Why the stuff in the bag really matters

A plant in a pot has only what you give it. Its roots cannot wander through the border in search of moisture or nutrients, so the growing medium you tip in has to do the whole job: hold water, drain freely, and feed the plant for weeks or months on end. Get the balance wrong and the signs show up fast — yellowing leaves, sudden wilting, stunted growth, or a soggy rootball that smells sour.

The good news is that matching compost to plant is not complicated once you understand a handful of ingredients. Think of it as cooking: the same base mixture can be enriched, lightened or made lime-free depending on who is going to live in it.

Know your base ingredients

• Peat-free multipurpose — the everyday workhorse, usually a blend of wood fibre, coir, bark and green waste. Light to carry, easy to work with, and perfectly good for a season of salads, bedding and vegetables. It dries out faster than peat and can be stubborn to rewet once it has gone bone dry, so water little and often.
• Loam-based compost (John Innes) — soil, sand and grit with added nutrients. Heavier and more structured, it holds water and food well and anchors roots in place. No. 1 suits seedlings and cuttings, No. 2 is a good general purpose mix, and No. 3 is made for mature shrubs and small trees.
• Ericaceous compost — lime-free and acidic, essential for blueberries, camellias, rhododendrons and pieris.
• Grit, sharp sand and bark — the additions that open up a mix and improve drainage when a plant demands it.

Vegetables and salads want richness

Tomatoes, peppers, aubergines and courgettes are hungry, thirsty plants that will reward you handsomely for a generous start. Use a rich peat-free multipurpose, ideally with a slow-release feed mixed through, or blend in a couple of handfuls of well-rotted garden compost. Plant tomatoes deeply so the stem roots along its length, and begin a high-potash tomato feed once the first truss has set. In grow bags, sink a pot beside each plant and water into that, so moisture reaches the roots rather than running off the surface.

Leafy crops such as lettuce, spinach, chard and rocket are far less fussy and thrive in plain multipurpose compost sown successionally through the season. Root crops are the exception: carrots and beetroot in containers prefer a lighter, free-draining mix and dislike fresh manure, which forks the roots. Runner beans and climbing French beans appreciate a deep pot and a moisture-retentive compost topped with a mulch.

Herbs fall into two camps

Mediterranean herbs — rosemary, thyme, oregano, sage and lavender — come from rocky, sun-baked hillsides and hate sitting wet. Mix roughly two parts multipurpose compost with one part horticultural grit, or use a loam-based compost lightened with grit, and choose a terracotta pot that breathes. Keep feeding to a minimum; too much nitrogen makes soft, floppy growth and dilutes the flavour you are growing them for.

Soft, leafy herbs are the opposite. Basil, parsley, coriander, chives and mint enjoy a moisture-retentive multipurpose compost, regular watering and an occasional dilute feed. Give mint its own pot, because it spreads ruthlessly and will swamp its neighbours.

Shrubs, trees and long-term plantings

Anything staying in its pot for years needs structure as well as food. A loam-based John Innes No. 2 or No. 3 is the sensible choice for acers, bay, olives, box, skimmia and camellias: it resists slumping, holds nutrients, and does not shrink away from the sides of the pot, which leaves water channelling straight down the gap. Olives and rosemary still benefit from extra grit. Acid-lovers such as camellia, blueberry and rhododendron must have ericaceous compost, whatever else you do.

Repot established specimens every two or three years in spring, moving up one pot size at a time and teasing out the outer roots. Replace roughly a third of the old compost with fresh, and always make sure there is drainage material or a good hole at the base.

Keeping compost working through the season

• Top-dress permanent pots each spring by scraping away the top 5cm and replacing it with fresh compost.
• Start liquid feeding four to six weeks after potting, once the compost's own reserves are spent.
• Water consistently — peat-free mixes need more attention, especially in a hot spell.
• Raise pots onto feet so surplus water drains away rather than pooling underneath.
• Avoid reusing old compost for the same crop; vine weevil, disease and depleted nutrients all build up. Use it instead as a mulch or for undemanding plants.

Choose well at the outset and your containers will repay you all summer — healthier roots, stronger growth and far less time spent fretting over wilting leaves.