What to do in the garden in September

Apple 'Gala' from T&M

Enjoy harvesting your apples this September
Image: Apple 'Gala' from Thompson & Morgan

September is here, and there’s plenty to be done while you’re outside enjoying the last of the year’s warmth. Harvesting crops will keep you busy this month, and there’s plenty of work to do tidying and maintaining plants and equipment.

Take a look at our 'what to sow & grow in September' page if you're looking for planting inspiration. Or, just read on for this month's garden jobs.

Timely tips

Echinacea SunMagic 'Vintage Red' from Thompson & Morgan

Keep perennial plants healthy by dividing them in September
Image: Echinacea SunMagic 'Vintage Red' from Thompson & Morgan

This month, there are lots of jobs to do in the garden. Here are our top four:

  1. Divide your herbaceous perennials. This will keep your plants healthy and vigorous year after year and multiply your stock.
  2. Net ponds now before autumn leaf fall gets underway to reduce the amount of debris entering the water.
  3. Clean out cold frames and greenhouses ahead of autumn sowing and growing.
  4. Plant spring-flowering bulbs now, such as daffodils, crocus and hyacinths, for glorious colour next year.

In the flower garden

Rose 'Our Beth' from Thompson & Morgan

Prune roses once they've finished flowering in September
Image: Rose 'Our Beth' from Thompson & Morgan

Here are your main jobs to do in the flower garden in September:

  • Continue to feed and deadhead hanging basket and container annuals — they will often keep going until the first frosts.
  • Keep deadheading annuals and perennials to extend their performance.
  • Deadhead your penstemons, dahlias, and roses to keep flowers coming.
  • Divide herbaceous perennials as the weather cools. Plant and water in the new divisions well.
  • Prune any late-summer flowering shrubs, such as the rock rose (Helianthemum).
  • Prune climbing roses and rambling roses once they've finished flowering (unless they are repeat-flowering varieties, in which case leave them until later in the year).
  • Keep camellias and rhododendrons well watered at this time of year to ensure that next year's buds develop well.

In the vegetable garden

Potato 'King Edward' from T&M

Maincrop potatoes, like 'King Edwards', can be harvested in September
Image: Photoshot

Here are your September jobs in the vegetable garden:

  • Keep harvesting crops. If you have a glut of fruit and veg try freezing, drying, pickling, and storing so that you can benefit from them later on. For a little inspiration, check out our recipes page for ideas on what to make with your produce.
  • Pinch out the tips of outdoor cordon tomato plants to concentrate the plant's energy into ripening fruits, and not producing further growth.
  • Harvest sweetcorn. To test if it’s ready, pinch a kernel — it will release a milky sap when ripe. If the kernels are starchy, you've left it too late. If they're watery they need a little longer to ripen.
  • Pull or cut off the foliage of maincrop potatoes at ground level three weeks before lifting them. This will prevent blight spores infecting the tubers as you lift them, and help to firm the skins of the potatoes.
  • Spread newly dug potatoes out to dry for a few hours before storing them in a cool, dark place. Store them in paper or hessian sacks, as this will allow the crop to breathe while it’s in storage. Only store undamaged, disease-free tubers — one rotten potato can ruin your whole crop!
  • Help pumpkins ripen in time for Halloween by removing any leaves shading the fruits.
  • Raise pumpkins and squashes off the ground to prevent rotting. Place them on a piece of slate or wood.
  • Keep feeding and watering French and runner beans to keep them producing. Continue harvesting little and often to prevent them setting seed.
  • Cut bean and pea plants away at ground level when they have finished cropping. Leave the roots, as this will slowly release nitrogen back into the soil as they break down.
  • Start the autumn cleanup. Remove any old crops that have finished, and clear away weeds to leave your plot clean and tidy for the winter.
  • Pot up some mint and parsley for the kitchen windowsill for fresh herbs through the winter.
  • Cover your brassicas with netting to prevent birds making a meal out of them.

