Gardening jobs for September

September is the official start of autumn but the frosts don't usually arrive for a month yet and the leaves are still on the trees. Enjoy the last of the summer displays and keep them going for as long as possible by feeding and deadheading. Some of the jobs that need doing vary according to whether the summer was dry or rainy in your area.

Flower borders and patio pots

Keep the patio going

Enjoy the summer display right to the end by keeping your containers in shape. 

Deadhead the plants regularly to encourage more blooms. To know if they need watering, push your finger into the compost and water if it feels dry. Give them a good soak and finally, give them an extra boost by feeding weekly with liquid fertiliser; controlled-release feeds will be running out of steam by now.

See our best liquid plant feeds

Buy spring bulbs

Most bulbs are ready to plant straight away, with the exception of tulips, which are best left until November to avoid the disease tulip fire. Whether you’re buying online, or in a garden centre or supermarket, look for named varieties and buy the biggest size bulbs you can for the best display of flowers. Plant the bulbs at three times the height of the bulb as this will give them moister conditions and make them more likely to reflower the following year.

Find out more about the best spring flowering bulbs

Stay on top of weeds

If you've had rain over the summer, weeds can grow quickly at this time of year, so don’t be ashamed if you find some whoppers. The best way to keep on top of things is by hoeing and hand weeding once a week. Only put annual weeds on the compost heap, as perennial ones, such as dandelions and bindweed, can regrow from their roots.

Try one of the best weed suppressing membranes

Sow hardy annuals

Get a head start on next year by sowing hardy annuals, such as poppies and nigella. Sow them in a coldframe or cold greenhouse and prick them out into indvidual pots once they’re big enough to handle. They can stay there until they’re ready to plant outdoors in March. They'll flower in early summer.

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Mulch bare soil

As crops and flowers are lifted and the ground becomes free, mulch it with 3-5cm of well-rotted manure, spent mushroom compost or garden compost. You can buy bags of soil improver at the garden centre or order a delivery of one-tonne bag online. 

Over the course of the year, worms will pull it into the soil, so there’s no need to dig or fork it in. This will improve any soil and help conserve moisture and suppress weeds weeds while it's on the surface. It's one of the most important jobs to do each year.

Learn how to make compost

Divide perennials

Once rain has come and your soil is damp again, autumn is a good time to divide clumps of perennials such as hardy geraniums. Lift the clump and then divide it into pieces, either by prising it apart with two forks or cutting it up with a spade or bread knife.

Each piece needs some leaves and roots. Older pieces from the centre of the clump should be thrown away, but newer pieces can be replanted or shared with friends. 

Some perennials, such as sedums, will benefit from being divided every few years to keep the clump growing vigorously.

Feed oriental hellebores

It may seem a strange time of year to be thinking of a spring-flowering plant, but now is the time to feed hellebores. Calcified seaweed is great for the job, but a general feed, such as Growmore, would also work well. After feeding, mulch around the plant, ideally with spent mushroom compost.

Learn how to grow hellebores

Watch out for powdery mildew

Powdery mildew often affects both ornamental and edible plants in early autumn, especially if the weather has been dry in your area over summer. You can use a fungicide on ornamental plants, but there are none available for edible plants. Once it's struck, there's not much you can do, but there are ways you can reduce the likelihood of it happening in the future.

Read more about how to prevent powdery mildew on plants

Remove greenhouse shading

Towards the end of the month as light levels begin to decrease, it’s time to remove shading from the greenhouse. Shade netting can simply be removed and stored for next year, while shading paint needs to be rubbed off. Whichever type of shading you used, give the outside of the glass a thorough clean afterwards to let in the maximum amount of light.

Our guide explains how to choose the right greenhouse for you

What not to do in September

It can be tempting to have a tidy up and cut back shrubs. However, if you prune spring-flowering ones, such as philadelphus and weigela, in September, you'll lose the flowers for next year. The shoots they've grow this summer will bloom next year. The best time to prune them is just after flowering, back shoots that have bloomed back to green shoots at their base.

