Why container compost is different

Plants in pots and raised beds cannot send their roots out hunting for water and nutrients. They live entirely in the volume you give them — a volume that dries out, heats up in the sun and runs out of food far faster than open ground ever does. That is why the bag you choose matters so much more than it does in a border. Get it right and your tomatoes, herbs and shrubs will reward you handsomely. Get it wrong and you will spend the summer watering twice a day, wondering why everything looks pale and sulky.

Three broad types dominate the garden centre shelves: loam-based, peat-free and multipurpose. Each has a job it does well.

Loam-based compost: the long-haul option

Loam-based mixes — often sold as a John Innes type — are built around sterilised loam, blended with sand or grit and enough organic matter to keep the mixture friable. They feel heavier in the hand, and that weight is the point.

• Best for: shrubs, small trees, bay, rosemary, olives, long-term perennials, and tall or top-heavy pots that would otherwise blow over.
• Strengths: holds water and nutrients steadily, resists collapsing, and buffers you against both drought and overwatering.
• Watch out for: heavy to carry, slow to warm up in spring, and it needs free drainage in the pot — crocks or pot feet help.

Because loam-based compost keeps its structure for years, it suits anything you do not intend to repot every spring.

Peat-free mixes: what you need to know

With the move away from peat well under way, nearly everything on the shelf is now peat-free, based on coir, wood fibre, bark or composted green waste. Quality varies more between brands and batches than it did with peat, so once you find one that suits your watering habits, stick with it.

• Best for: vegetables, salads, annuals, most herbs and any short-term cropping.
• Strengths: light to handle, often made from recycled materials, and generally free-draining.
• Watch out for: it can dry out and turn water-repellent, or the opposite — green-waste mixes sometimes stay stubbornly soggy. Water slowly and in stages rather than in one flood.

Multipurpose: the everyday workhorse

Multipurpose compost is the middle-of-the-road blend most of us reach for first. It usually carries enough feed for four to six weeks and enough structure for a single season in a pot. It is perfectly good for bedding plants, salad leaves, herbs you will replace, and bringing on young plants before they move somewhere more permanent.

Where it falls short is longevity. In a container it shrinks, compacts and breaks down, so by late August it holds less water and less air than you would like. Treat it as a one-season compost and plan to refresh or repot rather than expecting it to last for years.

Matching the mix to the plant

Vegetables are greedy. Tomatoes, peppers, aubergines and courgettes want a rich, moisture-retentive mix, so blend peat-free multipurpose with a few handfuls of well-rotted manure or garden compost. Once flowers set, switch to a high-potash tomato feed every week or so.

Herbs split into two camps. Mediterranean types — thyme, oregano, rosemary, sage — hate wet feet, so add 20–30% horticultural grit to multipurpose and go easy on the water. Soft, leafy herbs such as parsley, chives, mint, coriander and basil prefer something richer that holds moisture. Feed them lightly: too much nitrogen gives lush, soft growth with disappointingly weak flavour.

Long-term container plants — clipped box, acers, agapanthus, small fruit bushes — belong in loam-based compost. Top-dress each spring with fresh compost and a slow-release feed, and repot into a larger size every two to three years before the roots circle the pot.

Ericaceous plants such as blueberries, camellias and rhododendrons need a lime-free ericaceous mix. No amount of feeding will fix the yellowing leaves that come from the wrong compost.

Filling a raised bed without breaking the bank

The volume is what shocks people. A bed one metre by two metres, filled to 30cm, takes around 600 litres — roughly a dozen large bags. If you are filling from scratch, blend your own instead: about 40% good topsoil or loam, 40% compost (green waste or your own), and 20% sharp sand or grit for drainage. Filling the whole bed with bought multipurpose is expensive and it shrinks alarmingly in the first season.

In an established raised bed you need not replace everything. Top up with 5cm of compost each spring, scratch in a slow-release feed, and mulch the surface to slow evaporation. Worms and soil life will work the rest down for you.

Whatever you choose, buy fresh bags from a supplier that turns stock over quickly, store them under cover, and open them with a sniff — good compost smells sweet and earthy, never sour or chemical. Your plants will notice the difference within weeks.