Why Autumn Is the Perfect Moment to Pot Up

There is a particular kind of pleasure in pinching a few leaves of fresh mint for a lamb stew while the rain batters the window in January. That pleasure is entirely within reach, provided you do a little work before the first proper frost arrives. Mint, thyme and rosemary are all perfectly capable of carrying on indoors through the winter, but they need to be settled into pots and brought under cover before the cold bites — usually from late September through to the end of October in most parts of the UK, or a fortnight earlier if you garden in the north of Scotland.

The logic is simple. A plant that has spent the summer in open ground has built a sturdy root system, but its top growth is soft and accustomed to mild weather. Lift it gently, pot it up well before the first hard frost, and it will have several weeks of gentle autumn light to establish itself in its new home. Leave it too late and you will be nursing a sulking, half-dormant specimen all winter.

Choosing and Preparing Your Herbs

Not every herb takes kindly to a windowsill. Basil will sulk and stretch; parsley often bolts. Mint, thyme and rosemary, though, are reliable companions.

• Mint is the easiest of the three. It runs underground, so dig up a generous clump with plenty of root and it will barely notice the move. Be aware that mint is a thug — give it its own pot and never let it share with the others, or it will smother them.
• Thyme is a small, woody shrub that hates wet feet. Choose a compact, bushy clump rather than a sprawling old plant, and trim it back lightly by about a third before potting so it has less top growth to support while it settles.
• Rosemary is the trickiest indoors. It resents the dry, stuffy warmth of a centrally heated room and it detests soggy compost. A young, well-branched plant lifted with a good rootball will do far better than an ancient, woody bush.

If you would rather not disturb plants from the garden, garden centres and supermarkets sell small potted herbs cheaply in autumn. These are perfectly good starting points, though they are usually crammed into undersized pots and will need repotting straight away.

Pots, Compost and Drainage

Use pots that are only a size or two larger than the rootball — roughly 15cm to 20cm across for mint and thyme, and a little larger for rosemary. Terracotta looks lovely and breathes well, though it dries out faster; plastic holds moisture for longer. Either is fine.

Drainage matters more than anything else. Fill the base with a layer of crocks or gravel, then use a soil-based compost such as John Innes No. 2, which holds its structure through the winter far better than a lightweight multi-purpose mix. For thyme and rosemary, stir in a good handful of horticultural grit to improve the drainage further. Firm the compost gently around the roots, water once to settle everything in, and let the pots drain thoroughly before they go anywhere near a sill.

Finding the Right Spot

A south- or west-facing windowsill is ideal. These herbs want as much light as a British winter can offer — around six hours a day is the goal. Turn each pot a quarter turn every week or so, otherwise the plants will lean and stretch towards the glass and end up lopsided.

One caution: keep them clear of radiators. Blasting hot, dry air directly onto rosemary is the quickest way to lose it. If your only bright sill sits above a radiator, place a small saucer of water nearby to raise the humidity, or choose a cooler windowsill and accept slightly slower growth.

Watering Only When the Surface Feels Dry

Overwatering kills more windowsill herbs than cold ever does. In winter, plants use far less water than in summer, and compost that stays wet will rot the roots within a fortnight.

Push a finger about a centimetre into the compost. If it feels dry at that depth, water; if it feels cool and damp, wait. Always water with tepid, not cold, water, and let the excess drain away completely — never leave a pot standing in a puddle in its saucer. In a warm room you might water roughly once a week; in a cool porch or unheated conservatory, once a fortnight may be plenty.

Feed sparingly, if at all. A very weak liquid feed once a month from February onwards is enough. Winter is a time for resting, not for forcing growth.

Keeping Them Healthy and Harvesting

Check the undersides of leaves each week for aphids and whitefly, which thrive in warm, still indoor air. A gentle wipe with a damp cloth, or a spray of dilute washing-up liquid, sorts out a small infestation quickly. Pick off any yellowing leaves and remove dead stems to keep air moving through the plant.

Harvest little and often. Snip mint from the top to encourage bushy growth, take thyme sprigs from the outside of the clump, and cut rosemary stems no more than a few centimetres at a time. Regular picking keeps the plants compact and gives you a steady supply right through to spring, when they can go back outside once the last frosts have passed.