dahlias are great show flowers to plant in your garden

Garden writer Marc Rosenberg, who has been gardening for 30 years and has contributed to the STIHL blog since 2018, shares what he’s sowing and growing in September with a diary from his small suburban garden. Trimming unruly hedges, autumn lawn care and tackling mildew on the vegetable plot are on the list of gardening jobs for September… 

cut back your leylandii hedges this autumn1. Keep Leylandii in Check

The Leyland cypress, commonly known as leylandii, has an unenviable reputation as the thug of the horticultural world. Monster hedges growing at breakneck speed and blocking light from properties are the stuff of dreams for producers of neighbours at war-style television programmes, who relish the opportunity to capture suburban conflict on camera.

In reality, fast-growing leylandii hedges make a superb evergreen hedge that filters strong winds, provides privacy and creates a natural shelter for hedgehogs and wild birds. Fortunately, I’m on good terms with my neighbour and in early autumn, after checking that birds have flown their nests, we team up to trim his Leyland cypress hedge that borders my garden.

The secret to preventing brown, bare patches when trimming leylandii hedges in September is to avoid cutting too deep, moving the hedge trimmer in a sweeping, upwards motion against the sides first, ahead of cutting the top. Carried out once or twice a year, a neat, dense hedge will result – no neighbourly disputes, police presence required or ASBOs dished out!

dahlias are great show flowers to plant in your garden2. Let Dahlias Dazzle

The children’s author Dr Seuss famously wrote: “Don’t cry because it’s over, smile because it happened”. While few of us will shed a tear at the passing of this summer and its mediocre weather, there’s plenty to smile about as the seasons turn, with dahlia flowers dazzling in the golden September sunlight. 

The fact that dinner plate-size blooms are in full swing after a summer that was largely devoid of warmth is a miracle in itself, proving the resilience of these wonderfully showy flowers. Liquid feeding in September, however, is futile, because it’ll encourage fresh growth that’s unlikely to flower before the frosts – with new shoots susceptible to mildew.

To keep the show going, I’m staking tall, top-heavy stems to prevent them from toppling, while snipping off spent flowers regularly. Dead-heading your dahlia plants is the best way to encourage late-season blooms and it also prevents falling petals from settling on leaves and rotting, which can play a part in bringing flower displays to a premature end.

autumn lawn maintenance 3. Titivate Your Turf – Autumn Lawn Maintenance

By September, as the nights begin to draw in and the first chill of autumn hangs in the morning air, lawns can look patchy and tired. This year, due to regular rainfall and the lack of summer drought, my grass is looking unusually lush.

Skimping on autumn lawn maintenance would, however, be a false economy. Look closely and turf is likely to be packed with dead grass and moss that impedes fresh growth. The first job is to grab my electric lawn scarifier and let it work its magic; these machines are brilliant at effortlessly pulling a mountain of ‘thatch’ from the sward.

 With debris removed, I spike the lawn over with a garden fork, a process known as aerating. More than 80% of a grass plant sits below the soil surface in the form of roots, and allowing air and water to penetrate into the root zone holds the key to keeping turf healthy. Finally, any sparse patches of grass can be thickened up by overseeding – adding a fast-germinating lawn seed that’ll establish before the weather deteriorates.

clean greenhouse windows to allow maximum light through4. Let There be Light!

The lack of sunshine during August took its toll on my greenhouse tomatoes and sweet peppers, which have been reluctant to ripen. It’s imperative to maximise light under glass as the days shorten and the sun grows weaker, so I’ve removed the paint-on shading from the greenhouse glass as a priority, wiping it off with a duster on a dry day to let the light flood in.

Tomato plants produce lots of excess foliage which serves little purpose, so I’ve snipped lower leaves away to expose trusses of green fruit to the autumn sunlight – it helps tomatoes to ripen before temperatures fall.

To encourage chilli and sweet peppers to colour-up, I ease off watering in September while watching out for caterpillars that defoliate plants and burrow into fruits. These critters are masters of disguise and blend in against stems and leaves, so keeping eyes peeled for tell-tale signs of their tiny poo pellets on leaves (lovely!) is essential.

clean your pond of fallen leaves and debris5. Keep Ponds Leaf-free

I used to work for a gardening magazine editor who, unsurprisingly, had a pristine garden and his pride and joy was a fish pond that nestled in a mature border. Once, he told how his wife had lost her footing while gardening and plunged into the water. Fortunately, she was uninjured but she did emerge covered in smelly, brown pond sludge!

Mud in the depths of ponds is a natural element of aquatic ecosystems, resulting from decomposing organic matter, material from marginal plants and fish waste. Allowing excessive amounts of debris to build up, however, can pollute the water and that’s why I’m netting my pond ahead of leaf fall. Leaves and plant debris are easily wind-blown into garden ponds in autumn and reducing the amount of biodegradable material that enters the water will cut the frequency of having to remove excessive pond sludge – a whiffy job that I dread. 

watch out for mildew this autumn6. Watch Out, Mildew’s About!

There’s a menace that’s taking hold across the nation’s gardens – the unwelcome spectre of a musty smelling, powdery mildew. This year, it’s rife on my courgettes, with foliage blighted by rapidly spreading patches of white fungus. Powdery mildew is prevalent on edibles and flowers in late summer – its microscopic air-borne spores contain an abnormally high volume of water which enables them to infect plants even if conditions are dry.

As I prefer not to spray with fungicides, the first line of defence is to stop feeding plants, as a nutrient boost encourages fresh growth that’s susceptible to infection. Diseased leaves are cut off and binned (not composted) to prevent dormant spores of the fungus overwintering and re-infecting plants next season.

What jobs are you doing in your garden in September? How do you trim your leylandii hedges? How do you maintain your lawn in autumn? Let us know in the comments.

Marc Rosenberg bio
Marc Rosenburg at Powderham Plant CentreMarc Rosenberg is a freelance garden writer and editor. A former journalist with Amateur Gardening and Horticulture Week magazines, he holds seven Garden Media Guild Awards. Marc has written for publications including The Garden magazine, BBC Gardeners’ World and RHS online.