Your Garden In September

I’ve come to love September almost as much as May. It’s a month of gentle transition, bidding farewell to summer and ushering in autumn. September is soft, rich and characterful, like a fine wine or well-crafted cheese. Gardens become mellow, velvety and overblown; slightly frayed around the edges but still glorious. Heavy dew gives the appearance that diamond-laced nets have been thrown over lawns and hedges, spiders' webs glittering in the morning light.

A garden in September should be the pinnacle of your endeavour - the lazy, hazy look of swaying grasses, tall perennials and exotic foliage can’t be created overnight. However, the weather in 2024 has challenged even the most experienced gardener. A mild winter and wet spring followed by a relatively cool summer has made it a 'stop / start' year, with plants falling behind then growing like topsy, including weeds. If you have noticed some of your plants flowering at odd times or looking distressed, that could be the result of the unusual weather patterns we've experienced for the last three years. A decent spell of cold should provide the reset they need, but don’t be too quick to ‘winterise’ your garden: wildlife will benefit from a degree of disarray, and the best may yet be to come!

September at a Glance

Plan what plants to move and start creating autumn and winter container displays. You might even want to get ahead and write a Christmas list.

Watch out for mildew, blight and low temperatures at night.

Sow hardy annuals including calendula, larkspur, antirrhinum, cornflower and nigella (love-in-a-mist); salad leaves, radish, rocket, parsley, coriander and winter spinach.

Take cuttings of pelargoniums, impatiens, tradescantia, plectranthus, coleus, penstemon, berberis, ceanothus, choisya, hebe, rosemary, lavender, philadelphus, salvias and viburnum.

Plant container-grown shrubs and perennials. Divide overcrowded clumps and replant the vigorous outer sections. Move biennials and spring cabbages into their final positions.

Prune over-exuberant growth and continue trimming hedges, but not conifers (with the exception of yew). You can remove old stems of summer-fruiting raspberries at the base.

Harvest tomatoes, cucumbers, sweetcorn, beetroot, carrots, chillies, courgettes, French beans, runner beans, Swiss chard, figs, raspberries, blackberries and plums, plus seed from flowers and vegetables that you’d like more of or wish to share with friends.

Pick - sunflowers, dahlias, sweet peas, zinnias, cosmos, gladioli, pinks and chrysanthemums. It’s a good idea to pick any flowers you plan to dry before the weather turns damp and misty.

Make - jams, chutneys, preserves and pickles. Bag up seeds to give as gifts at Christmas.

Buy - pruning tools, shears, bulb planters, autumn and winter bedding plants and spring-flowering bulbs. Order bare-rooted plants for delivery from November onwards.

Enjoy the breathtaking autumn light; cool, misty mornings leading to bright days; watching butterflies and dragonflies; eating homegrown produce. September’s generosity is hard to beat.

Visit open gardens to gather ideas for your own. Many gardens make an effort to extend their appeal until the end of October with displays of tender tropicals or fiery perennials. Great Comp in Kent and East Ruston Old Vicarage (pictured at the top of this post) are favourites of ours. If you’re in Cornwall, a visit to the National Dahlia Collection is a must. Further north, Halls of Heddon put on a tremendous display of dahlias peaking later this month.

Further Advice From Dan Cooper Garden

Bathroom filled with house plants

Indoors

Hopefully, your cherished plants have survived the summer holidays in fine fettle. As the days get shorter and nights become cooler, it’s all change in terms of their care regime:

Move house plants that have spent the summer outside back indoors. I would do this as soon as nighttime temperatures dip below 10ºC. Check them for pests, including slugs and snails hiding beneath pots - you won’t want these making trails across your shag pile! If you’re in doubt, give them a spray with Andermatt's Bug Net before reuniting them with their plant pals.
You may need to do a little bit of juggling to ensure each plant gets the optimal amount of light. Any plants that you moved away from windows to avoid bright sunlight will need to move a little closer to the glass again, although preferably not touching and well away from cold drafts.
Ensure foliage is free of dust to maximise photosynthesis. Use a leaf-cleaning brush followed by a gentle leaf shine spray to keep plants in tip-top condition.
When the weather is cooler and duller, houseplants need watering less. Touch the compost surface; if it feels moist, do not water further. If it feels dry, give the plant a drink. If you struggle with houseplant watering, buy yourself a basic hygrometer (moisture gauge) to check the moisture of your pots, and you’ll soon develop a knack for knowing when to water and when to hold off.
The odd yellowing leaf is probably a sign that a plant is reducing its above-ground portions to help it survive the winter. This is not something to be concerned about - simply remove the leaves and compost them. If you notice lots of yellowing leaves, you may be over or under-watering - all the more reason to acquire a hygrometer!
Growth will start to slow considerably this month. Feeding can steadily be reduced and then stopped altogether from October.
Start dormant cyclamen corms back into growth by potting them in fresh compost, bringing them into a light position and watering them sparingly.
Pot up hyacinths for Christmas flowering. Look out for bulbs that have been ‘prepared’ - a period of chilling to fool them into thinking that winter has come and gone. Keep the hyacinths in the dark until the emerging shoots are 2-3cm long, and then bring them into a bright position. If they’re developing too quickly, move them into cooler conditions to slow them down.

