Why the bag you choose really matters

A plant in the ground can send its roots wandering in search of water and nutrients. A plant in a pot cannot. It lives entirely on what you tipped in at potting time, which is why container compost has to do four jobs at once: hold moisture, let air through, feed the plant and anchor the roots firmly. No single blend is brilliant at all four, and that is exactly why garden centre shelves look so crowded. Choose well and you will spend far less time watering, feeding and fretting over sulking foliage.

Multipurpose: your dependable all-rounder

If you buy one bag and one bag only, this is it. Multipurpose compost is a blend of organic matter — commonly bark, woodfibre, coir or composted green waste — with a base fertiliser and enough lime to bring the pH to roughly neutral. It is light, pleasant to handle and holds water well, which makes it the default choice for bedding plants, annuals, tomatoes, herbs, most houseplants, patio pots and hanging baskets.

Two things to remember. First, the feed included in the bag usually lasts only four to six weeks, so begin liquid feeding once that fades. Second, peat-free versions vary enormously between brands. Some are superb; others dry out in a strange, almost water-repellent way and become difficult to rewet. If yours behaves like that, fork through a handful of garden compost or a little loam before potting, and water in stages rather than all at once.

Soil-based compost: for the long haul

Soil-based blends, familiar under the John Innes name, contain loam, grit and a peat or peat-substitute element. That loam is the point: it brings weight, structure and a slow release of nutrients, so the compost keeps its shape rather than collapsing into a soggy slump. Heavier pots also stand up to wind far better, which matters on a breezy patio.

The numbering is worth knowing. No. 1 is fine and low in nutrients, ideal for seedlings and rooted cuttings. No. 2 suits established plants and summer containers that will be in place for months. No. 3 is the heavyweight, best for shrubs, small trees, roses and topiary that will live in the same pot for years.

The trade-offs are cost and effort — these bags are heavy to carry and pricier per litre. They also contain lime, so they are entirely wrong for acid-loving plants.

Ericaceous: for acid-lovers only

Some plants simply cannot cope with lime. Rhododendrons, azaleas, camellias, pieris, skimmia, heathers, magnolias, Japanese maples and blueberries all want a pH of roughly 4.5 to 5.5. Ericaceous compost is made without lime and is often bolstered with sulphur to keep the mix firmly on the acid side.

The classic sign of getting this wrong is lime-induced chlorosis: leaves that yellow between veins that stay green, followed by weak growth and no flowers at all. If you are growing a blue hydrangea, ericaceous compost is also the practical way to keep the flowers blue rather than drifting to pink.

Do not be tempted to use it everywhere. Most vegetables, herbs and bedding plants prefer neutral ground, and ericaceous compost costs more for no benefit. Keep it for the plants that genuinely need it.

Specialist blends worth knowing

• Seed and cuttings compost: finely sieved, low in nutrients and free-draining, so tender new roots are not burned or drowned.
• Gritty or Mediterranean mixes: extra grit and sand for lavender, rosemary, thyme, olives, alpines and cacti, which rot quickly in a moisture-retentive blend.
• Orchid bark: chunky and airy. Never plant an orchid in ordinary compost, however good it is.
• Citrus compost: slightly acidic and sharply drained, which suits potted lemons and limes far better than multipurpose.
• John Innes-based peat-free mixes: increasingly common, and a sensible middle path for shrubs and long-term pots.

Small habits that make a big difference

Whatever you buy, a few practical touches will get more from it.

• Check the date on the bag. Nutrients fade on the shelf, and a bag that has been sitting outside for two summers is not the bargain it looks.
• Dampen the compost before potting, then firm it gently. Roots need contact with the mix, not air pockets.
• Skip the traditional layer of crocks in the base. It does not improve drainage; good drainage holes and pot feet do.
• Top-dress perennials each spring by scraping away the top few centimetres and replacing it with fresh compost, or repot every couple of years before the mix turns sour.
• Start feeding six weeks after potting, and water until it runs out of the drainage holes rather than giving little sips.

Get these basics right and the compost becomes one less thing to worry about — leaving you to enjoy the pots themselves, from the first flush of spring bulbs to the last late-flowering dahlias on a mild October evening.