In the fruit garden

Blackberry ‘Loch Ness’ from Thompson & Morgan

Pick blackberries, like 'Loch Ness' as soon as they ripen
Image: Photos Horticultural

Here are the main tasks to be getting on with in your fruit garden in September:

  • Tidy up your strawberry plants and clear away any used straw, as this will harbour pests and diseases over winter.
  • Pot up strawberry runners to make extra plants for next year.
  • Pick ripe apples. To test when they’re ripe, gently lift them in the palm of your hand or give them a gentle pull — they should come away easily.
  • Pick off rotting fruits from pear, apple and stone fruit trees — they will spread disease if left on the tree.
  • Mow long grass under fruit trees to make it easier to spot windfall fruits.
  • Cover wall-trained peach trees to prevent peach leaf curl from taking hold. The fungus needs wet conditions to infect the plants. Learn more about peach leaf curl in our dedicated guide.
  • Harvest plums. If you have more than you need, freeze them by washing, halving and stoning them, before laying them out on a tray in the freezer. Once frozen, pack them into freezer bags.
  • If you haven't already, cut back the fruited canes of your summer raspberries, leaving the new green canes for next year's crop. Tie in next year's raspberry canes to support wires or fencing.
  • Pick blackberries as they ripen and use straight away or freeze some for use later on.
  • Take hardwood cuttings of currants, gooseberries and figs to increase your stock.

In the greenhouse

Watering plants in a greenhouse

Start your day by watering your greenhouse plants
Image: Shutterstock

  • Water greenhouse plants early on in the day so the greenhouse is dry by the evening. Damp, cool nights can encourage botrytis.
  • Close greenhouse vents and doors late in the afternoon to help trap in heat overnight. This will ensure your plants crop for as long as possible.
  • Empty pots — old compost and decaying plant material can harbour unwanted pests over winter.
  • Clean out your greenhouse to reduce the risk of pests and diseases next year.
  • Remove shading from your greenhouse towards the end of the month so that plants get the maximum light available.

Looking after your lawn

Garden Gear grass trimmer from T&M

Grass growth should start to slow down towards the end of September
Image: Garden Gear grass trimmer from Thompson & Morgan

  • Create a new lawn from turf or seed — autumn weather is favourable for good lawn establishment.
  • Raise the height of your mower blades as grass growth slows down.
  • Carry out essential lawn maintenance to avoid waterlogging and compaction. Aerate your lawn with a garden fork, removing thatch from the surface with a rake and repairing dead patches. Use a specialist lawn scarifier if you have a large area to cover.
  • Apply a special lawn top-dressing after carrying out maintenance work. Follow the instructions on the packet carefully.
  • Feed your lawn with an autumn fertiliser which is rich in potassium and low in nitrogen.
  • Control lawn chafers or leatherjackets with a biological control (nematodes).

Other jobs about the garden

Green manure 'Crimson Clover' from Thompson & Morgan

Sow green manure in September to improve the soil over winter
Image: Green manure 'Crimson Clover' from Thompson & Morgan

  • Improve clay soil before it becomes too wet or frozen by incorporating organic matter and/or horticultural grit.
  • Sow green manures such as buckwheat, clover and ryegrass on uncultivated areas to improve soil and keep weeds down over winter.
  • Create compost bins in preparation for all the fallen leaves and dead plant material which you'll be collecting over the coming months. Autumn leaves make a great addition to compost bins and are ideal for making leaf mould.
  • Burn diseased plant material or dispose of it in your household or green waste. Don’t compost it as the spores may remain in the compost and reinfect your plants.
  • Raise pots off the ground for the winter by using bricks or 'pot feet', to prevent waterlogging.
  • Clear pond weeds and net your pond in anticipation of autumn leaf fall.
  • Blitz perennial weeds — they are more vulnerable to weed killers in the autumn. Use a selective glyphosate free formula to kill both the leaves and the roots.
  • Install water butts to collect rain this autumn and winter. Rain water is great for watering ericaceous plants such as blueberries, rhododendrons and camellias.

From your armchair

Pinkcurrant ‘Gloire de Sablon’ from Thompson & Morgan

Order currant bushes this month to plant in their dormant season
Image: Pinkcurrant 'Gloire de Sablon' from Thompson & Morgan

  • Plan next year’s garden by making rough sketches of your flower borders and vegetable plot. This will help you spot what worked in your garden and what didn't!
  • Order spring-flowering bulbs now for planting this autumn.
  • Order trees and shrubs. They will grow vigorously next spring if planted this autumn.
  • Order your strawberries, raspberries, currant bushes and other berry fruits for cropping next year as these plants are best planted during their dormant season.

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