Why garden soil is the wrong choice

It is tempting, when there is a bare patch in the border and a spade to hand, to fill a pot with soil and hope for the best. Container growing is a different discipline entirely. A plant in a pot lives in a closed system: whatever you put in is all it will ever have until you step in. Garden soil, however good it looks, compacts quickly in a confined space. It loses the air pockets roots need, drains sluggishly against the sides of a pot, and can smuggle in weed seeds, slugs and vine weevil grubs. Choose a purpose-made compost instead, and think of it as a growing medium rather than dirt. Its job is to hold moisture, let air through and release nutrients steadily over the months ahead.

Drainage first, water second

Every container needs at least one drainage hole, and a saucer that never becomes a swamp. The old advice about filling the base with broken crocks does more harm than good: it raises the water table inside the pot and keeps the lowest layer of compost permanently wet. Fill to the base with compost and let the hole do its work.

For most plants, aim for a medium that is free-draining yet moisture-retentive. A peat-free multi-purpose compost straight from the bag is usually too dense on its own for long-term planting, so lighten it with around 20 to 30 per cent perlite, horticultural grit or composted bark. The mix should feel crumbly and open in your hand, not claggy. Terracotta pots dry out faster than glazed or plastic ones, which suits lavender and rosemary but demands more watering for ferns and hostas. Raising pots on feet keeps the drainage hole clear and stops slugs using the saucer as a hideout.

Match the compost to the plant

There is no single best compost, only the right one for the plant in front of you. A few reliable pairings:

• Acid-lovers such as camellias, rhododendrons, pieris and blueberries need ericaceous compost, or their leaves will yellow and flowering will falter. Use rainwater where you can, as tap water gradually pushes the pH back up.
• Mediterranean herbs, lavender and rosemary want a lean, gritty mix — roughly two parts compost to one part grit. Rich feed makes them soft and floppy, and they sulk in wet roots over winter.
• Cacti and succulents need sharp drainage: two parts grit to one part compost, in a pot that is only slightly wider than the plant.
• Tomatoes, peppers, courgettes and salads are hungry and thirsty. Give them a rich, moisture-retentive compost with slow-release granules mixed through, and keep them consistently watered.
• Ferns, hostas and moisture-lovers appreciate extra leaf mould or composted bark, which holds water without going sour.
• Citrus does best in a loam-based mix with added grit, plus a dedicated summer feed.

Loam-based composts, often sold as John Innes types, are heavier and hold their structure longer, which makes them a good choice for shrubs and small trees spending several years in the same pot.

Feeding through the growing season

Compost nutrients are largely spent after six to eight weeks of active growth, so feeding is not optional. Mix slow-release granules into the compost at planting, then begin liquid feeding in April as growth picks up. A balanced feed suits foliage plants; switch to a high-potash tomato feed once buds appear to encourage flowers and fruit. Feed little and often rather than in occasional heavy doses, and stop by early September so soft growth has time to harden before the cold arrives. If a white crust appears on the surface, you have overfed — scrape it away and water thoroughly to flush the pot.

Refresh, top-dress and repot

Even the best compost tires. Each spring, scrape off the top few centimetres of old compost and replace it with fresh, which refreshes nutrients and removes any build-up of salts. Every two to three years, move permanent plants into a pot roughly 5cm wider, teasing out the roots and trimming any that circle the edge. Early spring, just as growth restarts, is the kindest moment. Fruit trees, bay, and long-term shrubs benefit from root pruning and fresh compost even when the pot size stays the same.

Small habits that pay off

• Water with rainwater where possible, especially for ericaceous plants.
• Top-dress with a thin layer of gravel to slow evaporation and deter fungus gnats.
• Water until it runs from the drainage hole, then empty the saucer — occasional thorough soaking beats constant dribbling.
• Group pots together in summer to shade each other and reduce watering; spread them out in winter for airflow.
• Check rootballs in autumn for vine weevil grubs, a common cause of mystery collapse in container plants.

Get the compost right and container gardening becomes remarkably forgiving. Fresh medium, honest drainage and a steady feed will carry almost anything through a British summer, from a window-box of herbs to a patio olive in a very large pot.