September gardening guide: what to do in your garden this September
September marks the start of autumn, and the pace in the garden begins to slow. Yet, it’s far from the end of the show – many late-flowering perennials, climbers and shrubs still put on a colourful display. While some herbaceous perennials and annuals will have finished for the year, others – such as roses and sweet peas – may continue to bloom well into the month, depending on the spring and summer weather.
Dahlias are at their peak, delivering a spectacular late show, while sedums flower reliably at this time, attracting butterflies and pollinators. Roses often bloom until the first frosts, and Rosa rugosa is particularly rewarding – its fragrant blooms give way to large, vibrant hips. Hydrangeas also take centre stage in early autumn, their fading flower heads bringing subtle colour well into winter.
Looking for a handy summary? Click here to expand our quick reference guide for essential September gardening tasks at a glance.
Flowers and pruning
- Deadhead late bloomers such as dahlias, roses, and sweet peas to encourage continued flowering.
- Prune lavender lightly after flowering, avoiding cuts into woody stems.
- Give wisteria its final prune of the year, cutting back long shoots to promote flowering.
- Collect seeds from nasturtiums, violas, rocket, and beans for sowing next year.
Planting and sowing
- Plant daffodils now; wait until October or November for tulips.
- Sow final rocket and salad leaves under a cloche if temperatures are cool.
- Pot up tender herbs like basil, coriander, dill, and mint and bring them indoors before frost.
- Plan and order spring bulb combinations for next year’s displays.
Harvesting and preserving
- Harvest maincrop potatoes and store in hessian sacks in a cool, dark place.
- Lift and cure onions and garlic, then store or braid for winter use.
- Freeze beans and peas after blanching to preserve surplus crops.
- Pick courgettes regularly to encourage production.
- Dry and replenish herb supplies such as oregano, sage, and mint.
Care and protection
- Protect tender plants including pelargoniums, fuchsias, and osteospermums by moving them under glass or indoors.
- Raise pots on feet or bricks to improve drainage and prevent waterlogging over winter.
- Continue watering and feeding tomatoes, reducing as growth slows; thin leaves to aid fruit ripening.
Wildlife and garden environment
- Start feeding birds with fat balls and peanuts to support them through winter.
- Clean and disinfect nest boxes before rehanging for next season.
- Leave some seed heads and leaf litter in the garden to provide food and shelter for wildlife.
Lawn care
- Reseed bare patches on the lawn; cover seeds with netting to protect from birds.
- Lightly rake and feed the lawn to strengthen it before winter.
Planting
Time to think about spring bulbs
Autumn is prime time for planting spring bulbs. Start with daffodils in September, leaving tulips until October or November. The key to long-lasting blooms is correct planting depth – too shallow and they may not flower in future years. A simple rule of thumb is to plant at a depth three to four times the bulb’s height.
Browse bulb catalogues for tempting combinations – it’s hard to resist planning ahead when the promise of spring is only a few months away.
Last chance to sow herbs and salad leaves
In most areas, early September offers the final opportunity to sow rocket and salad mixes. If the weather is cool, use a cloche to encourage germination and growth. Tender herbs such as basil, coriander, dill, and mint should be potted up later in the month and brought under cover before the chill sets in.
Collecting seeds for next year
Late summer into early autumn is ideal for collecting seeds from plants like nasturtiums, violas, rocket, and members of the bean family. Store seeds in labelled packets in a sealed tin somewhere cool and dry – a garage or fridge works well. Avoid moisture, which is the enemy of seed storage, and consider adding a little milk powder to absorb humidity.
Care and maintenance
Last chance to prune wisteria
Early September is your final opportunity to prune wisteria, tying in growth needed for the plant’s framework and cutting back long, whippy shoots to around six buds from the main stems. Pruning twice a year encourages better flowering, and with luck, you may even get a second flush of smaller blooms in August.
Prune lavender for shape
Once flowering finishes, lightly prune lavender into a tidy shape, avoiding cuts into old woody stems, which do not regenerate well. This helps keep plants compact for next year.
Keep deadheading for extended blooms
Continue deadheading late-flowering perennials and annuals – even sweet peas may still be going strong. Removing spent flowers encourages more blooms unless you’re saving seed.
Care for tomatoes
Tomatoes need consistent watering (daily or every other day, depending on the weather) and feeding at least twice a week. As growth slows, gradually reduce watering and feeding. Thin out excess foliage to improve airflow and ripening, and give ripening trusses extra support with soft raffia ties.
Protect tender plants before frost
By late September, frost risk increases. Pelargoniums, fuchsias, and other tender plants can be overwintered under glass in a frost-free spot. Raise pots off the floor to improve air circulation and reduce the risk of grey mould. Taking cuttings from plants like osteospermums, petunias, and marguerites ensures a supply for next year.
Harvesting
The vegetable patch is often overflowing in September, with potatoes, onions, garlic, beans, courgettes, and herbs ready to be picked, stored, or preserved.
- Potatoes – Harvest maincrops this month, dry them thoroughly and store in hessian sacks in a cool, dark place. Avoid plastic, which traps moisture.
- Beans and peas – Freeze well after blanching for 2 minutes and cooling quickly in iced water.
- Onions and garlic – Lift in dry weather, leave to cure, then store in net bags or braid into strings.
- Herbs – Refresh your supply of dried oregano, sage, mint, thyme, and rosemary.
Wildlife and pests
September is a transition month for wildlife in the garden. Begin feeding birds with fat balls and peanuts, clean and disinfect nest boxes before putting them back up, and leave some seed heads for winter foraging. A layer of leaf litter will help shelter beneficial insects over winter.
Other garden tasks for September
Raise pots on pot feet or bricks to prevent waterlogging during wetter months. If your lawn has bare patches, autumn is an ideal time to re-seed – the soil is warm and damp, giving excellent germination. Cover seeded areas with netting to deter birds.
