1. Read the light honestly before you choose a single plant
Everything downstream depends on this, and it is where optimism does the most damage. A balcony described as sunny by an estate agent is often four hours of light in July and ninety minutes in September.
Pick a clear day and check the space every two hours from morning to evening. Note when direct sun actually lands on the floor and the railing, and for how long. Add it up. Then adjust for season: a south-facing balcony under an overhang can be in full sun in March when the sun is low and fully shaded in June when it is high, and a balcony with a building opposite loses hours as the sun’s arc shortens.
Sort your total into three bands. Six hours or more of direct summer sun is full sun: tomatoes, peppers, chillies, courgettes, beans, strawberries. Three to five hours is partial: salad leaves, spinach, chard, bush beans, radishes, spring onions, most herbs, dwarf carrots. Under three hours is shade, and that is not a failure — mint, parsley, chives, lettuce, rocket and sorrel all crop in it, and they are exactly the things that cost most per gram in a shop.
Check wind too, which kills more balcony crops than shade does. A high or corner balcony dries pots out in hours and shreds soft leaves. If a plastic bag tied to the railing snaps horizontal on an average day, you need a windbreak — a reed screen or a mesh panel cable-tied to the railing — before you need anything else.