Potting Shed & Greenhouse

My greenhouse door hasn’t been closed since May. Inside, it’s been far too warm to do anything other than water, but as the nights get colder, more attention needs to be paid to the under-glass conditions. Good hygiene, light and ventilation will help to stop fungal diseases from taking hold and spoiling your hard work.

September is the perfect time to clean a greenhouse and make space in the shed before autumn arrives. It should be warm and dry enough for any potted plants and equipment to be moved outside for an hour or two to give yourself ample wiggle room. Only put back inside what you need and dispose of the rest. If you’re feeling inspired, hold a garage sale to clear some space to store your garden furniture over winter.
Lift blinds and wipe away white shading to increase light levels. Remove detritus from gutters, clean the glass and scrub staging with hot soapy water. Your greenhouse will be ready to receive tender plants, pots of bulbs for forcing and tubs of potatoes to harvest at Christmastime. Make sure you have room to move with the door closed behind you so that heat is not lost.
Given the high cost of fuel, consider whether or not you need to heat your greenhouse this winter. If you must, start thinking about how to insulate the space and keep it warm without breaking the bank. Perhaps giving a spare room over to tender plants might be cheaper than heating a greenhouse? Might plants be able to fend for themselves, or could they be replaced in spring for less expense than 5 or 6 months’ supply of electricity, gas or paraffin?
Keep doors and windows open unless nighttime temperatures dip below 10ºC (15ºC if you’re growing tropical plants like orchids, caladiums and bromeliads).
Tomatoes and cucumbers will be starting to wind down soon, but chillies, sweet peppers and aubergines need a long season and still have more to give. Continue watering regularly and feed until the end of the month.
As light levels decrease, help tomatoes ripen by removing the lower leaves. If you haven’t already nipped out the growing point, do so now; any new trusses that develop are unlikely to ripen.
Rarely are we ready to plant spring-flowering bulbs immediately after they’re delivered or purchased at the garden centre? As soon as they arrive at home, take bulbs out of transit boxes and put the individual packages somewhere cool and dark with the maximum amount of ventilation around them: the drier the atmosphere and better the airflow, the healthier they’ll stay. Bulbs are generally safer in the ground than on the potting bench. Begin planting once there is moisture in the soil and before they produce roots and shoots. If you need more guidance, have a look at my guide to when and how to plant spring-flowering bulbs.

Outdoor kitchen sink with plants

Terrace & Balcony

After a long season in bloom, pots and containers may look jaded. You have three choices - leave them alone and accept their fading glory, replant them to give yourself a temporary autumn display or skip straight to winter and replace them with spring flowering bulbs, pansies and wallflowers.

Reduce the watering and feeding of annual plants as growth dwindles. Keep attending to potted shrubs and trees, as they’ll need the strength to overwinter. Don’t rely on heavy rain to water pots: the chances are that very little will get where you need it.
Powdery mildew, a whitish bloom that shows up as merging spots on a plant’s foliage, is a nuisance, taking advantage of humid, autumnal conditions to repress its host plant. Once you’ve got powdery mildew, it’s virtually impossible to control without chemicals that most of us are reluctant to use. Remove lower leaves to improve air circulation around the base of vulnerable plants such as dahlias and Michaelmas daisies. The rapid wetting and drying of compost in pots make plants growing in containers especially vulnerable.
Begin planting containers with autumn bulbs, starting with daffodils, crocuses, muscari and dwarf irises. I prefer to plant a single variety per pot. However, if you like to mix or layer your bulbs using the ‘lasagne’ method, hold off until next month as it’s too early to plant tulips.
Clear weeds from cracks in paving and driveways before they get established. As you’d expect, I have just the tool for the job!

