What Compost Should I Use for Sowing Seeds Indoors?

Using the wrong growing medium is one of the most common reasons seeds fail or seedlings struggle. Regular potting compost, garden soil, or multipurpose mixes can all cause problems for germinating seeds. Choosing the right seed compost at the start makes every subsequent step easier.

Why Seed Compost Is Different

Seeds contain all the energy they need to germinate — they do not need nutrients at this stage. In fact, high-nutrient composts can actually damage tender roots through salt burn. Seed compost is deliberately low in nutrients and has a very fine, open texture that allows tiny roots to push through easily and lets excess water drain away quickly. This matters enormously: waterlogged compost at germination time is the main cause of damping off disease.

What to Look for on the Bag

A good seed compost or seed-starting mix will typically say:

• "Seed and cutting compost" or "fine-grade seedling mix"
• Low nutrient levels (often listed as low EC value)
• Fine, sifted texture with no large lumps or bark pieces
• Peat-free options are widely available and work very well

Avoid anything labelled "enriched" or "with added fertiliser" for the initial sowing stage — save those for when you pot on.

Can You Use Multipurpose Compost?

In a pinch, yes — but sieve it first to remove large chunks and mix it roughly 50:50 with vermiculite or perlite to improve drainage and aeration. This DIY mix works well for larger seeds (courgettes, beans, cucumbers) but is less ideal for tiny seeds like celery, petunias, or antirrhinums which need a very fine surface to make good contact.

Garden Soil: Avoid It Completely

Never use garden soil for indoor sowing. It compacts in pots, drains poorly, and — most critically — carries weed seeds, fungal spores, and soil-borne pathogens. The warmth and moisture of a propagator turn garden soil into a petri dish. Invest in a proper bag of seed compost; it is inexpensive and the difference in results is remarkable.

Homemade and Peat-Free Options

Coir (coconut fibre) is an excellent peat-free base for seed starting. It is light, drains well, and has a neutral pH. Mix coir with a small amount of fine compost and perlite for a mix that performs as well as any branded product. Worm castings added at about 10% by volume provide gentle nutrition once the seedling has its first true leaves, without the risk of salt burn at germination.

Whatever mix you use, fill trays the night before sowing and let the compost settle so it is moist but not wet when seeds go in. Cold, freshly wetted compost slows germination noticeably.