What to sow and plant in September
September is one of the most important planting months of the year — garlic, overwintering onions, broad beans, and spring cabbage all go in now for harvests that will come weeks before anything spring-planted is ready.
Mid-September is the ideal planting window in most of the UK. Plant individual cloves 15cm apart, pointed end up, with the tip just below the soil surface. Hardneck varieties offer better flavour; softneck keeps longer.
Japanese varieties such as 'Senshyu Yellow' and 'Electric' planted in September will be ready in June — a month ahead of spring-planted onions. Plant in well-drained soil in a sunny spot.
Transplant seedlings sown in August now, 30cm apart. These will establish before winter and start producing hearts from March or April.
Sow 'Arctic King' lettuce, lamb's lettuce, winter purslane, and land cress in a coldframe or unheated greenhouse. They will crop slowly through winter with no additional heat.
Autumn-sown broad beans will be 4–6 weeks ahead of spring sowings. Choose an overwintering variety such as 'Aquadulce Claudia'. Sow directly where they are to grow.
An autumn sowing in pots placed in a coldframe will give you stronger, earlier plants than spring-sown ones. Sow 2–3 seeds per pot and overwinter under cover.
Sow winter rye, field beans, or phacelia on cleared beds to protect and improve soil over winter. Dig in before it flowers in spring.
September is a good time to divide and replant chives, mint, and other perennial herbs. New divisions establish well before the ground cools.
What to harvest in September
September brings the last of the summer crops and the first of the autumn ones. The priority is getting squash and remaining potatoes lifted before wet weather sets in.
Bring in before the first hard frost. The stem should be corky and starting to crack. Cure in a warm place for two weeks before storing — this hardens the skin for long storage.
If you haven't already, lift before sustained wet weather sets in. Allow skins to dry before storing in paper sacks in a cool, dark, frost-free place.
Harvest all remaining fruits before the first frost. Green tomatoes can ripen on a warm windowsill or be made into chutney. Remove plants once finished.
Early varieties will be at their best. Test by lifting the fruit — if it comes away easily from the spur, it is ready. Late keepers are still on the tree.
Early leek varieties are ready to start pulling from September. They will stand in the ground through winter and can be lifted as needed.
Kale is best after the first frosts, which improve flavour, but you can start harvesting outer leaves from September onwards.
Main-crop carrots can be left in the ground and lifted as needed through winter, or lifted now and stored in boxes of barely damp sand.
Lift and store before hard frosts — they become woody if left too long. Twist off the tops and store in boxes of barely damp sand in a cool shed.
Keep picking until frost finishes the plants. Any remaining pods left too long can be allowed to fully dry on the plant for seed saving.
Likely finishing now as nights cool. Pull plants once frost has blackened the leaves.
Key jobs in September
September is a clearing and preparing month. Work done now makes next spring significantly easier.
Finished tomato, courgette, and bean plants should come out as soon as they stop producing. Compost healthy material; bin or burn anything showing disease.
Cleared beds benefit from a good layer of compost or well-rotted manure dug in now. This improves soil structure over winter and is ready for spring planting.
Bring in a few pots of basil, parsley, and chives before the first frost. They will continue producing on a sunny windowsill through autumn.
Squash kept at room temperature in a dry place will store for months. Check monthly and use any that start to soften. A cool, dry shed is better than a cold garage.
September is the right time to plant tulips, daffodils, and alliums for spring colour. They are also excellent for attracting early pollinators to the plot.
Onions and garlic harvested in summer should be checked monthly through autumn. Remove any showing soft spots or mould before they spread to the rest.
Garlic needs well-drained, fertile soil with a pH of 6.5–7. If your soil is heavy, improve it with grit and compost before planting. Avoid recently manured ground.
September is the last realistic month for saving seeds from outdoor crops. Beans, peas, tomatoes, and courgettes all give excellent results if dried thoroughly before storing.
As softer pickings disappear, pigeons turn to brassicas. Netting now prevents significant damage through winter and early spring.
A soil pH test now gives you time to apply lime to acidic beds before spring. Brassicas need pH 6.5–7; most vegetables grow well at 6.5.
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