How To Choose The Right Potting Compost For Your Favourite Plants And Seeds

As spring approaches, gardeners typically plan their sowing and potting – and potting compost is an essential element of this. More than three million cubic metres of potting compost are used in the UK each year, three-quarters of it by home gardeners, the rest by plant nurseries.

Potting compost is not actually compost but is a “growing medium” consisting of a blend of ingredients such as bark and wood fibre that provides ideal conditions for potted plants.

Plants seldom thrive in pots filled with garden soil. Compost is, strictly speaking, composted materials from a compost bin or composting facility. Potting compost has the ideal blend of nutrients, solids and air spaces for seeds to germinate and roots to thrive.

Manufacturers blend ingredients to make products that are general purpose or specifically formulated to be ideal for certain uses, such as seed sowing, or certain plants, such as houseplants or ericaceous (lime-hating) plants.

Peat-based composts should be avoided due to the environmental damage caused by peat extraction. The RHS State of Gardening Survey 2025, found that just over half (51.2 per cent) of adult gardeners bought potting compost in 2024, 70 per cent of whom bought peat-free potting compost.

Peat-free potting composts are mostly made with bark and wood fibre, or coir, a by-product of coconut production in Asia. While peat‑free composts can differ, those made with higher‑quality ingredients generally provide reliable, consistent results, so you usually get what you pay for.

John Innes potting composts, developed in the 50s and still available, mix sterilised fibrous loamy soil with peat or, better, peat substitute and nutrients to make seed compost, and a range of composts numbered 1-3 for increasingly larger plants. Ericaceous John Innes composts are also available.

The loam acts as a buffer, protecting plants from sudden changes in nutrient supply, acidity and water, making plants easier to manage than in other potting composts.

These composts are relatively heavy and expensive and their quality has sometimes been questioned due to the replacement of scarce loamy soil with other soil or soil-like materials.

Some gardeners make their own soil-based potting compost, usually using unsterilised soil, as sterilising soil at home is laborious. A typical mix might be two-thirds garden soil to one-third home-made compost from the compost bin and a cupful of fertiliser for every 10L of mixture. This can work well and be a big saving for tubs and troughs, despite potentially containing disease spores, weed seeds and pests.

Others use sieved garden compost or leaf mould as potting compost for seed-raising or small plants at a very low cost, both in money and to the environment. Theoretically these have a higher risk of problems than commercial potting compost, but some gardeners swear they have reliable results.

Some potting composts are “peat-reduced”, where 30-50 per cent of the peat has been replaced by other materials – typically wood fibre or bark. Although a step in the right direction, gardeners can easily avoid all peat use by using only peat-free composts and make a better contribution to protecting the environment.
For the best results, buy the highest-quality peat-free potting composts and follow the manufacturer’s instructions, as different watering and feeding regimes are often needed compared to peat-based products.

Aim to buy fresh material, as quality falls off after six months. If
seed-raising is intended, be sure that the compost is sold for seed-sowing, as some products are unsuitable.

The RHS is a charity inspiring everyone to grow via its research, advisory, outreach, shows and gardens. For more information, visit: rhs.org.uk