There's a particular quality to the light in the third week of September. The sun sits lower in the sky, shadows stretch further across the field, and by six o'clock there's a nip in the air that simply wasn't there a fortnight ago. The autumn equinox falls around 22 or 23 September in the UK, the point where day and night draw level before the dark evenings properly take hold. You don't need a bonfire or a crowd to mark it. A few small, deliberate acts at home will do very nicely indeed.

Build a seasonal table from what you've gathered

The simplest way to mark the turning point is to clear a table, lay down a plain cloth, and arrange whatever you've collected on your walks that week. It needn't be elaborate or artful. A shallow bowl of haws, crab apples and rosehips, a jam jar holding two or three seedheads, a handful of hazelnuts still in their husks — that's plenty. Add a candle or two and light them at supper.

• Haws and hips — deep red and glossy, gathered from hawthorn and dog rose. Pick them whole and handle gently, as they bruise quickly.
• Crab apples — small, sour and often overlooked. A few on a dish look like jewels, and the rest will make jelly.
• Seedheads — teasel, honesty, wild carrot, the papery heads of yarrow. Cut with long stems and stand them in very little water.
• Fallen leaves and sweet chestnut husks — the spiky cases split open on the ground are a lovely thing to scatter along the centre of a table.

Two rules worth keeping. Leave feathers where you find them — under the Wildlife and Countryside Act it is an offence to keep the feather of a wild bird, however tempting the jay's blue is. And don't strip moss or lichen from walls and trunks; it grows far more slowly than you'd think.

Forage with care: what is ready now

The next three weeks are the richest of the hedgerow year, and everything comes at once. Take a basket, a pair of secateurs and a pocket guide, and pick only from plants you can identify with complete certainty.

• Blackberries — the best fruit is high up on the sunny side of the hedge, out of reach of dogs. Pick on a dry afternoon; wet berries mould within a day.
• Sloes — mouth-puckeringly sour until the first frost. Pick them now anyway and freeze overnight to soften them, then use for gin or jelly.
• Elderberries — strip the berries from the stalks with a fork. Cook them before eating, and discard any red stems.
• Hazelnuts and sweet chestnuts — drop the nuts in water; any that float are hollow or weeviled, so compost them.

Always leave plenty behind for the birds and small mammals that depend on the same harvest. Never uproot a wild plant without the landowner's permission, and remember that picking is restricted on Sites of Special Scientific Interest. If you aren't absolutely sure of a mushroom, leave it alone — no meal is worth the risk.

Cook a harvest supper

The equinox is a good excuse for a slow, generous meal eaten by candlelight. A one-pot barley and root vegetable stew, thickened with a handful of dried mushrooms and finished with chopped parsley, suits the mood. Serve it with bread and salted butter, and follow with a blackberry and apple crumble topped with oats and demerara rather than plain flour.

It's also the moment to start preserving, because the fruits you gather now will carry you through to midwinter.

• Sloe gin — prick 450g of sloes, add 200g of sugar and a litre of gin, shake weekly, and it will be ready for Christmas.
• Elderberry syrup — simmer the berries with water, mash, strain, then add sugar and a splash of brandy to keep it. A spoonful in hot water on a cold morning is a fine thing.
• Rosehip syrup — simmer, strain through muslin, and bottle. Traditional, tart and full of vitamin C.

Walk out to watch the light go

On the equinox, sunset in southern England falls around seven o'clock, and earlier the further north you are. Set off about forty minutes before, with a torch in your pocket for the walk home. This is the walk that rewards attention more than distance.

Notice how the low sun comes sideways through the hedge, turning the underside of leaves to gold. Watch shadows lengthen across the field and dew begin to form in the hollows. Spiders' webs strung between fence posts catch the last light and become briefly visible. Swallows mass on the wires before leaving, rooks drift towards their roost, and bats flicker out over the field edge as the colour drains from the sky.

If you can, walk a route you know well from June. The same path, in different light, tells you exactly what has changed.

Keep a note of the turning year

An inexpensive notebook kept by the door becomes, after two or three years, the most useful thing you own. Note the date you picked your first blackberry, when the last swift left, when the first frost silvered the lawn. Note, too, how you felt — the equinox often brings a mixture of relief and reluctance, and there is nothing wrong with either.

Then do something small and practical: string up onions and garlic in a cool, dark place, hang bunches of thyme and marjoram to dry, or simply put a lantern by the back door and light it each evening at dusk. These are modest acts, but they're the ones that keep you tethered to the season rather than merely watching it pass.