Why the stuff in the bag matters more than you think

Growing vegetables in containers is a bargain you strike with your plants: they give you a harvest, you give them everything else. In open ground, roots can wander in search of water and nutrients. In a ten-litre pot on a balcony or a windowsill, they are entirely dependent on what you put in there. Compost is not just somewhere for roots to sit — it is the water supply, the larder and the drainage system rolled into one. Choose well and watering becomes forgiving, feeding is straightforward and crops stay healthy for months. Choose badly and you will spend the summer fighting yellowing leaves and soggy roots.

There are three broad families worth knowing: peat-free blends, loam-based mixes and your own homemade compost. Most of us will end up using a combination.

Peat-free blends: the modern default

Peat-free composts are now the standard on garden centre shelves, usually based on composted bark, wood fibre, coir or green waste. They are light, renewable and pleasant to handle, and a good multi-purpose blend will raise most container crops perfectly well.

The catch is that they behave differently from old-style peat. They can look bone dry on the surface while still being damp an inch down, and they dry out quickly in wind and sun. They also hold fewer nutrients, so you will need to start liquid feeding sooner — often within three to four weeks. Water little and often rather than flooding occasionally, and consider a thin gravel mulch to slow evaporation. If a bag has been sitting open and dried out, tip it into a trug, add water, stir and leave it to stand for an hour before potting; dry peat-free mix can be stubbornly water-repellent.

Loam-based mixes: weight, body and staying power

Loam-based composts — sold as John Innes No. 2 or No. 3, or as "soil-based" mixes — combine sterilised soil with sand, a peat substitute and added fertiliser. They are heavier, darker and far better at holding both water and nutrients. They do not shrink away from the sides of the pot when they dry, which means water runs down the gap instead of into the rootball.

For long-season, hungry crops, this is the mix to beat. Tomatoes, peppers, aubergines, courgettes and overwintering brassicas all appreciate the steady fertility. Use No. 2 for general potting and No. 3 for mature plants in their final pots. The trade-offs are price, weight on balconies and window ledges, and slower drainage — so mix in a little peat-free multipurpose if you want to lighten it.

Homemade compost: wonderful, but know its limits

Nothing beats the feeling of potting up in compost you made yourself. It is free, it keeps waste out of the bin and it is teeming with the soil life that keeps plants ticking over. But it is variable, and it pays to be honest about that.

• Use only well-rotted compost, six to twelve months old, and sieve out lumps, sticks and unrotted scraps.
• Nutrient levels are unpredictable and generally lower than bagged mixes, so treat it as a base rather than a complete feed.
• Unless your heap gets properly hot, expect the odd weed seed and slug egg.
• Never use it neat for seed sowing — it is too coarse and too rich.

The reliable approach is a blend: roughly one part sieved homemade compost to two parts peat-free multipurpose. That gives you structure, fertility and moisture retention without the risks of going it alone. It is superb for hungry crops in large pots, and equally good as a mid-season top dressing.

Matching the mix to what you are growing

• Salad leaves, spinach and soft herbs: a lighter peat-free multipurpose, moderate fertility and steady moisture. Too rich and you get soft, sappy growth that bolts at the first hot spell. Rosemary, thyme and oregano prefer a leaner, grittier mix.
• Tomatoes, peppers and aubergines: a rich loam-based mix, pots of at least ten litres, and a weekly liquid tomato feed once the first truss has set.
• Carrots, beetroot and radishes: low-fertility, free-draining compost, sieved to remove lumps. Fresh manure or very rich compost encourages forked, hairy roots.
• Beans, peas and courgettes: thirsty, greedy plants. Use loam-based compost or multipurpose blended with sieved homemade compost, in the biggest pots you can manage.
• Winter greens and brassicas: a firm loam-based mix that will not run out of puff by November.

Small tweaks that make any compost work harder

Whatever you choose, a few habits will pay you back. Leave a two to three centimetre gap between the compost surface and the rim of the pot so you can water properly. Firm the mix gently as you fill — you want it settled, not squashed. Add a shallow layer of gravel or broken crocks to the base of large pots, but do not fill the bottom third with stones; you are creating drainage, not a reservoir.

Feeding matters more in containers than in beds because every watering leaches nutrients away. Start a liquid feed after four to six weeks, and top dress with a handful of sieved compost in midsummer. At the end of the season, spent compost can be refreshed with a third fresh mix and reused for undemanding crops such as salads, or spread as a mulch — but avoid growing tomatoes or potatoes in it two years running. Store part-used bags sealed and off the ground, and you will have a ready supply whenever the urge to sow strikes.