Why Container Compost Isn't Just Garden Soil in a Bag

A plant in a pot lives in a completely artificial world. In open ground, roots can travel metres in search of water and nutrients, and soil life quietly recycles goodness back into the mix. In a container, the rootball has whatever you gave it at planting time and nothing more. That's why compost for pots needs to be lighter, more moisture-retentive and more fertile than the soil in your borders.

Garden soil in a pot is a common mistake. It slumps, sets like concrete when it dries and drains poorly when wet. Container composts are built around a structure that keeps air moving through the root zone while still holding water like a sponge. Get that balance right and everything else — feeding, watering, planting — becomes far easier.

What's Actually in the Bag

Most composts on the shelf are a blend of a few key ingredients, each doing a specific job.

• Peat-free multipurpose — usually bark, wood fibre, coir and green waste compost. Light, free-draining and fine for a single season in baskets and bedding pots.
• Loam-based mixes — the John Innes range, with loam, peat or peat substitute and grit. Heavier, holds nutrients longer and suits shrubs, small trees and anything staying put for years.
• Perlite or grit — the white flecks and stone fragments that open up the mix and stop it compacting.
• Controlled-release fertiliser — small granules that feed slowly for six to twelve weeks. Handy, but not a substitute for liquid feeding by midsummer.
• Wetting agents — surfactants that help dry compost take up water again rather than letting it run down the sides of the pot.

Read the label and you'll usually find a rough breakdown plus a note on how long the feed lasts. That figure matters more than the picture of flowers on the front.

Matching the Mix to the Job

Different containers ask different things of their compost, and one bag rarely does everything well.

Hanging baskets and seasonal pots. Go for a light, peat-free multipurpose with added water-retaining granules. Baskets dry out faster than anything else in the garden, so water retention beats long-term fertility. Mix in a handful of perlite if the bag feels dense.

Large patio pots of annuals. Multipurpose with slow-release feed works well, but be generous with volume. A small pot of tall, thirsty plants will need watering twice a day in July, however good the compost.

Long-term planters. Shrubs, acers, bay trees and small fruit bushes deserve a loam-based mix such as John Innes No. 2 or No. 3. The loam holds nutrients and structure, so the compost won't collapse into a sour, airless mass after a season. Add extra grit if the pot is out in all weathers.

Acid-lovers. Camellias, rhododendrons, blueberries and pieris need ericaceous compost, and they need it refreshed. Tap water gradually pushes the pH up, so top-dress each spring with fresh ericaceous mix.

Drainage, Crocks and the Watering Habit

Good drainage is the difference between a thriving plant and a drowned one. Make sure every pot has drainage holes, and cover them with a single crock or a piece of broken slate rather than a thick layer of gravel. A deep gravel sump simply raises the water table inside the pot and keeps the roots wetter.

Stand pots on feet in winter so they don't sit in a puddle, and use saucers in summer only if you're prepared to empty them. Plants left standing in water for days will sulk, yellow and rot. If you're growing in a greenhouse or against a hot wall, consider double potting — a smaller pot plunged into a larger one lined with damp compost or newspaper — to buffer against drying out.

Feeding Through the Season

Once the slow-release granules are spent, usually by late June, your plants are entirely dependent on you. Start liquid feeding when new growth is clearly underway.

• Flowering and fruiting plants want a high-potash feed, roughly weekly from midsummer.
• Foliage plants and herbs prefer a balanced feed at half strength, more often.
• Never feed dry compost — water first, then feed, or you'll scorch the roots.
• Ease off in late summer so soft growth isn't caught out by the first frosts.

A top dressing of fresh compost in early summer gives a gentle boost and replaces some of the volume lost to settling.

Reusing and Refreshing Compost

Spent compost isn't rubbish, but it isn't the same product either. It has lost its structure, most of its nutrients and much of its water-holding ability. For pots of tomatoes, bedding or anything that grew hard and fast, fresh compost is worth the money.

For larger planters, take off the top five centimetres in spring, loosen the rest with a hand fork, then top up with a loam-based mix and a sprinkling of slow-release feed. If the compost smells sour or the roots are circling tightly, tip the plant out, tease the roots apart and repot into something fresh and slightly larger.

Whatever you choose, buy this season's bags where you can, store them under cover and open them with a bit of care — damp, warm compost is a favourite for vine weevil grubs. Get the foundation right and your containers will reward you all summer long.