Making Your Own Peat-Free Potting Mix: Recipes for Seeds, Pots and Mediterranean Herbs

A pot of growing mix has four jobs at once. It has to hold water so roots can drink, hold air so those same roots can breathe, hold nutrients so the plant can feed, and hold the plant upright against the wind. The first two pull hard against each other, because water and air occupy the same gaps between particles. Fine, spongy material such as compost and leafmould stores water and nutrients but packs down and stays wet. Coarse, gritty material drains and breathes but stores almost nothing. Every recipe below is a different compromise between those two camps, and the right compromise depends on the plant and, in Scotland, on how much rain will be landing on the pot: a wet spell fills the air spaces first.

What each ingredient actually does

Nobody grows in a single material, so it helps to know what each one contributes and what it costs you. What to buy in a bag is covered in the peat-free compost guide; the table below is about what goes in the bucket.

For which grit, sand or gravel to choose, see the drainage materials comparison.

Loam deserves a second look because it is the ingredient most home recipes lean on and the one most easily wrong. It should come from a part of the garden free of oxalis, horsetail and ground elder, which will happily move into your pots. It should be sieved, and taken from several spots rather than stripping one bed. If your soil is heavy clay, the fix is more sand or grit in the mix; on light sandy soil, more organic matter helps the mix hold nutrients (improving clay soil covers the garden side).

Recipes by job

These ratios are all by volume.

Garden Organic gives home recipes for seed sowing and potting on. The RHS advises that home-made mixes are best avoided for seed sowing, because they can carry fungi that harm seedlings or cause damping-off. Our advice: for seed, buy a peat-free seed compost, and if you do mix your own, sterilise the loam first, because that step matters more than the ratio; the sterilising guide has the one-line seed mix and the methods for cleaning up your ingredients.

You will also see the John Innes formulas quoted everywhere. The base is 7 parts sterilised loam, 3 parts peat substitute and 2 parts coarse sand by loose volume, with fertiliser added per 9-litre bucket. In a peat-free batch, coir, composted bark or wood fibre from the ingredient table takes the place of the peat substitute.

There are no legally binding standards for John Innes ingredients, so bagged John Innes varies from maker to maker.

Mel's Mix, the equal-parts blend popularised by square-foot gardening, is repeated on countless sites, but it belongs to that method, not to the loam-based recipes above. The square-foot gardening guide covers how the method works in a Scottish bed; for pots, stick with the published loam-based recipes above.

The gritty mix for Mediterranean herbs

Rosemary, thyme, sage, oregano and lavender come from dry, stony ground and rot in wet. A Scottish winter is the hard test: cold, saturated compost around a plant that wants none of it. Rosemary hates wet roots, especially in winter, and should never sit in waterlogged compost. Herb pots should not be allowed to freeze, get saturated by rain or dry out, and in a Scottish garden it is saturation you plan for.

There are two routes to a gritty mix, and they are built differently:

Start at the generous end of either range: a third sand in the first, the full quarter of grit in the second. That is our suggestion for a wet Scottish climate, not a published ratio.

A few practical points:

• Pick a pot at least 30cm wide and deep for rosemary, with plenty of drainage holes, and do not overpot, since a large mass of mix stays wet for longer.
• Lift the pot on feet or bricks so the holes never sit in a puddle. The lavender guide covers a gritty mound for the border and a container mix with grit up to a quarter of the volume. For a woody rosemary that has already gone wrong, see reviving woody rosemary.
• In the ground on heavy soil, use a raised bed or a container instead; rosemary is happier there.
• Group pots together and wrap or shelter them if a hard freeze is forecast; potted rosemary is particularly vulnerable to winter cold.

The gritty mix suits rosemary, thyme, sage and oregano alike. Do not enrich it to push growth; lean is what these plants are built for.

Method and quantities

Measure by volume, not weight, using one container as your "part". Three parts measured in a 9-litre bucket make a 27-litre batch. The pot size guide converts litres to centimetres so you can work out how many pots a batch fills.

1. Sieve the loam to remove stones and debris, and sieve the compost or leafmould to take out large lumps.
2. Tip everything onto a tarp or into a mixing tub and turn it with a spade until the colour is even.
3. Moisten it slightly before filling; bone-dry mix repels water for the first few waterings.
4. Fill pots loosely, then firm gently rather than packing down, since compaction squeezes out the air you built the mix to hold.
5. Store leftovers in labelled bags in cool, dry conditions.

