Choosing the Best Compost for Container Grown Plants

Why Compost Choice Matters More in Pots

Plants growing in the open ground can send roots wandering in search of water and nutrients. A plant in a container cannot. Everything it needs for the whole season — or for several years — has to come from the small volume of compost you tip into the pot. That is why the bag you choose matters far more than most gardeners realise. Get it right and your containers will look after themselves for months. Get it wrong and you will spend the summer watering twice a day and watching leaves turn yellow.

The good news is that you only really need to understand two types of growing medium, plus one or two additives that improve both.

Loam-Based Compost for Long-Term Pots

If you are planting shrubs, small trees, bay, olive, box, lavender, roses or anything that will live in the same pot for two years or more, choose a loam-based compost. These are the heavier, soil-based mixes, usually sold as a "john Innes" type in grades such as No. 2 or No. 3. They feel dense and gritty rather than fluffy, and that is exactly the point.

Loam-based compost holds water and nutrients far better than a pure peat-free multipurpose, and it has weight. That weight keeps tall or top-heavy plants upright in a windy garden, and it means the compost does not shrink away from the sides of the pot when it dries, which is the classic reason water runs straight down the gap and out of the bottom without ever reaching the roots.

Loam mixes also buffer feeding well, releasing nutrients steadily instead of all at once. A shrub in a decent loam-based compost, planted in spring, will usually sail through to late summer before it needs supplementary feeding at all. When you do feed, a balanced liquid feed every fortnight from midsummer onwards is plenty.

Peat-Free Multipurpose for Annuals and Bedding

For annuals, bedding plants, summer containers of petunias, cosmos, nasturtiums, busy lizzies and trailing foliage, a good peat-free multipurpose compost is ideal. These mixes are lighter, more open and free-draining, and their fine texture suits the quick, shallow roots of fast-growing bedding.

Peat-free composts have improved enormously over the last decade, but they still behave differently from the old peat mixes. Two practical points are worth remembering:

• Water more gently and more often. Peat-free mixes can become hydrophobic if they dry out completely. Once that happens, water beads on the surface instead of soaking in. Sit the pot in a tray of water for half an hour to re-wet it, then keep it consistently moist rather than bone dry.
• Feed sooner. Most peat-free multipurpose composts contain enough nutrients for roughly four to six weeks. After that, start a liquid feed. For hungry bedding, a high-potash tomato feed from midsummer encourages flowers rather than leafy growth.

Never reuse multipurpose compost from last year's annual containers for this year's bedding. It will have lost its structure, may harbour vine weevil larvae, and carries the risk of damping-off and other root diseases.

Perlite: The Small Addition That Makes a Big Difference

Whichever compost you choose, drainage and airflow around the roots are what keep container plants healthy. Waterlogged compost suffocates roots, and the plant dies of thirst while sitting in mud. The simplest fix is a handful of perlite.

Perlite is a lightweight, expanded volcanic glass, sold in large bags for very little money. Those little white granules create air pockets through the compost, letting excess water drain freely and allowing oxygen to reach the roots. As a rough guide:

• Use about one part perlite to four parts compost for most container plants.
• Increase to one part perlite to three for cacti, succulents, lavender, rosemary and other Mediterranean plants that hate wet roots.
• For a heavy loam-based mix used on a permanently outdoor pot, perlite is nearly essential — it stops winter rain turning the compost into a soggy brick.

Mix it through evenly with your hands or a trowel before potting, and always make sure the pot has crocks or a good layer of broken polystyrene at the base and at least one clear drainage hole. Perlite will not fix a pot with no drainage.

Matching Compost to the Planting Job

A quick summary to keep you straight when you are standing in the garden centre:

• Annuals and bedding: peat-free multipurpose, plus perlite, plus regular liquid feed.
• Perennials, shrubs and small trees: loam-based compost, plus perlite for outdoor pots.
• Acid-lovers such as camellias, rhododendrons and blueberries: ericaceous compost, which comes in both peat-free and loam-based forms.
• Herbs and alpines: a gritty, free-draining mix — multipurpose cut heavily with perlite or horticultural grit.
• Top-dressing existing pots: scrape off the top few centimetres of old compost each spring and replace it with fresh loam-based compost and a slow-release feed.

One last tip: buy fresh. Compost that has sat in a damp shed over winter loses its structure and its nutrients, and bags left open in the rain become a breeding ground for weeds and fungus gnats. Store yours dry, and use it within a season. Get the medium right at planting time and the rest of the year becomes far easier — just water, feed and enjoy the results.