There is something quietly satisfying about filling a seed tray with a mix you have made yourself, from stuff that was already sitting in your garden. It costs nothing, it keeps you off the garden centre queue in February, and once you have the knack, your seedlings get off to a noticeably stronger start than they do in a bag of general-purpose compost. Seed compost is not the same thing as potting compost. Seeds carry their own food supply, so they want a fine, open, low-nutrient medium that holds moisture and lets roots move freely. Rich compost can burn young roots and encourage the soft, leggy growth that damping off loves.

What You Actually Need

The classic recipe for a homemade seed mix has three parts to it, and each one does a job.

• Sieved leaf mould — two or three years old, crumbly and dark. It holds water beautifully and gives that fine, almost floury texture seedlings like. Fresh leaf mould is too acidic and too coarse.
• Well-rotted garden compost — sieved to remove twigs, stones and half-rotted lumps. Use it sparingly, as it is far richer than leaf mould.
• Horticultural sand or fine grit — sharp, not builder's sand, which contains too much fine silt and sets like concrete. This is what keeps the mix open and free-draining.

If you have a patch of decent loam — old, well-worked topsoil that has never been heavily limed — that is worth digging and sieving too. A loam-based mix holds its structure far longer than anything bought in a bag.

The Basic Mix

For a reliable, general-purpose seed compost, combine roughly two parts sieved loam or leaf mould, one part sieved garden compost, and one part horticultural sand. If you have no loam, use two parts leaf mould to one part compost to one part sand, and accept that it will dry out a little faster.

Sieve everything through a 6mm riddle first, then a finer one if you have it. The aim is a mix with the texture of damp cake crumbs — no lumps, no visible twigs. Mix it on a tarpaulin or in a big tub, turning it over several times until the colour is even. Add a handful of lime per barrowload if your leaf mould is acidic, which most is, and a light dusting of rock dust or wood ash if you want trace minerals, though this is optional.

Moisten as you go. Squeeze a fistful: it should hold together and then break apart with a light prod. Dust-dry mix repels water, and soaking wet mix smothers seeds.

Sterilising: Worth It or Not?

Homemade mixes carry weed seeds, fungus gnat larvae and the pathogens that cause damping off. For most hardy vegetables and flowers, that risk is manageable. For precious seed — a rare chilli, some hand-saved heritage tomato, anything you only have a dozen of — it is worth sterilising.

The simplest method is the oven. Spread the damp mix in a roasting tin no more than 5cm deep, cover with foil, and heat at 80–90°C for about 45 minutes, stirring once halfway. You want the centre to reach roughly 70–80°C. Any hotter and you cook the organic matter into a sour, claggy mess. Be warned: it smells like a wet allotment bonfire, so do it when the household is out or the windows are open.

A microwave works for small batches — a couple of minutes per kilogram on full power, stirring between bursts. Some gardeners pour boiling water through a bucket of mix and leave it to drain, which is gentler but far less thorough. Whichever route you take, use sterilised mix fresh. It is biologically dead, so it has no defences against moulds once it sits around damp for weeks.

Test Germination Before You Commit

This is the step most people skip, and it is the one that saves a season. You have a mix of unknown fertility, unknown pH and unknown pathogen load, so run a quick trial.

• Fill a small pot with your mix, firm it lightly and water it.
• Sow ten or so fast, cheap seeds — mustard, cress or a pinch of lettuce.
• Keep at 15–20°C in good light, and count how many come up after seven to ten days.
• Eight or more healthy seedlings means the mix is good. Five or fewer, and something is wrong.

If germination is poor, suspect the compost is too acid, too wet, or has been contaminated by a mouldy batch of garden compost. Adjust and retest rather than gambling your best seed on it.

Storing and Using It

Keep finished mix in a lidded bin or old compost sack in a cool shed, and use it within a season or two. Damp stored mix grows algae and liverwort on the surface, so store it just moist rather than wet. When sowing, fill trays loosely, firm gently, sow, cover thinly, and water from below with a fine rose or by standing the tray in a shallow bath. Sieve a little dry mix over the top to cover the seed, and label everything — you will not remember which tray was which in April.

Make a batch now, test it, and you will have a free, dependable seed compost ready for the earliest sowings under cover. It is one of those small allotment skills that pays back every single spring.