Choosing the Right Compost for Pots and Planters

Walk into any garden centre and you will find bags stacked to the rafters, each promising bigger blooms and bumper crops. For anyone growing in pots, planters, hanging baskets or a small courtyard, the choice matters far more than it does in open ground. A container is a closed system: the plant cannot send roots searching for water or nutrients, so what you tip into the pot is effectively its entire world for the season. Get the compost right and everything else becomes far easier.

Why container compost is different from garden soil

Garden soil is a mix of mineral particles, organic matter, air and living organisms, and it holds together in a structure built up over years. Tip it into a pot, however, and that structure collapses. It slumps, compacts and drains poorly, and it brings weed seeds and pests along with it.

Container composts are engineered instead. They are light, open and free-draining, with enough air pockets to keep roots healthy and enough water-holding material to see plants through a warm afternoon. They are also sterile enough to start seeds and young plants off cleanly. The trade-off is that they contain limited nutrients and break down over a season, which is why feeding and refreshing matter so much.

Multipurpose compost: the reliable all-rounder

For most gardeners, most of the time, a good multipurpose compost is the sensible default. It is typically a blend of peat or peat-free alternatives such as coir, bark, wood fibre and green waste compost, with added fertiliser to get plants going. It is light to carry, pleasant to handle and works well for the majority of patio displays.

It suits:

• Annual bedding in hanging baskets, troughs and window boxes
• Tomatoes, peppers and chillies in grow bags or large pots
• Herbs, salads and quick crops on a balcony or doorstep
• Young shrubs and perennials being potted on before planting out

Its weakness is water retention. Multipurpose mixes dry out quickly in wind and sun, and they shrink away from the sides of the pot, which means water runs straight down the gap and out of the drainage holes. A thin layer of grit or gravel on the surface, or a mulch of bark chips, slows evaporation considerably. If you are buying peat-free, choose a bag that feels substantial rather than fluffy — cheap mixes high in uncomposted wood fibre can dry out alarmingly fast and rob nitrogen from the soil as they break down.

Loam-based compost: when moisture retention matters

Loam-based composts, often labelled as John Innes-type, blend sterilised loam with peat or a peat substitute and sand or grit. They are heavier, hold water and nutrients far better, and settle into a stable structure that supports plants for years rather than months.

Reach for loam-based compost when you are growing:

• Long-term specimens such as acers, bay trees, olives and topiary
• Shrubs and climbers that will stay in the same pot for several seasons
• Plants in terracotta, which loses moisture through its porous walls
• Anything sitting in a windy, sun-baked spot where drying out is a constant battle

The extra weight is a genuine advantage for tall or top-heavy plants, as it makes pots far less likely to blow over in a gale. The downside is cost and heft: a large planter filled with loam-based compost is a two-person job to move, so decide on its final position before you fill it.

Feeding: the difference between a good display and a great one

Almost all bagged composts contain a starter fertiliser, usually enough for four to six weeks. After that, the plants are on their own. For a long patio display, mix a controlled-release feed into the compost at planting time — the granules release nutrients steadily for three to six months as temperatures rise, which matches the growing season neatly.

Even then, hungry plants in small pots will benefit from a liquid feed. Tomatoes, courgettes, dahlias and petunias in particular respond well to a weekly high-potash feed once flowers or fruit begin to form. A simple rule: leafy crops and foliage plants prefer a balanced or nitrogen-rich feed, flowering and fruiting plants prefer potash.

Matching compost to plant, pot and place

Think about three things before you fill a pot. First, how long will the plant stay there? A season of bedding wants a light multipurpose mix; five years of topiary wants loam. Second, how thirsty is the plant? Ferns, hostas and moisture lovers do better in a mix with extra water-retaining material, while lavender, rosemary and succulents need sharp drainage — add grit at roughly one part in four.

Third, how big is the pot relative to the plant? Oversized pots of soggy compost are a common cause of root rot, while cramped pots dry out twice a day in summer. As a rough guide, most patio plants are happiest with a root run of at least 30cm of compost, and tomatoes need closer to 40cm per plant.

Finally, do not reuse old compost for a new crop without thought. It has lost its structure and nutrients, and may harbour vine weevil grubs or disease. Spread spent compost on the garden border as a soil conditioner instead, and start fresh in the pot. Your plants — and the birds and bees visiting your little patch — will thank you for it.