The choice of ‘eco-friendly’ and non-peat-based potting composts has never been wider, as manufacturers experiment with different materials. Some products are excellent, others however do not always live up to the claims made on the bag.
Potting composts are critical to raising and growing-on plants for most UK gardeners. For decades the traditional organic matter used in compost has been peat; commercial and home growers still use 2.5 million cubic metres of peat a year.
Growing media need to provide an ideal environment for rooting – good composts hold water and nutrients, provide air spaces, and are sufficiently robust to allow plants to root securely in the pot. They should also be free of pests, diseases and weed seeds. Peat fulfils all these roles.
Sustainability
However, peat is primarily sourced from lowland raised bogs, an increasingly rare habitat in the UK. In recent years society has recognised the need to conserve this diminishing natural resource, and the flora and fauna that depend on it.
‘Eco-friendly’ potting composts and growing bags contain mixtures of other organic materials – composted or chipped bark, coconut fibre, wood fibre and recycled household bio-waste, for example – together with inorganic materials such as grit, sharp sand, rock wool and perlite. A mix of coarse and fine particles is needed to create the balance of large and small-pore spaces necessary to hold both water and air.
Completely peat-free media are said to be the best choice for the environment, but are not available for all crops. There is as yet no peat-free ericaceous compost on the market for acid-loving plants (although reduced-peat brands are available from AHS, Westland, and J Arthur Bowers).
For non-ericaceous plants, there is a range of peat-free media. Common brands include Westland, Bulrush, Gem, Scotts, Godwins, Lakeland and New Horizon.
All tend to be slightly more expensive than peat-based composts. Bulrush and Gem performed particularly well in the most recent assessment of peat-free growing media at RHS Garden Wisley.
Consistency
Product consistency can be problematic with peat-free media, so many brands also offer ‘reduced-peat’ choices, blending into the formulation up to 20 percent composted bio-waste or wood waste. Manufacturers are getting better at blending non-peat organic matters to overcome the disadvantages of any single material.
Materials
Ideally, choose a peat-free compost that contains a mix of different organic materials, and follow the instructions given on the packaging about the suitability of the mix for particular purposes. Peat-free brands often recommend specific fertilisers for use with their compost: this is not a marketing ploy, as different formulations have different balances of nutrients. Use either the recommended product, or one with a similar nutrient balance – compare fertilisers by checking the N-P-K ratio and the trace-element content quoted on the packaging.
Wood: Most peat-free composts contain chipped or composted bark, sawdust, wood or paper waste as their primary ingredient. There is an industry standard (British Standards Institution PAS 100) for wood-waste composting, which tends to encourage
more consistent products.
Coir: Coconut fibre or ‘coir’ is mainly imported from Sri Lanka. Coir is a waste product, does not require additional wetting agents to absorb water, has excellent natural water-holding ability, and a sufficient mix of fine and coarse fibres to hold air in its pore spaces, making it a good growing medium.
It does not hold nutrients well, however.
The environmental credentials of coir-based products are damaged by the distance they must be transported to the UK – their ‘compost miles’ – and the energy used in their production, both of which rely on fossil fuels.
Municipal waste: Composted sewage slurry and municipal bio-waste is increasingly being recycled by Local Authorities and made into potting composts and soil improvers. Products vary widely between areas, but the more locally-made your compost, the fewer miles it is likely to have travelled and so the more sustainable its production. It is advisable to ask questions about ingredients, contaminants such as glass and heavy metals, and quality control. If in doubt, use as soil improver or mulch.
Arable waste and bracken: Arable straw waste, wool waste and paper and cardboard production waste make suitable additional ingredients for use in blended products. Composted bracken is promising as a sustainable and effective base for peat-free media, particularly for ericaceous composts. Brands include Dalefoot’s Lakeland Gold and Wool Compost (the latter a mix of bracken and wool claimed to be carbon-neutral), and Bracken Down.
Home-made
Gardeners can mix mature, home-made compost, leafmould and inorganic materials to make their own peat-free growing media, but results tend to be variable. It is difficult to standardise pH, moisture retention and available nutrients, and to ensure that the final mix is weed-free. Home mixes are best used as mulches and soil improvers rather than potting compost, and are probably best avoided altogether for seed sowing.
More information
Download the RHS leaflet 'Peat and the Gardener' here (Adobe Acrobat pdf 203KB)
More on peat-free growing media