James Whitcombe
Growing vegetables in pots, troughs and grow bags is one of the most rewarding ways to garden in a small space. But the compost you tip in is doing far more work than it would in open ground. In a border, roots can wander in search of moisture and nutrients. In a ten-litre pot, everything a plant needs has to be within a few centimetres of the stem. Get the mix right and you will be picking courgettes and tomatoes for months; get it wrong and you will spend the summer watching plants sulk.
What Container Compost Actually Has to Do
A good container mix has to juggle four jobs at once, and every ingredient you add tips the balance one way or another.
- Hold water without turning into a soggy brick that rots roots.
- Drain freely so roots get oxygen between waterings.
- Hold nutrients in a form roots can reach, rather than washing straight out of the drainage holes.
- Stay open and structured for a whole season, not just a few weeks.
Multipurpose compost is a compromise between all four. It is fine for a quick crop of salad leaves, but heavy-feeding, long-season vegetables will want something more substantial.
Peat-Free Multipurpose: The Everyday Workhorse
Most peat-free mixes in the UK are built from composted bark, coir, wood fibre and green waste. They are light, pleasant to handle and perfectly good for lettuce, rocket, radish, spinach and herbs. The catch is that they dry out fast at the surface and their nutrients leach away quickly in our rainy summers.
Two easy improvements make a real difference. First, stir in roughly a fifth of your mix by volume of sieved garden compost or worm castings to boost moisture retention and slow-release feeding. Second, add a handful of a granular slow-release fertiliser per ten litres of compost, because peat-free mixes are often hungrier than their labels suggest.
If a mix feels dusty and bone-dry, dampen it thoroughly the day before sowing. Peat-free composts can be stubborn about re-wetting once they have dried out completely.
Loam-Based Mixes for Hungry, Long-Season Crops
Loam-based composts, the traditional John Innes style, combine sterilised loam with a peat or peat-free substitute, grit or sand, and a base fertiliser. They are heavier, which is a virtue in a pot: the weight keeps tall plants upright and the loam acts as a nutrient reservoir, holding on to feed instead of letting it drain away.
This is the mix to choose for tomatoes, peppers, aubergines, cucumbers and aubergines in large pots. Fill at least ten to twelve litres per plant, water steadily rather than in fits and starts, and switch to a potassium-rich liquid feed once the first flowers set. You will get firmer growth, fewer blossom end rot problems and a longer cropping window.
You can blend your own version with roughly seven parts sieved loam, three parts composted bark or coir and two parts horticultural grit, plus a balanced base fertiliser.
Homemade Compost and Garden Soil: Free, But Needs Adjusting
Your own compost heap is a brilliant ingredient, but it should never be the whole mix. Sieve it to about 10mm to remove twigs and lumps, then use it at around a third of the total volume. Pure garden soil in a pot slumps, compacts and drains poorly, so always cut it with something open.
A dependable home blend is one part sieved garden compost, one part peat-free multipurpose, a generous handful of leafmould and a handful of grit per bucket. Two warnings: homemade compost can carry weed seeds and slug eggs, so keep it away from seed-sowing mixes, and never use manure or compost that may contain herbicide residues on crops you plan to eat.
Matching the Mix to the Crop
- Salad leaves, radish, herbs: fine peat-free multipurpose, sown thinly and kept moist.
- Tomatoes, peppers, aubergines: loam-based mix in large pots, fed with a high-potash liquid feed.
- Carrots, parsnips, beetroot: stone-free, sandy mix; avoid fresh manure, which causes forking.
- Potatoes: half multipurpose, half garden compost, in deep bags, earthed up as they grow.
- Beans and peas: multipurpose enriched with compost, in deep pots with good drainage.
- Kale, chard, brassicas: loam-based or enriched multipurpose, firmed well at planting.
- Courgettes, squash, cucumbers: rich, moisture-retentive mix in the largest pots you can manage.
Watering, Feeding and Refreshing Through the Season
Peat-free composts dry unevenly, so water little and often rather than flooding occasionally. A two-centimetre mulch of compost or gravel on the surface slows evaporation and keeps roots cool. Begin liquid feeding once plants are established, and for fruiting crops switch to a tomato-type feed when the first truss sets.
At the end of the season, do not reuse spent tomato compost for tomatoes again, as disease builds up. Instead, tip it onto borders or into the worm bin, and start next spring with fresh mix for anything in a container. Refresh the top five centimetres of permanent pots each March with new compost and a slow-release feed, and your small space will keep earning its keep.
