Why Compost Matters More in a Pot Than in the Border

In open ground, a plant can send its roots searching for water and nutrients, and it has a whole soil ecosystem to fall back on. In a container, it has only what you give it. The compost you choose is arguably the single biggest decision you make for your patio pots, and it will dictate how much watering you do, how well your plants flower, and how they cope when the weather turns hot and dry in July.

A good container compost needs to do three jobs at once: hold moisture, drain freely, and supply nutrients for long enough to matter. Peat-free multi-purpose compost with a generous addition of grit does all three, which is why it is the mix most growers come back to year after year.

Peat-Free Multi-Purpose: Your Sensible Starting Point

Peat-free compost has improved enormously in recent years. The best blends are built from composted bark, coir, wood fibre and green waste, and they perform every bit as well as the old peat-based bags when they are handled properly. Look for a bag labelled "peat-free multi-purpose" and you have a mix that suits the vast majority of patio plants: pelargoniums, petunias, cosmos, fuchsias, salad leaves and herbs will all settle in happily.

Do bear in mind that peat-free mixes can vary between batches, and some dry out more quickly than others. Wood-fibre blends occasionally lock up a little nitrogen in the first few weeks, which shows as pale, slightly sulky growth. A feed after a month or so sorts that out. When you are in the garden centre, give the bag a squeeze. It should feel moist and crumbly rather than sodden, and it should smell earthy, not sour.

Grit: The Small Addition That Changes Everything

Drainage is where most container displays quietly fail. Compost alone compacts over a season, holds water like a sponge, and leaves roots sitting in the wet. Adding horticultural grit opens up the structure so that excess water runs straight through, while the compost around it still holds plenty for the roots to draw on.

Use washed horticultural grit at around 4–6mm, or sharp sand if you prefer something finer. Builder's sand and beach sand are not suitable, as one is too fine and the other carries salt.

• 3 parts compost to 1 part grit — a good general mix for most patio pots and baskets.
• 4 parts compost to 1 part grit — for thirsty plants such as courgettes, tomatoes and fuchsias.
• Equal parts compost and grit — for lavender, rosemary, thyme, sedum and alpines.

While you are at it, forget the old advice about a layer of crocks in the bottom of the pot. It simply raises the water table inside the container. A single broken piece of pot over the drainage hole, or a scrap of mesh, is plenty to stop compost washing out. Standing your pots on feet so air can circulate underneath matters far more.

Matching the Mix to What You Are Growing

Once you have your base mix, small adjustments make a real difference:

• Tomatoes, peppers and aubergines: multi-purpose with grit and a slow-release feed, then switch to a high-potash liquid feed once the first flowers set.
• Mediterranean herbs: a grittier, leaner mix and a light hand with the feed. Rich compost makes basil and oregano floppy and flavourless.
• Acid lovers: blueberries, camellias, rhododendrons and heathers need ericaceous compost, and rainwater to drink wherever you can manage it.
• Succulents and alpines: very free-draining, in shallow pots that never sit in a saucer of water.
• Hanging baskets: a scoop of water-retaining gel helps, though skip it for succulents and drought-tolerant herbs.

Feeding, Watering and Refreshing Through the Season

Multi-purpose compost contains enough nutrients for roughly four to six weeks. After that, your plants are relying on you. Start liquid feeding weekly from early summer, and increase to twice weekly for heavy croppers in small pots. Slow-release granules are a useful backup, but they release faster in hot weather than the packet suggests, so do not rely on them alone.

Watering is a skill in itself. Little and often beats a rare soak, and mornings are better than evenings because foliage dries before nightfall, which helps keep mildew at bay. Peat-free compost can become water-repellent if it dries out completely, so if a pot has gone bone dry, stand it in a tray of water for an hour rather than pouring from the top.

Each spring, scrape off the top few centimetres of old compost and replace it with fresh. Every two to three years, repot properly into a pot one size larger, teasing out the roots as you go.

A Few Mistakes Worth Avoiding

• Reusing compost from last year's tomatoes for this year's tomatoes, which invites disease and vine weevil.
• Filling pots right to the rim. Leave a 2–3cm gap so you can actually water without it running off.
• Sitting pots flat on paving, which blocks drainage and encourages roots to escape through the holes.
• Overfeeding herbs and Mediterranean plants, which produces lush leaves and very little scent or flavour.

Get the basics right and container growing becomes far more forgiving. A peat-free multi-purpose compost cut with grit, fresh each spring and topped up with regular feeding, gives you pots that hold their colour, crop steadily and carry on looking good right through to the first frosts.