Depth Comes Before Anything Else

On a balcony, the compost in your container is the whole of your soil. Roots cannot wander off in search of moisture or nutrients, so whatever you provide in that pot is all the plant will ever have. Before you fall for a handsome planter, measure its depth from the inside rim to the base. Outer heights often include a decorative lip, and a deep saucer can quietly steal a few centimetres of growing room.

As a rough guide for edible crops:

• 15–20 cm: salad leaves, radishes, spring onions, basil and other soft herbs.
• 25–30 cm: bush beans, chard, dwarf peas, strawberries, parsley and mint.
• 30–40 cm: peppers, aubergines, bush tomatoes, short carrots such as 'Paris Market'.
• 45 cm and deeper: courgettes, cucumbers, climbing beans, maincrop potatoes, full-size tomatoes.

Width matters just as much. A tall, narrow column of compost holds less than a wide bowl of the same height, and roots spread outwards as readily as they go down. For most vegetables, a pot at least as wide as it is deep will comfortably outperform a slender, elegant one.

Pots: Match Diameter to Appetite

Round pots are the easiest containers to live with, because they are simple to rotate, group and lift when you need to move them. A 30 cm pot holds roughly 10–12 litres of compost, enough for a single pepper or a generous clump of herbs, but it will leave a tomato permanently hungry. Step up to 35–40 cm, giving 25–30 litres, and you have room for one proper tomato plant or three cut-and-come-again crops.

Material is a trade-off between weight and moisture. Terracotta looks wonderful and is heavy enough to stay put in a gust, but it dries out quickly and can crack in a hard frost, so line it with a plastic pot if you want the look without the drawbacks. Glazed ceramic holds water better and weighs a great deal once filled. Plastic is light, inexpensive and easy to drill, which makes it the sensible workhorse of a balcony. Fabric pots air-prune roots beautifully, but they lose moisture through the sides and demand far more of your watering can.

Troughs and Window Boxes: Making a Long Run Work

Troughs use horizontal space elegantly and sit neatly along a railing or windowsill. The catch is that shallow ones hold surprisingly little compost. A trough 90 cm long, 20 cm wide and 20 cm deep holds only about 30 litres, and once you have spaced plants properly that is not much per root system.

Choose depth to suit the crop: 20–25 cm for salads, herbs, radishes and chard; 30 cm or more if you want carrots, bush beans or dwarf tomatoes. Wider troughs are always better than narrow ones of the same length, because each plant gets a proper share of compost. Space carefully rather than cramming — crowded roots compete for every drop of water, and crowded leaves trap damp, which invites mildew.

Grow Bags: Cheap, Light and Thirsty

Grow bags offer 15–18 litres of compost in a layer only 12–15 cm deep. That suits salads, herbs, dwarf tomatoes and peppers, but not thirsty, deep-rooted crops. Because the compost is shallow, plants depend on you for water almost constantly, and the dark plastic warms up fast in sun, which can cook delicate roots.

Get the best from them by cutting drainage slits in the base, standing the bag on a tray, and planting no more than three tomatoes across it. If you want more depth, cut the bag in half across the middle and stand each half upright like a squat pot — you keep the same compost but gain a much deeper root run. Sinking a bottomless pot into the bag and growing in that also works well.

Drainage, Weight and Wind

Every container needs holes in its base, ideally several at least a centimetre across. Contrary to old advice, a layer of crocks is not necessary and can actually slow drainage by raising the water table inside the pot. Instead, raise containers on feet or battens so water runs away freely, and empty saucers after heavy rain to avoid soggy roots.

Weight deserves honest thought on any balcony. Water weighs a kilogram per litre, so a 30-litre pot, saturated compost and a full-grown tomato can easily reach 30–35 kg. Spread heavy pots along supporting walls rather than clustering them at the centre of a span, and consider lighter composts blended with bark or coir. Broad-leaved plants act as sails in the wind, so favour squat containers over tall ones, group pots together for mutual shelter, and never trust a railing bracket with more than it was designed to hold.

Feeding and Watering a Small Root Run

Bagged compost feeds for around six weeks. After that, begin liquid feeding: a high-potash feed weekly for tomatoes and peppers once the first flowers set, and a general feed for leafy crops. A 2 cm mulch of compost or gravel on the surface slows evaporation, and grouping pots so they shade one another's compost helps too, as long as air still moves between the leaves.

Water in the morning where you can, and check pots again in the evening during a heatwave — balcony containers may need two drinks a day in July. Self-watering containers with a reservoir take the guesswork out of a busy week, but check the reservoir regularly and top it up before it runs dry. Give roots the depth, drainage and steady moisture they need, and even a modest balcony will keep you in salads, herbs and tomatoes right through the season.