Why Compost Choice Matters More in a Pot

A plant in the ground can send its roots searching for water and nutrients. A plant in a pot cannot. It has whatever you gave it, in a volume that might be no bigger than a bucket, and it has to survive there for months or years. That is why the compost you tip into a container matters far more than the bag of soil you dig into a border.

Container compost has three jobs to do at once: hold water, hold air, and hold nutrients. A mix that does one of those well but the others badly will cause problems. A heavy, sticky compost holds water but suffocates roots. A light, open mix drains beautifully but dries out by teatime on a sunny day. The trick is matching the mix to the plant and the length of time it will stay in that pot.

Loam-Based Mixes for Long-Term Plants

If a plant is going to live in its pot for two years or more, you want a loam-based compost. These are the heavier, darker mixes, often sold as John Innes-style formulations in numbered grades. They contain sterilised loam, which is simply rotted turf, blended with peat or a peat-free alternative, grit and a base fertiliser.

Loam gives the mix body. It holds nutrients rather than letting them wash straight through with every watering, and it buffers changes in moisture so a missed watering is not instantly fatal. The extra weight is a bonus, too: a mature bay tree or a standard rose in a loam-based mix is far less likely to topple in an autumn gale.

• Best for: shrubs, small trees, topiary, bay, citrus, olives, agapanthus, long-lived perennials, anything overwintered in the same pot.
• Numbering: the lower numbers are lighter and finer, suited to smaller plants; the higher numbers are coarser and heavier, better for established specimens.
• Watch out for: compaction. Loam-based mixes can slump and lose air if you water heavily and never repot. Loosen the top inch with a hand fork each spring.

Loam-based compost is not usually ideal for tiny seedlings or delicate alpines, which prefer something leaner and sharper-draining. It is the workhorse for everything with staying power.

Peat-Free Multipurpose for Seasonal Displays

For bedding plants, annual climbers, summer containers and salad crops, a peat-free multipurpose blend is usually the better and cheaper choice. These mixes are built from bark, coir, wood fibre, composted green waste or a combination of all four, and they are light, open and easy to work with.

The catch is that peat-free mixes vary enormously between brands and even between batches. Some are beautifully consistent; others are lumpy with half-composted woodchip that dries out in patches and leaves the rest of the pot sodden. Buy from a supplier that turns its stock over quickly, and if a bag feels suspiciously heavy and damp, leave it on the shelf. Squeeze a handful: it should feel crumbly and slightly moist, not wet enough to wring out.

• Best for: petunias, cosmos, begonias, trailing plants, herbs, lettuce, anything lifted and replaced within a season.
• Feeding: peat-free mixes often run short of nutrients within four to six weeks. Start liquid feeding earlier than you would with a loam-based compost.
• Watering: coir-rich mixes hold moisture well; wood-fibre mixes dry faster and may become water-repellent if allowed to dry out completely.

Improving Any Bag of Compost

Straight from the bag, most multipurpose compost is fine but not perfect. A few small additions make a real difference.

Mix in horticultural grit or perlite at roughly one part to nine for most containers, and more generously for lavender, rosemary, cacti and other plants that hate wet feet. A handful of slow-release fertiliser added at potting time will carry a seasonal display for weeks. For long-term plants, a layer of gravel or bark mulch on the surface slows evaporation and keeps weeds and liverwort down.

Skip the old tradition of putting crocks in the base of the pot. It raises the water table and keeps roots wetter, not drier. Instead, use a pot with good drainage holes and stand it on feet so water can escape freely.

Repotting, Top-Dressing and Keeping Compost Fresh

Compost does not last forever. Over a season or two it collapses, loses structure and stops draining. Look for roots pushing out of the drainage holes, water running straight through the pot, or a plant that sulks no matter how much you feed it. Those are signs it is time to move it on.

When you repot, go up one pot size at most and tease out the outer roots before settling the plant into fresh compost at the same depth as before. Water in thoroughly and keep it out of strong sun for a week.

If a full repot is not practical, top-dressing helps: scrape away the top two or three centimetres of old compost in spring and replace it with fresh, gently forking it in. Store leftover compost in a sealed bag in a cool, dry place, and use it within a season. Damp, open bags grow weeds, fungus gnats and algae, and none of those do your containers any favours.