Why Raised Beds Need Their Own Compost Recipe

A raised bed is not simply a giant plant pot, and it is not quite open ground either. It warms up earlier in spring, drains faster than the soil beneath it, and loses nutrients more quickly to winter rain. The compost you fill it with has to hold moisture without turning into a bog, feed heavy-cropping plants for months, and keep its structure for years rather than weeks.

Depth matters as much as the mix. Twenty to twenty-five centimetres suits salads, radishes and most herbs. Aim for thirty to forty-five centimetres for hungry crops such as tomatoes, courgettes, brassicas and climbing beans, whose roots want room to roam and a steady supply of food.

Whatever you choose, the golden rule is a blend rather than a single bag. A bed filled with nothing but multipurpose compost will slump, shrink and lose fertility by midsummer. A bed filled with nothing but garden soil will bake hard in August and starve your plants by June.

Loam-Based Mixes: The Long-Term Backbone

Loam-based composts, usually sold as John Innes No. 2 or No. 3, are built from sterilised loam, organic matter and sharp sand or grit. They feel heavier in the hand and hold water and nutrients far better than the fluffy peat-free bags on the same shelf.

• Best for: permanent plantings, brassicas, fruit bushes, courgettes and anything that stands in the bed for a full season.
• Strengths: excellent structure, good anchorage for tall crops, buffers against drought and over-watering.
• Watch out for: the weight when barrowing it to a back garden, and the fact that it still benefits from extra garden compost dug through.

For a new bed, a sensible approach is a base of loam-based mix topped with five centimetres of peat-free multipurpose. You get structure below and a fine, seed-friendly tilth above.

Peat-Free Multipurpose: Light, Versatile, Thirsty

Peat-free composts are now the norm, and they have improved enormously. Most are built around coir, woodfibre, composted bark, green waste or anaerobic digestate. They are light, pleasant to handle and ideal for sowing, potting on and short-term crops.

They are not, however, a complete bed-filler on their own. Expect to feed after four to six weeks, because the nutrients in the bag are quickly used up or washed away. Woodfibre blends can also shrink noticeably in a dry spell and become reluctant to rewet once they have dried out completely. Mixing them with loam or topsoil, and mulching the surface, solves most of that.

Look for clear labelling showing the ingredients and the intended use, and choose composted green waste carrying a recognised quality standard so you know it was properly matured. Cheap, immature green waste can burn young roots or carry herbicide residues.

Homemade Blends: Cheap, Effective and Very Forthcoming

You can build an excellent raised bed mix yourself. A rough, reliable recipe by volume is:

• Four parts good topsoil or sieved loam
• Three parts well-rotted manure or garden compost
• Two parts coir, leafmould or spent mushroom compost
• One part sharp grit or horticultural sand
• Plus a handful of blood, fish and bone per barrowload for hungry crops

Mix it on a tarpaulin, break up any clods and water it as you go. Never use fresh manure — it will scorch roots and attract foxes. Your own compost heap is brilliant as a surface mulch or blended into the mix, but be honest about its quality: it is often low in nutrients and rich in weed seeds, so sieve it and use it where seedlings will not be smothered.

Matching the Mix to What You Are Growing

One bed does not suit everything, so adjust as you plant.

• Hungry vegetables: tomatoes, courgettes, squash and brassicas want the richest blend — plenty of well-rotted manure and a slow-release organic fertiliser. Most prefer a pH between 6.0 and 7.0, so test occasionally and lime only where brassicas need it.
• Herbs: rosemary, thyme, oregano and sage hate rich, soggy conditions. Give them a leaner mix with extra grit and plenty of drainage, and go easy on the compost.
• Flowering plants: sweet peas, dahlias, cosmos and hardy annuals thrive in a loam-based mix with a little bonemeal for root development. They flower harder in moderate fertility than in a heavily manured bed.
• Root crops: carrots and parsnips resent fresh manure, which forks the roots. Use a lighter, older mix and avoid stones.

Topping Up Year After Year

Raised beds settle, so plan on topping up each autumn or early spring with five centimetres of garden compost or well-rotted manure. This replaces what the season's crops have taken and keeps the structure open. Every three or four years, fork the top fifteen centimetres to break any pan and work in extra organic matter.

Rotate your crops between beds, keep the surface mulched through summer, and water deeply rather than little and often. Get the mix right at the start and your raised beds will reward you with fuller harvests, healthier herbs and flowers that simply keep going.