Allotment dahlias

Flower Garden

I find it sad that so many people consider it ‘game over’ for their garden by the end of September. A well-tended, carefully planned garden can be a joy to behold well into October. If you feel the battle is already lost, start to clear and tidy, making space for bulbs, biennials and hardy annuals. Bare earth is not an attractive sight and an invitation for weeds to colonise, so try to have something planned for when beds and borders become vacant, even if it’s a simple green manure crop like grazing rye or winter vetch.

Any experienced gardener will tell you that autumn is a terrific season for planting. The earth is warm, inviting plants to produce strong root systems before winter. Autumn weather is also comfortable for gardening - neither too hot nor too cold and not too soggy underfoot.

Once it starts to rain again, conditions will be ideal for dividing clumps of perennials, saving the most vigorous sections for replanting. Container-grown plants will settle in fast if planted now.
If you sowed wallflowers, pansies and violas in early summer, you should start moving them into their flowering positions. Give them plenty of water to help them get re-established. If you forgot, or the young plants perished in the drought, you’ll find that garden centres are already jam-packed with garden-ready plants to get you up to speed.
It’s the last opportunity to sow hardy annuals such as calendulas, opium poppies larkspur, antirrhinums, cornflowers, aquilegia and nigella. The seeds will germinate and form bushy plants before winter. Next year they’ll start flowering earlier than spring sowings, perhaps in May or June. Sow in marked rows to distinguish the seedlings you want from the weeds you don’t.
Look out for self-sown seedlings - I’ve already spotted hollyhocks, honeywort (Cerinthe), violas, verbena and sweet williams popping up on the allotment. Protect and nurture those you want to keep and pull out the surplus. Nature can be very generous so don’t be afraid to thin seedlings out so that there are 20-30cm between each one.
If you live in colder regions of the U.K., prepare to lift and store tender plants such as bananas, begonias, angel’s trumpets (brugmansia), cannas and pelargoniums. In warmer regions, you can hold off until October or even November. The main thing is to avoid any chance of plants being frosted.
Continue picking flowers as often as possible. As someone who prefers to enjoy flowers in situ, I pick more enthusiastically as the days get shorter and I spend more time indoors.
Collect flower seed as it ripens, choosing a dry day. Remove remnants of old petals, stems and seed pods before storing the clean seed in labelled envelopes or sachets.
Dahlias, chrysanthemums and sunflowers must be tied securely to stakes or strong canes to take their flowering weight. Do not assume they will continue to do so because they’re standing upright on their own: we are already getting our first visits from the local parakeets, and they’re snapping off every unsupported sunflower head!
September is the best month to plant daffodil bulbs in the ground. Unlike tulips, daffodils need time to establish themselves in warm soil before winter. You can also plant smaller bulbs like crocuses, irises, muscari and anemones. Practically speaking it can be challenging to find space in borders until other plants have died down, but in lawns and meadows, you should be able to get cracking provided the soil is not too hard and dry. Use a good quality bulb planter to make the job easier.

The Gin and Tonic Garden Courtyard

Trees, Shrubs & Lawns

Hedge trimming continues this month, and with autumn rain comes the opportunity to move evergreens, re-seed threadbare lawns or sow new ones. By now, my garden has become very jungly, some might say unruly, with plants spilling and arching over paths and seating areas. I like it that way, but if you prefer your garden to look tidy, there’s nothing wrong with trimming here and there to keep things neat and usable for a few more weeks. Avoid being overzealous as birds and insects will appreciate the cover.

Follow my ‘In The Know’ guide for hedge trimming advice but don’t set about conifer hedges with your shears now as it’s too late. Yew is an exception to that rule and can still be cut for a few more weeks.
Keep camellias, azaleas and rhododendrons fastidiously well watered. This is when next year’s flower buds are formed, and a lack of water now can mean no flowers in spring.
Move evergreen trees and shrubs if you wish to. They’ll need time to settle and establish new roots before winter, so complete this job by early October.
It's been a good year for our lawns as there's been regular rainfall across most of the country. However, if your lawn has developed bare, scuffed patches, now's the time to reseed.

Trug of tomatoes

Kitchen Garden & Allotment

September is typically a month of harvest and this year crops have been abundant.

