Sophie Barnes
Why Container Compost Is a Different Beast
Plants in the ground can send their roots deep and wide, hunting for water and nutrients. A plant in a pot cannot. It lives entirely within the handful of litres you give it, so every decision about what fills that pot matters enormously. Get the compost right and your tomatoes, herbs, salads and chillies will romp away. Get it wrong and you will spend the summer watering and fretting over plants that never quite thrive.
The good news is that there is no single "best" compost. There is only the best compost for what you are growing and how you intend to look after it. Once you understand what each type does well, choosing becomes straightforward.
The Main Types of Bagged Compost
Walk into any garden centre and you will be faced with a wall of bags. In practice, most of them fall into a few clear categories.
- Multipurpose compost. The workhorse of container growing. It is usually peat-free these days, built from wood fibre, coir, composted bark or green waste. It holds moisture, drains reasonably well and contains enough nutrients for roughly four to six weeks of growth. Perfect for most salads, herbs and young plants.
- Loam-based compost. Heavier and darker, made with sterilised soil, a peat substitute and sand or grit. It holds water and nutrients far better than multipurpose and provides weight, which helps top-heavy plants stay upright. Excellent for long-term containers, fruit bushes and anything you plan to keep for years.
- Ericaceous compost. An acidic mix for blueberries, cranberries and other lime-hating plants. Using ordinary compost for these causes yellowing leaves and stunted growth, no matter how generously you feed them.
- Seed and cutting compost. Low in nutrients and fine in texture. This is what you want for germinating seed, because rich compost can scorch tender young roots. Once seedlings have true leaves, move them into something more substantial.
Peat-free composts have improved dramatically, but they do behave differently. They tend to dry out faster at the surface and can run short of nutrients sooner than older peat-based mixes, so plan to water more attentively and begin feeding earlier.
Match the Mix to What You Are Growing
Think about how long a plant will occupy its pot and how hungry it is likely to be.
- Salad leaves, radishes and herbs: multipurpose is plenty. These are short-lived, light feeders.
- Tomatoes, peppers, chillies and aubergines: multipurpose works well, but they will need regular feeding once flowers appear. Many growers prefer a loam-based mix for its steadier moisture supply.
- Courgettes and cucumbers: thirsty plants that demand plenty of moisture. A rich, water-retentive mix with added garden compost or worm castings suits them beautifully.
- Blueberries and cranberries: ericaceous compost, without exception.
- Fruit bushes and small trees in long-term pots: loam-based compost, topped up each spring.
- Carrots and beetroot in deep containers: a fine, stone-free mix, ideally with some sharp sand or grit worked through to encourage straight roots.
Improving Your Mix: Drainage, Moisture and Air
Bagged compost rarely needs to be used exactly as it comes. A few simple additions can transform how it performs.
- Perlite is lightweight volcanic glass that opens up the mix and improves drainage. Add a handful per pot for plants that resent soggy roots.
- Vermiculite does the opposite, holding water and releasing it slowly. It is useful in hanging baskets and anything that dries out quickly.
- Horticultural grit or sharp sand adds weight and drainage, and a layer on the surface can deter slugs and vine weevils.
- Composted bark or green waste improves structure and supports beneficial soil life.
- Worm castings or well-rotted garden compost add slow-release nutrients and help the mix hold moisture. Use them as up to a quarter of the total volume.
Always check that containers have drainage holes. A pot without them is really a bucket, and roots sitting in water will soon rot.
Feeding: Compost Alone Will Not Last the Season
This is where many container growers come unstuck. The nutrients in a bag of multipurpose compost are largely spent by midsummer, just as your tomatoes are setting fruit and your courgettes are working hardest.
There are two sensible approaches. You can mix a slow-release fertiliser into the compost at planting time, which will feed steadily for around three months. Or you can feed with a liquid weekly once plants are established, switching to a high-potash feed when flowering begins to encourage fruit rather than leafy growth.
Herbs are the exception. Basil, thyme, rosemary and parsley all prefer leaner conditions, and overfeeding makes them soft, floppy and short on flavour.
Refreshing Compost for Another Year
You do not have to buy fresh compost for every pot every spring. Old compost can be rejuvenated, provided the plants grown in it were healthy and free of pests or disease.
Tip it out, break up the lumps and remove old roots and debris. Mix in roughly a third fresh compost, plus a handful of slow-release fertiliser and some worm castings or garden compost. This refreshed blend works well for less demanding crops such as salads, herbs and beans.
Do not reuse compost that has hosted diseased plants, vine weevil grubs or white mould on the surface. For tomatoes, peppers and anything prone to soil-borne problems, fresh compost in a clean pot is the safer choice. It costs a little more, but it saves a great deal of disappointment.
