What to do in September

September marks the transition from summer to autumn in the UK garden. It’s a month of harvesting the last of the summer crops, preparing for cooler weather and getting your garden ready for the cooler months:

Things to do in September:

Start clearing autumn debris to prevent pests and diseases over wintering.
Net ponds to keep out falling leaves.
Sow or turf new lawns and scarify and aerate established ones.
Move evergreen shrubs.
Reduce feeding of plants in containers.
Take hardwood cuttings from roses.
Begin dividing overgrown perennials.
Plant spring-flowering bulbs, plant out spring-flowering biennials and plant up containers with spring bedding.
Lift tender perennials and bring them under cover.
Harvest the last of the marrows and courgettes and lift main crop potatoes.
Plant out spring cabbages and sow winter lettuce.
Continue to harvest apples and pears and pick autumn fruiting raspberries.
Reduce watering and ventilation in the greenhouse.

Last Chance:

Force hyacinths and narcissi for Christmas flowering.
Plant onion sets and sow spring cabbages.

Getting Ahead:

Start to prune climbing roses as the flowers finish.
Clean the greenhouse in preparation for winter.
Dig over heavy clay soil before the autumn rains make it less workable.
Sow hardy annuals to flower next year.

The Weather

Milder temperatures, with cooling evenings as the month progresses into autumn. Average day time temperatures ranging from 16°C to 18°C and night time temperatures between 9°C and 12°C. Early autumn can still feel quite warm, but cooler weather starts to return later in the month.
Generally moderate to high, with occasional heavy rainfall.
The days start to shorten, but there are still plenty of sunny days, especially at the beginning of the month.

Here’s a detailed checklist of what to do in the garden during September, broken down into categories for clarity. This is a key month: summer is winding down, the soil is still warm and many plants are preparing for dormancy - so it’s a smart time to both harvest and prepare for next year.

General Tasks

Weeding:	Continue to remove weeds; particularly as they start to set seed, this will help reduce weed problems in the next season.
Watering:	Although the weather may be cooler, if the weather remains dry, continue to water plants, especially newly planted ones and vegetables.
Mulching:	Add a layer of mulch around plants, particularly perennials, fruit trees and shrubs, to help retain moisture and protect plant roots as the weather cools.
Pest control:	Look for pests such as aphids, slugs and snails, which can be active in September. Use organic treatments like slug pellets, or pick pests off by hand.

Vegetables & Herbs

Harvest late summer vegetables:	Continue harvesting vegetables such as courgettes, tomatoes, beans and cucumbers. Check for any overripe or damaged produce and harvest regularly to encourage further growth.
Harvest root vegetables:	You can start harvesting early root vegetables like carrots, beetroot and turnips. Leave a few in the ground to store for later in the season.
Support:	Ripening pumpkins and other squashes should be lifted off the ground to prevent rotting.
Sowing:	Now is the time to sow autumn vegetables like spinach, kale, mustard greens and radishes for a late-season harvest. Also over-wintering turnips, onions, spinach, oriental vegetables and garlic.
Feeding:	Continue feeding tomatoes, pumpkins and other fruiting vegetables with a liquid feed high in potassium (K) to increase colour and flavour.
Bare soil:	Areas left vacant after crops have been harvested can be dug over and planted with winter green manures.
Prune and harvest herbs:	Harvest herbs like basil, mint and thyme and trim back any leggy growth. You can freeze or dry any surplus herbs for use in winter.

Fruit

Harvesting:	September is a busy time for harvesting fruit. Autumn raspberries should now be ready as well as early apples and pears.
Thin fruit trees:	For apples and pears that still have too much fruit on the tree, thin out any excess. This will help the remaining fruit grow to a better size.
Store fruit:	If you have apples or pears that need storing, pick them before they are overripe and store them in a cool, dry place. Check for bruising before storing.
Prune raspberries:	After harvesting, cut back the (woody) spent canes of summer-fruiting raspberries to ground level, leaving the newer canes to grow for next year’s crop.
Watering:	Water fruit trees, especially those that are still growing fruit. As fruit trees prepare for winter dormancy, they need plenty of water to ensure a good crop next season.