The lawn

Prepare your lawn for winter

If your lawn was baked dry over summer, there's not much you can do until spring. If you had a damp summer and the grass is green, it's a good time to do some TLC to get it ready for winter.

• Mow the grass Cut it a bit shorter than usual to make the next steps easier.
• Rake or scarify Remove moss, thatch and other debris. 
• Oversow Thicken up turf by applying grass seed at the same rate recommended by the manufacturer for sowing a new lawn – typically 25-50g per sq m.
• Top dress Buy from the garden centre and apply at 2kg per sq m. Work it into the turf with a brush. Mow after two or three weeks. The delay will give the new grass seedlings time to establish. 
• Feed the grass Use an autumn lawn food as these encourage root growth and general plant health.

Lay turf

If the summer has been rainy over summer in your area and the soil is damp, September is a great month for laying turf to create a new lawn or to repair an existing one. The soil is warm enough for the roots to grow before the colder weather arrives.

If the summer has been dry for you, wait until spring before you do any lawncare.

Learn how to lay turf, plus the best turf suppliers

The veg patch

Harvest veg

Tomatoes

You can pick tomatoes at any stage from half-colour to fully ripe. The longer you leave them on the plant, the more tender they become, with less bite to the flavour. Handle them carefully and lift upwards with your thumb on the calyx, to keep this on the tomato after it’s picked. Doing this helps them keep for longer, with no skin broken where detached from the calyx. Remove the lower leaves under the bottom truss of fruit to channel the plant’s energy into cropping and to improve air circulation to reduce the likelihood of fungal diseases.

Pumpkins and squash

Pumpkins and squash should be raised off the ground to ripen in the sun before harvesting. If the weather is wet, cut them early and bring them in to ripen in a greenhouse or sunny windowsill. To cure the fruits for storage, keep them in a warm room for a fortnight, then put them somewhere dry and cool but frost-free.

Leeks

Take as many leeks as you need at one time. The remaining plants can be left in the ground to carry on growing. If you planted multi-sown leeks, you can use a trowel or knife to thin out the clumps, taking the biggest one while leaving the rest undisturbed.

Potatoes

Maincrop potato varieties benefit from a long growing season to get to a good size. Now is the time to lift them before keel slugs tunnel into the tubers underground. If you do find some slug damage, use these tubers first and cut out the damaged parts.

Pick early apples and pears

Different varieties ripen at different times, with the earliest starting in August and the latest coming in October. Check if your fruit is ready by cupping it in your hand and gently twisting it to see if comes off. Remove any fruit with brown rot, which turns them brown with rings of white fungal bodies.

Sow winter salads

Early September is the best time to sow salad plants for harvests between November and early May. Grow them out of the worst of the weather under a protective covering, such as glass, polythene, fleece or insect-proof mesh. Although no extra heat is needed, without any protective covering harvests are small in winter and of lower quality.

Sow in modular trays using a Best Buy compost for sowing seeds and then plant outside when they're big enough to handle next month.

Winter salads to try:

• Chervil – sow asap for small harvests of fine flavour.
• Chicories and endives– small harvests of bitter and pretty leaves.
• Claytonia – very hardy, and prolific in mild winters.

• Corn salad – for harvests from November to April.
• Land cress – sow asap for small harvests of strong flavours, and beware pigeons.
• Lettuce – to stand as small plants then heart up in spring or for regular picking of outer leaves from November to May.
• Mizuna – for plentiful leaves in autumn and occasional winter harvests.
• Mustards – large leaves in mild weather.
• Salad rocket – crops in small amounts through the winter.
• Spinach ‘Medania’ – sow asap for small, steady and sweet harvests.

Make late sowings

There is just time in the first week of September to sow spring onions and cabbage, either direct or in modules for planting outside in early October: sow three cabbage seeds per module, and thin to one or two; and up to 10 spring onion seeds per module to plant as a clump that you can gradually thin next spring as you harvest the onions. These will grow through the winter and give crops next year.