The Best Compost for Containers

What goes in the pot matters more than the pot itself. Get the compost right and most container crops more or less look after themselves. Here's exactly what to buy, what to mix in, and the one thing never to use.

Peat-free multipurpose as the default

For nine out of ten crops, a good peat-free multipurpose compost is all you need. It holds water, holds nutrients and is light enough for roots to push through. Peat-free is now the UK standard — peat sales to home gardeners have been phased out, and the better brands perform every bit as well.

The catch: quality varies a lot between bags. Cheap multipurpose can be lumpy, full of woody bits, or dries out fast. It's worth paying a little more, and it's worth checking the bag is fresh — compost that has sat in a wet garden centre yard all winter loses structure.

Multipurpose suits tomatoes, courgettes, salad leaves, beans, herbs and almost everything else you'd grow on a patio. For the full picture of pots, watering and feeding, see the container growing guide.

Adding perlite or grit for drainage

Container compost needs to drain — roots sitting in soggy compost rot. The single best upgrade is to mix in perlite, the white volcanic granules that open up the mix and let water through.

A good rule of thumb:

• Most crops: about 20% perlite by volume (roughly one part perlite to four parts compost).
• Mediterranean herbs like rosemary and thyme, which hate wet feet: up to 30%.
• Thirsty crops in summer, like courgettes: less perlite, so the pot holds more water.

Horticultural grit does a similar job and adds useful weight to tall, top-heavy pots that might blow over. Use grit for Mediterranean herbs and perlite where you want to keep things light, such as a balcony or hanging basket.

Ericaceous for blueberries

A few crops won't tolerate ordinary compost because they need acidic, lime-free conditions. The big one for beginners is the blueberry, which must be grown in ericaceous compost — a special acidic mix sold separately.

Plant a blueberry in standard multipurpose and the leaves slowly yellow and the plant fades, because it can't take up iron in non-acidic conditions. Ericaceous compost fixes this. In a pot it's easy: just fill with ericaceous compost from the start, top up each year, and water with rainwater where you can, as hard tap water is slightly alkaline and nudges the mix the wrong way over time.

John Innes for permanent plants

Multipurpose compost is brilliant for a single season, but it breaks down over a year and loses structure — fine for annual crops you replant anyway. For anything that stays put for years, you want a loam-based compost: the John Innes range.

John Innes is made with sterilised loam (good soil), so it holds its structure, drains steadily and carries nutrients far longer. Reach for it when a plant lives in the same pot long-term:

• Fruit trees and bushes in pots — use John Innes No. 3, the richest in the range.
• Rhubarb, perennial herbs and other long-stayers.
• A 50/50 mix of John Innes and peat-free multipurpose is a good all-rounder for big, permanent containers — structure from the loam, lightness from the multipurpose.

Never garden soil

It's tempting to save money by digging up soil from the garden. Don't. Garden soil is wrong for pots in almost every way:

• It compacts in a container, squeezing out the air roots need.
• It drains badly, so pots stay waterlogged.
• It carries weed seeds, pests and diseases straight to your crops.

Good open ground works because worms, weather and a whole soil ecosystem keep it loose — none of which happens inside a pot. Always start with fresh potting compost. (Garden soil and homemade compost are great in the ground, just not in containers — more on that in how to make compost.)

Refreshing each year

Compost in a pot is a yearly consumable. Over a season the plant uses up the structure and most of the nutrients, so don't expect a second year out of the same tired mix.

Each spring:

• Annual crops (tomatoes, salads, courgettes): tip out and replace with fresh compost.
• Permanent plants (fruit, perennial herbs): scrape off the top 5cm and top up with fresh John Innes — a "top-dress" — and add a slow-release feed.
• Old compost isn't wasted — spread it as a mulch on beds or add it to the compost heap.

Get into this once-a-year habit and your containers stay productive year after year. For pot sizes, feeding and watering, head back to the container growing guide.