Can I Use Potting Mix to Start Seeds? Yes, With Caveats

Potting mix works for larger seeds but is too coarse and too rich for fine ones. Sieving it and cutting it with perlite closes most of the gap.

By Zakaria October 12, 2026 3 min read Save to Pinterest

Rows of seedlings emerging from compost in biodegradable fibre pots — Photo: Pexels, Pixabay Content License, via Pixabay

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Yes — with caveats. Potting mix is coarser and usually fed, and both matter at germination. Large seeds cope easily; fine seeds do noticeably better in a proper seed mix. Sieving it and cutting it with perlite closes most of the gap.

The short answer

Potting mix can be used to start seeds but is a compromise, because it is coarser than seed mix and usually contains slow-release fertiliser. Large chunks leave air gaps a thread-thin root cannot bridge, and fertiliser salts can scorch the first root. Large seeds such as beans, squash and sunflowers cope easily while fine seeds like lettuce and most herbs do better in a proper seed mix. Sieving the mix and cutting it with perlite and fine coir lightens the texture and dilutes the feed at once.

Coarse texture and added fertiliser are the two problems
Large seeds cope; fine seeds suffer
Sieve it, then cut with perlite and fine coir, 2:1:1
Seeds need no nutrients until the first true leaves
Damping off is the risk — sow thin, water from below, move air
Homemade mix: equal parts coir, perlite, sieved leaf mould

Why seed mix is different

This table answers the question on its own

	Seed starting mix	Potting mix
Texture	Fine and even	Coarse, with bark and lumps
Nutrients	Almost none — deliberately	Slow-release fertiliser
Sterile	Usually	Not necessarily
Water behaviour	Even, consistent	Patchy around chunks
Suits	Fine seed, any seed	Large seed; potting on

The low nutrient level is the design, not a shortcoming: a seed carries its own food until the first true leaves, and salts in the medium can scorch the first root instead.

Making potting mix work

Sieve it. Remove bark chunks and lumps — that is most of the problem solved.
Cut it: roughly two parts sieved mix to one part perlite and one part fine coir or sieved leaf mould. This lightens the texture and dilutes the fertiliser together.
Fill and firm gently. Air pockets are what strand a thread-thin root.
For very fine seed, top the surface with a thin layer of pure sieved material so the seed sits in something even.

Which seeds do not care

Beans, peas, squash, courgette, sunflowers, sweetcorn — big seeds with big reserves and robust first roots. These germinate happily in straight potting mix and are not worth buying seed compost for.

Damping off

A fungal collapse where seedlings topple at soil level. Too rich, too wet, or not sterile all contribute.

Prevention is the whole battle

Once it starts in a tray it usually takes most of it. Sow thinly. Water from below. Keep air moving across the tray. Take the humidity dome off as soon as seedlings emerge. Use clean trays. There is no reliable cure after the fact.

Your own mix

Equal parts fine coir or peat substitute, perlite, and sieved compost or leaf mould — with the compost reduced or left out for very fine seed. It should feel light, hold together when squeezed damp and crumble when poked.

Sterilising is not essential for most sowings but reduces damping off; a short spell in a hot oven or a scald with boiling water does it.

When to start feeding

At the first true leaves — the pair after the seed leaves. Before that, feeding achieves nothing. After that, a half-strength liquid feed every week or two, or pot on into ordinary potting mix, which already carries what they now need.

Common questions

Can I use potting mix to start seeds?

Yes, and plenty of gardeners do, but it is a compromise rather than an equivalent. Potting mix is coarser and usually contains slow-release fertiliser, both of which matter at germination — large chunks leave air gaps that fine roots cannot bridge, and fertiliser salts can scorch the first root a seed puts out. Large seeds such as beans, squash and sunflowers cope easily. Fine seeds such as lettuce, petunia and most herbs do noticeably better in a proper seed mix.

What is the difference between seed starting mix and potting soil?

Seed mix is fine, light, low in nutrients and usually sterile, while potting soil is coarser, heavier and fed. The low nutrient level is deliberate rather than a shortcoming, because a seed carries its own food reserve and needs nothing from the medium until the first true leaves appear. The fine texture matters because it holds consistent moisture against a tiny seed and lets a thread-thin root find its way without hitting a void.

How do I make potting mix work for seeds?

Sieve it to remove the lumps and bark chunks, then cut it with perlite and fine coir or sieved leaf mould at roughly two parts mix to one part each. That lightens the texture and dilutes the fertiliser at the same time. Fill the tray with the sieved blend, firm it gently, and if you are sowing something very fine, top the surface with a thin layer of pure sieved material so the seed sits in something even.

Can you make your own seed starting mix?

Easily, and the standard recipe is equal parts fine coir or peat substitute, perlite and sieved compost or leaf mould, with the compost reduced or omitted for very fine seed. The result should feel light, hold together when squeezed damp and crumble when poked. Sterilising homemade mix is not essential for most sowings but reduces damping off, and a short spell in a hot oven or a scald with boiling water does it.

What is damping off and does the mix cause it?

Damping off is a fungal collapse where seedlings topple at soil level, and the mix contributes when it is too rich, too wet or not sterile. The practical defences are sowing thinly, watering from below rather than overhead, keeping air moving across the tray, removing any humidity dome once seedlings emerge, and using clean trays. Once it starts in a tray it usually takes most of it, so prevention is the whole battle.

When do seedlings actually need feeding?

When the first true leaves appear, which are the pair after the initial seed leaves. Until then the seed's own reserve supplies everything and feeding does nothing useful, which is why a nutrient-free starting mix costs you nothing. From true leaves onward a half-strength liquid feed every week or two is enough, or pot the seedlings on into ordinary potting mix, which carries the nutrition they now need.

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A handful of dark potting mix with white perlite granules visible through it

Lawn & Soil · Sep 9, 2026