Why the sunniest sill in the house is worth claiming

A south-facing windowsill is one of the most productive bits of gardening space in any UK home, and most of us waste it. Culinary herbs evolved on dry, sunny Mediterranean hillsides, so they actively prefer the warm, bright, slightly neglected conditions a windowsill provides. Give them six hours of direct light, decent drainage and regular picking, and a row of pots will out-perform a shaded patch of garden all summer long.

Position matters more than pot size. South-facing gives the strongest light and the most heat; east-facing suits softer herbs such as parsley, chives and mint, which appreciate a break from fierce afternoon sun. A north-facing sill will limp along in June but turns gloomy and damp by October, so keep those for houseplants instead. In a heatwave, glass can magnify the sun enough to scorch tender leaves, so ease pots back 10cm from the pane or crack a window during the hottest part of the day. Turning each pot a quarter-turn every week keeps growth even rather than lopsided.

Pick herbs that earn their space

You will get far more from a few well-chosen pots than from a crowded row of struggling seedlings. Soft, leafy herbs grow quickly from seed and are cheap to replace; woody herbs are best bought as young plants, because they are slow and fiddly to raise indoors.

• Basil — the star of a sunny sill, but the thirstiest and the first to sulk if chilled. Sow little and often from April.
• Flat-leaf parsley — slow to germinate (three to four weeks is normal), so sow in early spring or buy a pot and divide it.
• Chives — almost indestructible, happy in a slightly shadier spot, and quick to regrow after cutting.
• Mint — always in its own pot, as it will romp through everything else. It likes more water than the rest.
• Coriander and dill — sow directly, as they resent root disturbance, and expect to resow every few weeks as they bolt.
• Thyme, rosemary and oregano — compact, drought-tolerant and happy on a hot sill, provided the compost is gritty.

Compost, pots and drainage

Use a good-quality peat-free multipurpose compost and improve it by mixing in roughly a quarter by volume of horticultural grit or perlite. Herbs loathe sitting in soggy, airless roots, and this gritty blend stops compost compacting over the season. Every pot must have drainage holes — a decorative outer pot with no holes is a drowning trap unless you empty it after watering.

Size things sensibly: 15–20cm pots suit basil, parsley and coriander, while mint, rosemary and bay want 25cm or more. Stand pots on saucers filled with a layer of pebbles so roots never wallow, and raise them a couple of centimetres above the sill if you can, which improves airflow underneath and reduces damp. If you are sowing seed, a 50/50 mix of compost and vermiculite on the surface helps germination, and a warm sill usually provides all the heat you need.

Watering and feeding through the summer

Windowsill pots dry out far faster than garden soil, especially in a warm room with a radiator nearby. Check daily from May to September — twice a day in a heatwave — by pushing a finger 2cm into the compost. Water in the morning with water at room temperature, letting the excess drain away rather than leaving the pot standing in a puddle.

Feed leafy herbs with a liquid seaweed or general-purpose feed every fortnight from late spring, diluted to about half the recommended strength. Little and often beats a single strong dose, which pushes soft, flavourless growth and can burn tender roots. Once flowering herbs such as thyme and oregano come into bloom, hold back a little; slightly lean conditions concentrate the aromatic oils that give herbs their flavour.

Harvest little and often

The single most common mistake is being too timid with the scissors. Regular cutting is what keeps a windowsill herb bushy and productive rather than tall, leggy and sad.

• Basil — pinch out the growing tip just above a pair of leaves, always leaving two sets of leaves below the cut. Never strip a stem bare.
• Parsley and coriander — take the outer stems at the base, working inwards, and let the centre keep growing.
• Chives — snip whole leaves down to about 3cm from the compost, a few at a time.
• Thyme and rosemary — take the soft, young tips and never cut back into old, woody stems, which rarely regrow.

Pick just before you cook, in the cool of the morning if you can, and remove any flower buds on basil and coriander the moment they appear to keep leaf production going.

Keeping plants healthy on a windowsill

Hot, dry, still air encourages red spider mite, which shows as fine webbing and pale speckling on leaves. Raise humidity by grouping pots together, misting in the morning and keeping the compost consistently moist. Aphids and whitefly can be squashed by hand or washed off with a firm spray of water. Small black flies hovering around pots are fungus gnats, a sign of overwatering — let the surface dry out between drinks. Finally, watch for pale, stretched growth, which means light is too low, and simply move the pot to a brighter spot. With a bit of rotation, watering and ruthless harvesting, that sunny sill will keep you in fresh herbs right through to the first frosts.