Why container compost is a different beast

In a raised bed or border, roots can wander in search of water and nutrients. In a pot, they are entirely dependent on what you put in that pot. Compost must anchor the plant, hold moisture without becoming a bog, let air reach the roots, and supply food for weeks or months. Get it wrong and you will spend the summer fighting yellow leaves, blossom end rot, or a pot that dries out by lunchtime. Get it right and even a small balcony can give you a generous harvest.

The building blocks of a good mix

Most peat-free multipurpose composts are based on wood fibre, coir, and composted bark. They are perfectly workable, but they run out of steam faster than old-style peat mixes. That means you need to think about three things: body (something to hold nutrients, such as loam or worm castings), drainage (grit, perlite, or sharp sand), and food (slow-release fertiliser or well-rotted manure).

• Multipurpose peat-free compost: your base. Buy fresh each spring if you can.
• Loam-based compost (John Innes No. 2 or 3): heavier and good for long-term crops. Mix it with multipurpose to stop it setting like concrete.
• Grit or perlite: opens up the mix. Horticultural grit is cheap and lasts for years.
• Worm castings or composted manure: gentle, slow food. Not fresh manure, which burns roots.

Tomatoes, peppers and aubergines: hungry, thirsty, long-term

These are the gluttons of the container garden. They set fruit over many weeks and need a steady supply of water and potassium. A good mix is roughly 60 per cent peat-free multipurpose, 20 per cent composted bark, and 20 per cent worm castings or well-rotted manure. Add a handful of horticultural grit per ten litres to keep it open. Plant into a pot at least 30 centimetres across, and bury the stem up to the first true leaves — tomatoes will root along it.

Start feeding when the first tiny fruits appear. A high-potassium liquid tomato feed once a week, according to the bottle, is plenty. In hot spells, you may need to water twice a day; irregular watering causes blossom end rot, not a lack of calcium in the compost.

Salads and leafy greens: light, quick, and not too rich

Lettuces, rocket, spinach, and chard are fast crops. They do not need a heavy, rich mix — too much nitrogen gives soft, sappy leaves that aphids adore. Use 80 per cent multipurpose compost and 20 per cent perlite or grit. No extra manure. Sow little and often in shallow pots or troughs, and feed with a weak liquid seaweed every fortnight. Keep them well watered; dry compost makes salads bolt in days.

Herbs: split them by their home climate

Mediterranean herbs — rosemary, thyme, oregano, sage, winter savory — hate sitting wet. Give them 50 per cent multipurpose and 50 per cent grit or sharp sand. Terracotta pots help wick away moisture. Feed sparingly; rich compost dilutes their flavour. Soft herbs — basil, parsley, coriander, chives, mint — prefer a richer, moisture-retentive mix: 70 per cent multipurpose and 30 per cent worm castings or compost. Always grow mint in its own pot, or it will march through everything else.

Root vegetables: deep, stone-free, and low in nitrogen

Carrots, radishes, beetroot, and turnips fork and split if they hit stones or fresh manure. Use 50 per cent multipurpose and 50 per cent horticultural grit or sand, and choose short, round varieties bred for pots. Do not add manure or rich compost; too much nitrogen gives leafy tops and tiny roots. Water consistently — dry spells then a soaking will split beetroot and make carrots woody. A pot 30 centimetres deep is enough for most round types.

Drainage, watering and feeding through the season

No matter which mix you choose, drainage matters. Cover the holes with a piece of broken pot or mesh, but skip the traditional layer of gravel — it just raises the water table. Water thoroughly, then let the top centimetre dry before watering again. Little and often is fine for salads; tomatoes and peppers prefer a good soak.

Remember that peat-free compost runs out of nutrients after about four to six weeks. From midsummer, top-dress pots with a centimetre of worm castings or fresh multipurpose, and use a liquid feed every week or two. If you reuse compost from year to year, refresh it with 30 per cent new compost and a slow-release fertiliser, and never reuse it for tomatoes if you had blight. A little planning now means your containers will work hard right through to the first frosts.