Start with the light, not the herbs

Before you buy a single seed packet, spend a day watching your windowsill. Herbs are sun-loving plants, and the difference between a south-facing sill and a north-facing one is the difference between a flourishing basil plant and a sad, leggy stick. A south-facing window gives you the strongest, longest light of the day; east and west are decent, with gentler morning or afternoon sun; north-facing sills are best kept for mint and parsley, which will tolerate lower light but still won't romp away.

Also check for draughts. A windowsill above a radiator can cook roots and dry leaves, while a single-glazed sash window in January can chill them. Ideally, aim for a spot with at least five to six hours of direct sun in summer, and move plants slightly back from the glass in the depths of winter to avoid cold damage at night.

Rotate your pots a quarter-turn every few days. Herbs lean towards the light, and turning them keeps growth even rather than stretched to one side.

Which herbs actually thrive indoors

Not every herb enjoys life in a pot on a sill. These are the reliable performers:

• Basil — the classic windowsill herb. Needs warmth, plenty of sun and regular pinching. Grow from seed in spring or buy a supermarket pot and split it into several smaller pots, as the seedlings are crammed together.
• Mint — almost too easy. Always give it its own pot, because it spreads aggressively and will smother neighbours.
• Chives — happy in a modest pot, tolerant of partial shade, and they regrow quickly after cutting.
• Parsley — slow to germinate but long-lived. Flat-leaf types are more flavoursome; curly types look cheerful on a sill.
• Thyme, oregano and sage — Mediterranean herbs that want sharp drainage and a sunny spot. They sulk if overwatered.
• Coriander — fast and rewarding, but it bolts quickly in heat. Sow small batches every few weeks rather than one big pot.
• Rosemary — possible, but it prefers a cool, airy spot and dislikes sitting in damp compost.

Skip dill and chervil indoors unless you have very bright light; they stretch and flop disappointingly.

Pots, compost and drainage

Use pots at least 15cm across, with drainage holes — always. A saucer underneath is fine, but tip away any water that collects after ten minutes, because soggy roots are the number one killer of windowsill herbs.

Fill with a peat-free multi-purpose compost, or a soil-based mix for Mediterranean herbs. Terracotta dries out faster than plastic, which suits thyme and rosemary but means more frequent watering for basil and mint. Group pots together to create a slightly more humid microclimate, which helps leafy herbs through dry, centrally heated rooms.

Watering: little and often, but check first

There is no fixed schedule, because it depends on pot size, room temperature and the season. Instead, test the compost with your finger: push about 2cm into the surface. If it feels dry at that depth, water until it runs out of the base. If it's still damp, wait.

As a rough guide in summer, small pots on a sunny sill may need water every day or two; in winter, once a week may be plenty. Signs you're overwatering include yellowing lower leaves, a sour smell from the compost, and fungus gnats hovering about. Underwatering shows as drooping stems, crisp leaf edges and compost pulling away from the sides of the pot.

Feed sparingly. A diluted liquid feed every fortnight through spring and summer is enough for most herbs. Too much food produces soft, flavourless leaves. In winter, stop feeding altogether.

Keeping leaves productive

The trick to a long-lasting windowsill herb is to harvest often and never strip a plant bare. Regular picking signals to the plant that it should keep making new growth.

• Basil: pinch out the growing tip just above a pair of leaves. Two new shoots will appear from that point. Remove flower buds as soon as they form, because once basil flowers the leaves turn bitter.
• Mint and chives: snip stems about 3–5cm from the base. Chives can be cut right back and will regrow within a few weeks.
• Parsley and coriander: take the outer stems first, leaving the young centre to develop.
• Thyme, oregano and sage: harvest lightly and frequently, always leaving at least half the foliage on the plant.

If a plant becomes leggy, cut it back by a third — most will bush out again within a fortnight.

Carrying herbs through the cooler months

British winters are hard on windowsill herbs. Light is weak and the days are short, so growth slows dramatically. Move pots to your brightest window, even if that means rearranging the kitchen. Keep leaves clear of cold glass on frosty nights.

Water far less, and only in the morning, so excess moisture has a chance to evaporate before the cooler evening. Stop feeding until you see fresh growth in early spring. If light is genuinely poor, a small LED grow light for a few hours a day can make a real difference, particularly for basil.

Watch for pests: greenfly on soft new tips, whitefly clouds when you brush a plant, and tiny fungus gnats rising from wet compost. Wipe leaves, remove affected stems, and let the top of the compost dry out between waterings.

Finally, plan for succession. Even the best-tended basil will tire after a few months, so sow a fresh pot in late summer to carry you through to spring. A windowsill of herbs is not a one-off project but a small, steady rotation — and once you get the rhythm, you'll rarely need to buy a plastic punnet again.