Why do my seedlings do badly in peat-free compost?
Four specific things go wrong when small seed meets a coarse mix — poor contact, surface drying, nitrogen drawdown and over-rich multipurpose — and each has a direct fix.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
Use a proper peat-free seed compost rather than multipurpose, or sieve the top 20mm of what you have. Cover seed with vermiculite rather than compost. Bottom-water by standing trays in a shallow tray of water rather than watering from above. And start liquid feeding at true-leaf stage, because peat-free runs out of nitrogen sooner than you expect.
Do this first
Sowing is where peat-free is genuinely harder rather than merely different, because small seed is unforgiving about the physical scale of the material it is sitting in. Chunks of bark that make no difference to a shrub in a 5-litre pot are boulders to a lettuce seed, and the surface drying that a large pot shrugs off kills a germinating seedling in an afternoon.
Safety
What you'll need
- Peat-free seed compost, or a riddle to sieve multipurpose
- Vermiculite
- A shallow tray for bottom watering
- A propagator lid or clear bag (optional)
- Liquid feed
Step by step
- Use seed compost, not multipurpose.Seed compost is milled much finer and is deliberately low in nutrients. Multipurpose is coarse and contains enough fertiliser to inhibit germination of small seed. Using multipurpose for sowing is the commonest cause of poor results and it is not specific to peat-free — it is just more punishing there.
- Sieve the surface if you only have multipurpose.Fill the tray with multipurpose to within 20mm, then riddle a fine layer over the top for the seed to sit in. It costs two minutes and fixes the seed-contact problem almost entirely.
- Firm very lightly, then level.Tap the tray down on the bench and strike the surface level with a board. Do not press hard — the whole point of the coarse structure is air, and compressing it out gives you a dense wet layer that rots seed.
- Cover seed with vermiculite rather than compost.Vermiculite holds moisture at the seed's level, does not cap, lets light through for seed that needs it, and is fine enough to give perfect contact. For small seed this single change transforms germination rates in peat-free.
- Bottom-water, always.Stand the tray in 20mm of water until the surface darkens, then lift it out and let it drain. Watering from above washes small seed about, compacts the surface and wets unevenly. Bottom watering is more reliable in any compost and close to essential in a coarse one.
- Cover to hold surface humidity.A propagator lid or a clear bag until germination. Peat-free surfaces dry within hours, and a germinating seed that dries once is dead. Remove the cover the moment the seedlings are up, or you will get damping off.
- Start feeding at true-leaf stage.Peat-free seed compost holds very little nitrogen, and wood-fibre products can tie some of that up as they break down. Once the first true leaves appear, feed weekly at quarter to half strength. Seedlings that go pale at three weeks are hungry, not sick.
- Prick out sooner than you would have with peat.The fine seed compost has almost no reserves. Move seedlings into a richer mix as soon as they can be handled by a seed leaf, rather than leaving them in the tray until they are large.
- Check the compost is fresh.A bag opened last spring and left in a shed is a different material now — the structure has slumped, the feed has gone, and the nitrogen drawdown is worse. Buy a fresh bag for sowing even if you have an old one for potting on.
Tips
- Peat-free seed composts vary a lot between brands. If germination is poor with one, it is genuinely worth trying another rather than concluding the category does not work.
- Adding perlite or vermiculite at around 20 per cent to a peat-free seed mix opens it up and improves both drainage and rewetting.
- Larger seed — beans, peas, courgettes, sweetcorn — has no difficulty at all in coarse peat-free compost. The problem is specific to small seed, so do not overhaul a system that is working for beans.
- Damping off is a real risk in the humid conditions that peat-free surface drying pushes people towards. Use clean trays, mains water rather than butt water for seedlings, and ventilate as soon as germination starts.
- Coir-based seed composts hold water more evenly than wood-fibre ones and are the easiest peat-free option for sowing.
If it doesn't work
Patchy germination, some seed never came up
Cause: Poor seed-to-compost contact on coarse material, or the surface dried out during germination — Fix: Sieve the surface layer, cover with vermiculite, and keep a lid on until germination. Bottom-water rather than watering from above.
Seedlings germinate then go pale and stall at three or four weeks
Cause: Nitrogen exhausted, and possibly drawn down by wood fibre still breaking down — Fix: Liquid feed at quarter to half strength weekly from true-leaf stage, and prick out into a richer mix sooner.
Seedlings collapsing at soil level
Cause: Damping off — too humid, too wet, too crowded, poor ventilation — Fix: Ventilate immediately, sow thinner, use clean trays and mains water, and stop bottom-watering into standing water. Once it starts it cannot be cured, only prevented next time.
Surface green with algae or liverwort
Cause: Consistently wet surface with poor air movement — Fix: Water less often and more thoroughly, improve air movement, and cover the surface with a thin layer of vermiculite.