How do I stop seed trays drying out in the greenhouse?
Small cells in a sunny greenhouse can dry out in a single morning. Capillary matting, sand benches, covers and positioning — what actually holds moisture without drowning the seedlings.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
Short answer
Stand trays on damp capillary matting or a sand bench so they draw water from below, keep them out of direct midday sun until they are up, cover freshly sown trays until germination, and water from underneath rather than over the top.
A module tray holds a tiny amount of compost with a very large surface area, sitting in the sunniest place in the garden. Once a peat-free compost has dried it becomes water-repellent, so a tray that dries out once is much harder to keep moist afterwards. Prevention is the whole of the answer.
Step by step
- Water from below wherever you can.Stand the tray in a shallow bath of water until the surface just darkens, then lift it out and let it drain. It wets the whole cell evenly, does not wash the seed about, and does not compact the surface.
- Set up a capillary bench.Capillary matting on solid staging, kept damp, with the trays standing directly on it. The compost draws water up through the drainage holes as it needs it. It is the standard nursery method and it transforms how often you need to intervene.
- Or use a sand bench.A shallow tray of sharp sand, 30 to 50mm deep, kept damp. It holds far more water than matting, evens out the swings even better, and doubles as a stable surface for small pots. It is heavy, so check the staging will take it.
- Cover freshly sown trays until germination.A clear lid, a propagator top, a sheet of glass or a clear plastic bag holds humidity so the surface does not dry. Remove it the moment the first seedlings appear, or you get damping off and drawn growth.
- Keep trays out of direct midday sun.A tray on a south-facing bench in April can go from moist to bone dry in a morning. Shade the propagation bench, or put it where it gets morning sun and afternoon shade.
- Use deeper modules for anything you are growing on.A deep cell holds more compost and buys hours. Small plug trays are for germination only — prick out or pot on promptly rather than trying to keep a full tray of seedlings alive in tiny cells.
- Mulch the surface of larger pots with vermiculite.A thin layer of vermiculite over sown pots and modules reduces surface evaporation, keeps the seed in contact with moisture and lets light through for seeds that need it.
- Group the trays together.Trays standing in a block shade each other's sides and create a more humid microclimate. Trays scattered singly across a bench dry out from every edge.
Tips
- A tray that has dried out completely will not rewet from the top. Stand it in water for half an hour — you will see air bubbling out as the compost takes water back up.
- Peat-free composts are more prone to becoming water-repellent when dry than the old peat-based ones. Never let them dry fully, rather than trying to recover them.
- A cheap timer and a battery-powered drip system fed from a water butt covers you for a weekend away, which is when most trays are lost.
If it doesn't work
Water sits on the surface then runs off the sides
Cause: The compost has dried and become water-repellent, so it no longer absorbs from above. — Fix: Stand the whole tray in water until it stops bubbling, then keep it on damp capillary matting so it never dries fully again.
Seedlings collapse at soil level with a thin, dark stem
Cause: Damping off — a fungal problem encouraged by overwatering, still air and overcrowding, often from a permanently wet cover. — Fix: Remove the cover once germinated, improve air movement, sow thinner, water from below with mains water rather than stored rainwater.
Seedlings pale, stretched and leaning
Cause: Too little light, usually from a cover left on too long or a shaded position, not from watering. — Fix: Remove covers at germination, move to the brightest bench, and turn trays daily.
Compost is wet but seedlings wilt anyway
Cause: Waterlogged cells with no air at the roots, often from a capillary bench that is too wet or a tray standing in water permanently. — Fix: The matting should be damp, not flooded. Lift the trays off and let them drain, then keep the matting moist rather than saturated.
Questions people ask
Should I leave trays standing in water?
No. Stand them in water to wet them, then lift them out and let them drain. Permanently standing in water excludes air from the roots and causes damping off.
How often should I check seed trays in spring?
Twice a day on a sunny day — morning and mid-afternoon. On a capillary bench, once a day to top up the matting is usually enough, which is why the bench is worth setting up.