If you want to sterilise the loam yourself, the published oven method for moist, sieved soil is to heat it in a baking tray up to 8cm deep for 10 minutes at 71 to 79°C. The sterilising guide explains the fire and steam risks and the alternatives, so read it before you turn the oven on.

Weight, wind and the balcony

Loam and sand make heavy, stable mixes, which helps a tall tomato in a gale and hurts a fourth-floor balcony. Swapping loam for coir or bark lightens a mix, and perlite is light too, but the loam-based recipes above assume loam, and any lighter variant is your own experiment. Test one pot before you commit a whole balcony. The balcony growing guide covers weight limits and wind, and the pot materials comparison helps if the pots themselves are part of the weight problem.

Feeding: why a home mix runs out

Seeds carry their own food, so seed mixes should be lean. Anything that will live in a pot for a season is different. Bagged composts hold enough nutrients for roughly four to six weeks, and container plants should be fed from about six weeks after planting. A home mix does not escape this. Compost and leafmould supply some nutrients, and loam some more, but they are variable, and a pot has a small root run that empties quickly.

For a shrub or perennial in a mix with plenty of organic matter, extra fertiliser is often unnecessary. Crops that fruit heavily in a pot are different, and the feeding table below applies.

The fertiliser types guide explains what the numbers on a bottle mean. Bear in mind that coir holds nutrients poorly, so any mix with a lot of coir needs feeding sooner and more often.

pH: worth a quick check, not a project

A pH of 6.5 is the best general-purpose figure for most plants apart from lime-haters, and nutrient availability is at its highest in the moderately acid range of 6.1 to 7.0. The catch with home mixes is that pH is hard to standardise: it depends on your loam and your compost, and the mix will not match a label. Test a handful of the finished mix with a kit, and if it is far outside that range, adjust the ingredients rather than the pot (soil pH testing and adjustment explains how). A test taken within three months of adding lime, fertiliser or organic matter can mislead, so test the loam on its own before it goes in the mix. Lime-haters such as blueberries want an acidic mix, so the 6.5 target does not apply to them. For lavender and Mediterranean herbs, drainage matters far more than pH.

Hygiene: keep garden soil out of seed trays

Damping-off is caused by soil-borne fungi and fungus-like organisms, and it kills seedlings in a tray fast. Commercial growing compost is typically free of these organisms; unsterilised garden soil is not. Sow thinly, avoid overwatering, keep air moving, and use new pots and trays or wash reused ones and disinfect them. If you use rainwater, keep the butt covered so leaves do not rot in it; mains water is safer for seedlings. There are no fungicides available to home gardeners for damping-off, so prevention is the only protection.

For raw garden soil, the safe rule is simple: it goes into pots for established plants, not into seed trays. Spent compost can be reused for pots and mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → but not for seedlings without care; see reusing spent compost.

Scottish springs and cold mixes

A wet, loam-heavy mix warms slowly, and seed sitting in cold, damp mix through a Scottish April can stall or rot. If you are sowing indoors, a lighter, more open seed mix (the loam-and-sand option, or a bought seed compost) is the sensible choice, and the germination temperature guide sets out what temperatures different seeds want. Once seedlings are ready to leave the windowsill, hardening off covers the move outdoors.

When a home mix is not worth it

Buy instead when you need only a few litres. A single pot, a windowsill of seedlings or a small batch of cuttings costs more in time and ingredients than a bag does, and a bag is consistent where your compost heap is not. Buy for seed sowing, where sterilised commercial seed compost with low nutrient levels is the safer route. Buy if you have no source of clean loam, or nowhere dry to store a batch, or a balcony where a bag of lightweight peat-free mix is easier to carry up the stairs than a tub of loam. Peat-free composts can start to break down if kept beyond the season they were bought for, so buy only what you will use.

Make your own when you already have compost and leafmould, need large volumes for beds of pots or raised planters, and want a proper gritty mix for herbs that no general bag provides. Run the numbers on litres rather than bag prices: count how many litres your pots hold using the pot size guide, then decide. For growing with no potting mix at all, see hydroponics for beginners and the Kratky method; for tiny quantities, microgreens need very little mix. A wormery can add to your compost supply. Once your mixes are in the pot, a top layer of mulch helps steady watering.

Common questions

Sources