What to Do on Your Allotment in September

September is the month of transition. The glut begins to wind down, nights grow cooler, and the sensible allotment holder shifts focus from harvesting summer crops to storing, clearing, and establishing what will carry them through autumn and winter.

What to sow in September

• Garlic — can be planted from late September onwards in most areas. Break bulbs into cloves and push into prepared soil, 10–15cm apart, with just the tip showing.
• Overwintering onion sets (Japanese types, 'Senshyu Yellow') — plant now for a June harvest next year.
• Hardy green manures — sow grazing rye, winter tares, or phacelia on cleared beds to prevent nutrient loss, suppress weeds, and improve soil structure. Dig in next spring.
• Hardy lettuce and lamb's lettuce — for a continuous autumn and winter supply.
• Spinach — a September sowing under a cloche or fleece will crop through winter.

What to harvest in September

• Pumpkins and winter squash — wait until the stalk is woody and begins to crack. Skin should be hard enough to resist a fingernail. Leave a generous length of stalk when cutting — it seals the fruit during storage.
• Sweetcorn — the final cobs should be harvested before the first serious frost.
• Tomatoes — green tomatoes ripen perfectly indoors. Bring them in before the first frost threatens.
• Autumn raspberries and blackberries — often the best flavoured fruit of the year.
• Apples and pears — test by cupping a fruit and twisting gently. If it comes away cleanly, it is ready.

Key jobs for September

Cure pumpkins and squash for storage. After harvest, "cure" squash in a warm, sunny spot for 10–14 days. This hardens the skin and extends storage life from weeks to months. A properly cured squash stored in a cool, dry room will last until February.

Plant garlic in well-prepared soil. Garlic prefers a slightly alkaline soil; add lime if yours is acidic. Avoid freshly manured beds — they encourage leaf growth at the expense of bulb development.

Divide strawberry runners. Peg healthy runners into pots of compost positioned around the parent plant. Once rooted, cut from the parent, let them establish for 2–3 weeks, then plant in a new bed. Strawberries are most productive in their 3rd year; replace beds every 3–4 years.

Pests to watch

• Slugs — cool, damp September nights are ideal slug conditions. Check under squash leaves, around late salads, and on brassicas.
• Late blight on any remaining tomato and potato foliage. Remove and bag; do not compost.
• Aphids — second flush on autumn brassicas. A jet of water handles most infestations.

Quick win: ripen green tomatoes indoors, not in the dark

Green tomatoes ripen perfectly on a warm, bright windowsill. The common advice to put them in a bowl with a banana is helpful (ethylene speeds ripening) but not necessary. Avoid storing them in a dark drawer — they ripen without flavour developing. Light and warmth are what they need.