspect roots and stems rather than automatically adding water.

Keep a simple fallback: a frost-safe indoor space, a clean coldframe with appropriate protection for hardy crops, or a plan to bring tender trays inside before a cold night. Fleece can add limited protection but cannot turn an unheated greenhouse into a reliably warm one. Check the forecast and the thermometer each morning until the move is established. Once plants are growing strongly, use the UK greenhouse calendar to plan their next stage.