What Makes a Good Seed Raising Mix?

A seed raising mix has one job: hold enough moisture for a seed to germinate without ever going soggy, and stay loose enough for a new root to push through. It is not potting mix and it is not garden soil, and the differences are the reason home made mixes either work beautifully or fail completely. A simple blend of a water holding base, an aerating mineral and a little sieved compost will raise almost anything.

Before you start: a real safety note

Why make your own

Two honest reasons. It is cheaper if you sow a lot, because bagged seed raising mix is expensive by volume. And you can adjust it, which matters if you are raising something that wants sharper drainage, or if the mix you can buy locally is too coarse for fine seed.

If you sow two trays a year, buying a bag is perfectly sensible. This is for people who have outgrown that.

What goes in, and what each part does

Most of the confusion in home made mixes comes from treating perlite and vermiculite as interchangeable. They are close to opposites.

Coir or peat, the base

This is the bulk of the mix and the part that holds water. Coir is made from coconut husk, comes as a compressed block you rehydrate, and has a near neutral pH. Sphagnum peat holds water superbly and is naturally acidic, which suits some crops and not others.

Which to choose is a genuine trade off rather than an obvious win. Peat is slow to form and peat bogs are significant carbon stores, which is the real argument against it. Coir is a by product of an existing coconut industry, but it is shipped a long way, its processing uses a lot of fresh water, and unbuffered coir can be salty and can lock up calcium and magnesium, so it is worth buying a buffered product or rinsing a block before use. One point worth correcting: good quality sphagnum peat is not a common source of plant disease, and is generally low in weed seed and pathogens. The case against peat is environmental, not hygienic.

Perlite, for air

The white volcanic granules. Perlite creates air space and lets excess water drain away. It holds very little water itself, which is exactly the point. Use it when you want a more open, free draining mix.

Vermiculite, for water

The soft, flaky, golden brown mineral. Vermiculite does the opposite of perlite: it holds water and it holds nutrients, because it has a high cation exchange capacity. Use it when you want a mix that stays evenly damp, and it is particularly good as a thin covering over seed that needs to stay moist.

You can use one or the other, or a little of both. Just do not expect perlite to keep your mix moist or vermiculite to open it up.

Sieved compost or worm castings, a small amount

Seeds carry their own food supply and do not need a rich mix to germinate. A modest amount of well finished, sieved compost gives seedlings something to grow into after the first true leaves, but too much will hold water, encourage disease and can burn fine roots. This is the ingredient people most often overdo.

Coarse sand, optional

Sharp or washed river sand adds weight and drainage, which helps for larger seed and for trays that get knocked about. Do not use fine builder's or beach sand, which packs down and sets hard.

A mix that works

By volume, not weight:

• 2 parts coir or peat
• 1 part perlite or vermiculite, or half of each
• 1 part sieved compost or worm castings

Mix it dry, wearing a mask, then wet it and let it sit for a few hours so the coir or peat absorbs properly. It is ready when a squeezed handful holds together and releases a drop or two of water, not a trickle. Sieve out anything lumpy, because a clod beside a small seed can block it entirely.

For very fine seed, sieve a portion of the mix finer still and use that for the top layer.

Keep it clean, and keep it moving

Damping off is the disease that flattens seedlings overnight, and it is caused by soil dwelling fungi and water moulds. Overwatering does not create them, but it creates the conditions they need, so the practical controls are the same either way: clean trays, fresh mix, sow thinly, water from below where you can, and give the trays some air movement rather than leaving them sealed under a lid once they are up.

Wash and dry reused punnets before the season starts. It is a ten minute job that prevents a fortnight of disappointment.

Frequently asked questions

Can I use garden soil in a seed raising mix?

Better not to. Garden soil compacts in a tray, drains poorly, and brings weed seed and disease with it. Keep it for the garden bed.

Do I need to sterilise my mix?

Usually not, if you use fresh ingredients and clean containers. Sterilising also kills the useful organisms in compost.

Perlite or vermiculite?

Perlite for drainage and air, vermiculite for holding moisture and nutrients. If you tend to overwater, use perlite. If your trays dry out on a windowsill, use vermiculite.

Is coir better than peat?

Each has drawbacks. Coir avoids peat extraction but carries shipping, processing water and salinity considerations, and is best bought buffered. Choose on what matters most to you rather than on a claim that one is simply better.

How rich should the mix be?

Barely. Seeds germinate on their own reserves. Start feeding lightly once the first true leaves appear, or pot on into a richer mix then.

Related growing guides

• A step by step guide to starting seeds indoors
• Damping off: what it is and how to prevent it
• Sow to the Seed, Not to the Spade: How Deep Seeds Really Want To Go
• Sandy, clay or loam? Understanding and improving your soil type

Ready to plant? Browse our Vegetable Seeds.

Health guidance on potting mix follows NSW Health and SA Health advice on Legionella longbeachae.