Shorter days and cooler nights mean that growth is starting to slow dramatically. Lower light levels reduce the rate at which fruit ripens, and unsettled weather may result in blemishes or damage. Harvest your fruit and vegetables in their prime, freezing, preserving or pickling them when you have too much. Friends and family rarely say no to a big tub of tomatoes, although courgettes can be notoriously hard to give away. Try not to let anything go to waste - what’s tiresome now will taste like manna from heaven in January.
You can start picking autumn brassicas now. Cut cauliflowers and cabbages through the stem close to ground level. The stumps can be left to resprout, producing edible leaves before winter. Alternatively, they can be dug out and composted, making space for a new crop.
Dig the bed over once more to check for missed tubers when you've finished lifting your potatoes. Even potatoes the size of a pea can produce a new plant next year, usually in the middle of a different crop. If you don’t have anything to take their place immediately, sow green manure.
Apples are ready for picking when they sit in the palm of your hand and come away with a slight twist. If you have to pull hard, they’re not quite ready yet. Store apples in a cool, dry, well-ventilated spot until you’re ready to use them.
Sow spinach, radishes, rocket, salad leaves, oriental leaves (including pak choi, mizuna, mustard and Chinese cabbage), parsley and coriander for cropping in autumn and winter.
Keep on top of the weeding as often as you can. Shortening days signal plants to hurry up and produce seeds before winter. You may find that early-sown parsley and coriander run to seed and that weeds race to complete their lifecycle before winter.

Ivy flowers in bud

Wildlife & Sustainable Garden

When I think about my garden in September, the humble spider is the first creature that springs to mind. I am grateful that these prolific creatures don’t cause me anxiety because they stage a complete takeover every autumn, inside and out. As I write this post, I see at least three spiders from my desk. They go about their business, I go about mine, and we’re all happy. In the garden, they create much mischief by crisscrossing every path and doorway with their invisible webs. Rarely do I make it far without catching the gossamer threads in my hair. If you haven’t stopped reading in horror, carry on to discover what other delights we might find in the garden this month.

Having made the most of summer, insects are now preparing for winter. Butterflies will flock to late summer flowers, building up reserves for hibernation or migration back to mainland Europe. Queen bumblebees may already be looking for places to hibernate. Now’s the time to set up a bee hotel or make one by bundling hollow stems together and placing them in a sheltered spot. Hang bee hotels in a sunny position, preferably catching early morning rays to warm emerging adult bees when they hatch in springtime.
It’s changeover time in the skies, with birds such as swallows, swifts, willow warblers, black caps and pied flycatchers preparing to depart our shores. Next month, ducks, geese, redwings and fieldfares will start to arrive for winter.
Clean bird feeders and birdbaths thoroughly in readiness for the months ahead. This summer’s drought is likely to result in a scarcity of seeds, nuts and berries so be prepared to supplement birds’ natural food sources until spring.
In some situations, ivy (Hedera helix, pictured above) can be considered a nuisance, but wildlife adores it. In its shrubby form, ivy produces sputnik-shaped flower heads that bees and hoverflies love so much you can often hear the plant before seeing it. Birds are likely to roost within, so avoid any disturbance. Trim ivy, if you must, in winter before birds start nesting again.
Hoglets (young hedgehogs) need fattening up before they hibernate next month. Put out dishes of water and meat-based pet food if they’re visiting your garden. If you spot a late-born hoglet that’s too small to hibernate, contact your local hedgehog rescue for advice.
Frogs, toads and newts are starting to look for shelter in compost heaps, log piles or at the bottom of ponds. Avoid disturbing all three habitats so that they can overwinter undisturbed.
Hedgerows will be dripping with berries, haws and hips, providing food for birds, small mammals, moths and insects. Avoid trimming or tidying until the leaves have fallen and all the fruits have gone.
It’s the last chance to cut lawns and meadows where long grass lingers. Carefully remove the bulk using shears or a strimmer and leave it where it falls for a day or two so that insects can crawl out and find alternative shelter. Then remove the hay and mow the grass to about 7.5cm.
Don't be hasty and harvest every pear, plum or apple you can reach. Leave a few fruits on each tree for birds and butterflies to enjoy.
Leave seedheads on plants such as teasel, lavender, sunflowers and Verbena bonariensis. These will provide food for birds and small mammals through autumn and winter. Stop deadheading roses if you wish the hips to develop.
Material for composting will mount up rapidly during autumn. Ensure you have ample compost bins and net sacks for collecting leaves and making leafmould. If you have space in your greenhouse, consider moving a compost bin inside to generate a small amount of background heat as the contents decompose.