Why the pot matters as much as the herb

On a balcony, every plant lives in a microclimate. Wind, reflected heat from brickwork and a wall that blocks the rain all change how quickly compost dries out. A pot is not simply a vessel for compost — it is the entire root zone. It governs how fast water drains away, how hot the roots get during a July heatwave, and how much room a rosemary has before it begins to sulk.

Get the pot right and half the watering problems people blame on their own forgetfulness simply disappear.

Terracotta: charming, breathable and thirsty

Unglazed terracotta is porous. Water moves through the walls and evaporates from the surface, which is why the outside of a terracotta pot goes dark after watering. The upside is that roots stay cooler and the compost is far less likely to sit soggy through a wet British spell. The downside is speed: on a sunny, breezy balcony a terracotta pot can dry out a full day or two faster than a glazed one.

That makes it a lovely match for Mediterranean herbs that actively resent wet feet — thyme, oregano, sage and rosemary. It is a poor choice for basil, coriander or mint, which want consistent moisture. Two practical notes: buy terracotta in sensible sizes, because a small pot is a daily-watering commitment; and in a hard frost, terracotta can crack, so either move pots into shelter or choose a frost-resistant type and raise them onto feet so they are not sitting in standing water.

Glazed ceramic and plastic: moisture that stays put

Glazed ceramic is sealed, so almost no water escapes through the sides. Plastic behaves much the same, but far more cheaply and at a fraction of the weight. Both suit soft-leaved herbs that bolt or yellow when they dry out: basil, parsley, chives, chervil, coriander and mint.

Two things to check before you buy. First, drainage holes — some glazed pots come with only a tiny central hole, which is not enough, so drill or choose a pot with several. Second, weight. A 15-litre glazed pot filled with damp compost can weigh the best part of 20kg, and balcony weight limits are worth respecting. Dark plastic also heats up quickly in full sun, so shade the pot itself with a neighbouring planter or a low-growing companion if the roots are shallow.

Fabric pots and air-pruning: root health without the circling

Fabric grow pots are made from a felted, breathable material. When a root tip reaches the wall, it meets air and stops, then branches into side roots instead of spiralling round the base. The result is a dense, fibrous root system and a plant that establishes unusually well. Mint thrives in them, and they are brilliant for herbs you want to grow to a decent size.

The trade-off is fast evaporation, especially in wind. Choose a larger volume than you would for a solid pot, water more often, and stand fabric pots on a tray so run-off is not wasted. They are not the prettiest things on a balcony either — slip one inside a decorative outer pot if you like, but leave a gap so air can still reach the fabric.

Match pot size to the mature plant, not the seedling

The most common mistake is planting a small herb in a small pot because it looks neat. Within a few weeks the roots have filled the space and every watering becomes a crisis. Aim for the size the plant will need at maturity.

• Basil: 3–5 litres per plant, roughly 15–20cm across.
• Parsley, coriander, chives: 15–20cm and 3–5 litres. Sow coriander little and often, as it bolts quickly.
• Thyme and oregano: 20–25cm and 5–8 litres, in gritty, free-draining compost.
• Mint: 20–25cm and 5 litres minimum — always in its own pot, or its runners will invade everything nearby.
• Rosemary, sage and bay: 30cm or more, 10–15 litres. Rosemary dislikes being moved, so plant it once into a generous pot and leave it be.

Long troughs work beautifully for a row of smaller herbs, provided they are at least 20–25cm deep. Tiny windowsill pots of about a litre are best treated as temporary cut-and-come-again crops rather than permanent homes.

Drainage, weight and the realities of a British balcony

Every pot needs drainage holes, and a saucer beneath it — not least because drips onto a neighbour's balcony below are a quick route to complaint. Traditional crocks in the base do little except raise the water table, so use a free-draining compost with added grit or perlite instead, and empty saucers after heavy rain.

Wind is the other great balancer of moisture. High, exposed balconies dry pots far faster, so choose squat, heavy-bottomed containers that will not topple, and avoid tall, narrow ones. A sheltered, north-facing balcony holds moisture much longer, which means glazed pots there can easily stay too wet, and drainage matters more than water retention.

Whatever you choose, mulch the surface with gravel or a couple of centimetres of loose compost to slow evaporation, and water in the morning so the foliage dries before evening — a small habit that pays off handsomely when mildew season arrives.