Sowing seed is the moment compost has to work hardest. A seedling's roots are a few millimetres long, the seed sits close to the surface, and there is nothing to buffer a dry afternoon on a windowsill or a cold, wet week in the greenhouse. Get the texture and moisture right and germination is quick and even. Get it wrong and you are left with a tray of gaps and a nagging feeling that peat-free compost simply doesn't work.

It does work. But coir, bark and wood fibre are three very different materials, and they behave differently in the fortnight that matters most.

What seeds actually need from a compost

Seeds carry their own food store, so a sowing mix does not need much nutrition. A rich compost can actually slow germination and encourages algae, moss and damping-off. What matters is structure and consistency:

• A fine, even texture, so seed makes close contact with damp particles rather than sitting in an air pocket.
• Steady moisture without saturation — a waterlogged mix starves seeds of oxygen.
• Enough open structure to let air move through, particularly if you water from above with a can.
• A pH near neutral; most vegetable and flower seed germinates happily around 6 to 6.5.
• Cleanliness: fresh compost, scrubbed trays, and no bag that has been open in a damp shed since last spring.

Coir: consistent and forgiving

Coir is the pith left after coconut husks are processed, sold loose, in blocks or as compressed bricks. It is fine, uniform and holds a lot of water, and unlike peat it rewets reasonably well once dry. That even texture is its real strength for sowing: seeds sit in close contact with moisture, and fine seed such as nicotiana or foxglove does not fall between lumps.

The drawbacks are the flip side of the same coin. Coir can sit wet if drainage is poor, and it slumps into a sloppy, airless surface when overwatered. It also contains almost no nutrients, which is fine for germination but means seedlings need moving on promptly. Good coir is washed and buffered with calcium and magnesium; cheaper unbuffered grades can carry a lot of natural salts. Bricks need rehydrating with warm water, breaking up by hand and then a few hours to even out before you sow.

Bark: airy, free-draining and best in a blend

Composted pine bark fines are open and quick-draining, which is exactly what you want when potting on a shrub and not quite what you want in a seed tray. Used alone, bark dries out fast and the coarser chunks leave seed suspended in air rather than pressed against damp material. Once it has dried right out, bark can also shed water off the surface instead of absorbing it.

Treat bark as a structural ingredient. Sieve it through a 6 mm sieve, keep the fine fraction for sowing and put the chunky bits aside for mulching or potting on. Uncomposted bark tends to be acidic, and as soil organisms break it down they take up nitrogen, so it is better fully composted. In a seed mix, bark earns its place at roughly one part in four, adding the air spaces that keep a coir-heavy mix from turning to sludge.

Wood fibre: the home-grown option

Wood fibre comes from softwood, often as a by-product of UK timber processing, so it usually travels far fewer miles than coir and is easier to buy from British growers and smaller suppliers. It looks a little like pale, chunky bran: spongy, light and surprisingly absorbent. Many of the better peat-free blends now use it as the bulk ingredient, with coir and bark for balance.

Two things to watch. First, it must be properly matured; fresh wood fibre feeds the microbes that break it down, and they compete with your seedlings for nitrogen. Second, it can be reluctant to rewet if it dries completely, so keep trays consistently damp rather than letting them swing between bone dry and soaked. Sieve the finer material for sowing and use the rest in potting mixes.

Side by side

• Water retention: coir holds the most and releases it slowly; wood fibre is close behind; bark holds the least.
• Drainage and air: bark is the most open; wood fibre sits in the middle; coir is the tightest and can become airless when wet.
• Rewetting: coir is the easiest to bring back from dry; wood fibre is slow; bark can turn water-repellent at the surface.
• Germination: coir suits fine seed straight from the bag; wood fibre works well sieved and blended; bark needs screening and a partner.
• Nutrition and pH: all three are lean, which is what seed wants. Bark is the most acidic; wood fibre can tie up nitrogen until matured.
• Sourcing: coir is imported, bark mostly comes from the UK and Europe, and wood fibre is often British-grown.

Choosing a bag in the UK

The word "peat-free" on the front tells you very little. The ingredient panel on the back, with proportions, tells you almost everything — and it is worth reading before you lift the bag into a trolley.

• Look for a stated breakdown rather than a vague list. A blend of wood fibre, coir and bark fines is a good sign; a bag of pure coir or pure bark is a specialist purchase, not a general seed mix.
• Squeeze the bag if there is a clear panel. It should feel like crumbly, slightly damp cake, not a brick and not wet putty.
• Check whether coir is described as washed or buffered. Compressed bricks store for years in a dry garage and give you a lot of compost for the space, so check the stated yield.
• Buy fresh for sowing and keep part-used bags sealed and dry. Compost left open in a damp shed grows liverwort and algae on the surface.
• Small bags of a dedicated seed and cutting mix are worth the extra for a first sowing; bulk bags tend to be coarser and are better for potting on.
• Try one bag from a local nursery before committing to several from a national brand. Peat-free mixes genuinely vary between batches and seasons, so one disappointing bag is not the whole story.

A blend worth trying, and a quick test

If you would rather mix your own, this blend is fine enough for seed and open enough to stay sweet:

1. Two parts by volume of fine coir, rehydrated and broken up.
2. Two parts sieved wood fibre, using the finest fraction.
3. One part sieved composted bark fines for structure.
4. A small handful of vermiculite or fine perlite per bucket for extra air.
5. Moisten the whole mix the day before sowing, aiming for the dampness of a wrung-out sponge, and firm it lightly into trays. Sow thinly and cover with sieved mix or vermiculite.
6. Water from below for the first fortnight. It keeps the surface from drying in patches and stops you dislodging seed with a can.

If you are using a lot of bark, a light dusting of garden lime will nudge the pH back towards neutral, but check with a cheap pH test kit before sowing rather than guessing. And to judge any ingredient, fill a small pot, water it, leave it overnight and squeeze a handful the next day: if water runs out, it is too wet for seed; if it crumbles and will not hold together at all, it is too dry. Aim for the middle, and your sowing will reward you.

Photo: Thirdman / Pexels