The Flower Garden

Prune late-flowering plants:	Prune back spent blooms from late-summer or autumn-flowering plants like dahlias, asters and chrysanthemums to encourage further flowering.
Divide and move perennials:	If you have overcrowded perennials, this is a good time to divide them and replant them. You can also move plants to new spots to improve growth or extend flowering.
Prepare beds for next season:	Clear any dead plants or weeds from flower beds, add compost or manure and prepare the soil for planting new perennials.
Deadheading:	Continue deadheading summer flowers like roses, dahlias and marigolds to encourage any last blooms before the end of the season. Trim back any faded or diseased foliage on perennials, annuals and roses to keep the garden looking tidy.
Collect seed:	September is a great month for collecting seeds from your favourite flowers to sow next year. Gather seeds from annuals, such as marigolds, sweet peas and nasturtiums, as well as from biennial and perennial plants.

The Greenhouse

Harvesting:	Continue harvesting ripening crops from the greenhouse. Tomatoes, peppers and cucumbers may still need to be harvested regularly before the weather cools.
Ventilation:	With cooler weather coming, ventilate the greenhouse during the day to maintain airflow and prevent humidity build-up. Close windows or vents at night to retain heat.
Watering:	Be sure to continue watering greenhouse plants like tomatoes, cucumbers and peppers regularly, especially if the weather remains warm.
Prune and tie-in plants:	As crops like tomatoes and cucumbers finish producing, prune away any dead or overgrown branches and tie in new growth to keep the plants healthy.
Prepare for winter:	Clean any greenhouse equipment, pots and tools and remove any old plant debris to reduce the risk of pests and diseases over winter.

Containers

Watering:	Containers can dry out quickly, especially in the warmer weather. Continue to water regularly, but make sure the containers have good drainage to avoid waterlogging.
Feeding:	Feed plants in containers with a liquid fertiliser to help them stay healthy and productive throughout the month.
Refresh container planting:	Consider refreshing containers with autumn-flowering plants like pansies, violas or chrysanthemums, or add some autumnal colours with ornamental kale or cabbages.
Deadhead and tidy up:	Remove dead flowers and tidy up the plants to encourage continued blooming in containers.

Trees & Shrubs

Prune dead or damaged growth:	September is a good time to prune away any dead or damaged branches from trees and shrubs. Avoid heavy pruning of healthy trees and shrubs at this time, as it can stimulate new growth that may be damaged by early frosts.
Watering:	Continue to water trees and shrubs, especially if they are still establishing or have been planted recently. Deep watering encourages strong root development going into winter.
Mulch trees:	Apply mulch around trees and shrubs to protect the roots from cold weather and to help retain moisture. Ensure that mulch is kept away from the base of the plant to prevent rot.
Prepare for winter:	If you have tender shrubs or small trees, start to think about protection for the coming frost. You can use fleece or cloches to cover them when temperatures drop.

Hedges

Trim hedges:	Trim back any growth on hedges that has become too overgrown, but be cautious not to cut back too hard, as some hedges may start to form new growth that could be damaged by frost.
Water newly planted hedges:	Ensure newly planted hedges are watered regularly, especially if the weather is dry, to help them establish before winter.
Pests:	Inspect hedges for any signs of pests or diseases, especially aphids or mildew and take action to control them.

The Lawn

Mowing:	Mow the lawn if it’s still growing, but reduce the frequency as the weather cools. Raise the mower blade slightly to allow the grass to grow a little longer in preparation for autumn.
Watering:	If there has been little rainfall, give the lawn a deep watering. Don’t overwater, but make sure the grass roots are well-hydrated before cooler temperatures set in.
Feeding:	September is a good time to apply a high-potassium autumn lawn feed. This will strengthen the grass before winter and promote healthy growth in the spring.
Aerating:	If your lawn is compacted, consider aerating it with a garden fork or lawn aerator to allow air, water and nutrients to penetrate deeper into the soil.

Wildlife and the Pond

Clear debris:	Keep the pond clean by removing fallen leaves, twigs and debris. This prevents organic matter from decaying in the water, which could reduce oxygen levels and encourage algae growth.
Check water levels:	Ensure the pond water level remains consistent. Top it up with rainwater or dechlorinated tap water if necessary.
Clean the filter:	If your pond has a filter, clean it thoroughly to ensure that it continues to function efficiently.
Wildlife:	As temperatures drop, make sure amphibians and other wildlife have easy access to the pond for overwintering. Place large stones or logs near the water’s edge to provide shelter.
Prepare for autumn and winter:	If you have pond plants that are sensitive to frost, move them to a sheltered spot or remove them from the pond for winter storage.

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