2. Buy fewer, bigger pots
The single most common mistake is a row of small pots. Small containers dry out in hours, swing wildly in temperature, and run out of root room in weeks, so the plant stalls and never recovers.
Go by volume, not by width. Salad leaves and radishes need about 6 inches (15 cm) of depth. Herbs, spring onions and bush beans want 8 to 10 inches. Dwarf carrots and chard want 10 to 12. Tomatoes, peppers, chillies and courgettes want a minimum of 10 litres — about 2.5 US gallons — per plant, and they do noticeably better at 15 to 20 litres. A potato sack wants 30 to 40 litres.
Two large troughs plus three 15-litre pots will out-produce fifteen small ones, cost less in compost, and need watering half as often. If budget is tight, food-grade buckets from a bakery or restaurant are free and hold 20 litres; drill six to eight drainage holes in the base and two in the lower sides.
Drainage is not optional. Every container needs open holes and needs to sit on feet, small tiles or bottle caps so water can actually leave. A pot standing flat in its own saucer of water will drown roots and turn sour in a fortnight. Saucers are for indoors only.
Weight matters on a balcony. Wet compost is heavy — a 20-litre pot weighs roughly 40 lb or 18 kg once watered. Keep the heavy pots against the building wall or over the structural edge rather than clustered in the middle of a cantilevered slab, and check your building’s rules if you are planning more than a few.