Gardening in September

This is the perfect month to sow a new lawn from seed.

• Dig the area over - check out our Gardening jobs for August for seed bed preparation tips)
• Tread the area to firm the soil then rake the site level, removing any debris. 
• Sow grass seed evenly then lightly rake it into the surface, keeping the seed bed well-watered as grass germinates and takes root. 

With established lawns, cut grass short under fruit trees, so you can spot and remove windfall fruits before they rot. Also, for mature lawns only, apply an autumn lawn feed. Unlike spring feeds which are high in nitrogen to encourage growth, autumn feeds are rich in potassium which helps lawns to survive the ravages of winter.

Learn more about how to lay a lawn >

They say the early bird catches the worm, and the same theory applies to gardeners shopping for spring-flowering bulbs such as daffodil and crocus. Visit garden centres now to get hold of the biggest, firmest, plumpest bulbs, and get bulbs in the ground while the soil is still workable. Don’t be tempted to plant tulips this month though – wait until late October or ideally November, because early planting can encourage a fungal disease called tulip fire that results in distorted leaves.

For many gardeners, spring marks the start of the seed-sowing season, with every windowsill crammed with propagators by Easter! However, by direct-sowing hardy annuals outdoors now – plants that bloom and die in the space of a year but can survive winter frosts – gardeners will benefit from earlier flowers next summer and are often rewarded with more vigorous plants. Prepare a weed-free, raked seed bed then sow popular hardy annuals such as poppies, calendula, nigella and larkspur.

Blanket weed can build-up to unacceptable levels over summer, so give ponds a tidy-up before winter. Carefully insert a bamboo cane into the water and twirl dense patches of weed around it before lifting the bundle of growth from the water. Leave blanket weed on the edge of ponds for a few days before disposing of it, so any wildlife inadvertently removed can crawl back into the water. Ponds should be netted before autumn sets in, to prevent leaves from falling into the water and affecting nutrient levels as they decompose.

As the days grow shorter and sunlight weakens, gardeners can give edibles a helping hand to ensure crops ripen.

• Pinch out the tops of cordon tomatoes
• Reduce watering and remove leaves that are overshadowing trusses of green fruit. 
• As pumpkins and squashes swell rapidly, cut off leaves that cast shade over fruits to assist with ripening. 
• Hungry birds (especially pigeons) can wreck brassicas as autumn turns to winter, so cover crops such as Brussels sprouts and cabbages with netting to prevent damage.

Seeing trees laden with fruit is a treat, but don’t go yanking crops from branches or you risk damaging the tree! Hold apples and pears in the palm of your hand, lift the fruit and gently twist it so it comes away with the stalk. Pears may need a week or two indoors to fully ripen before eating. While apples and pears will appear firm at harvest time, plums should be soft and may need to be picked over a period of weeks as fruits gradually ripen.

With the nights drawing-in fast, gardeners need to make the most of every hour of sunlight, so remove greenhouse shading to maximise light penetration through the glass. Temperatures can drop sharply at night in September, and the risk of frost increases too, especially in the north. Get into the habit of closing greenhouse vents and doors before dusk – and if temperatures are forecast to dip, bring tender plants under cover.

Once summer crops such as runner bean and tomato plants have been consigned to the compost heap, veg plots often lay barren over winter, but there’s no need for your kitchen garden to be unproductive during the cold months.

Onion sets for autumn planting are perfect for getting into the ground in September – onion ‘Shakespeare’ is an excellent variety for good size bulbs. Order seeds of broad bean ‘Aquadulce Claudia’ now (although it’s best sown from October) while autumn sowings of hardy salad leaves such as rocket, mustard and land cress are a must for winter nutrition.

If clump-forming perennials look tired and flowering displays have been lacklustre, plants may benefit from autumn division – a proven technique that rejuvenates growth and provides an opportunity to expand your stock of plants. Traditionally, gardeners were taught that hardy herbaceous plants which bloom before Midsummer’s Day should be divided in autumn, while those that flower after mid-summer can benefit from spring division. There are exceptions, so check your variety in a reference book.

• Use a fork to lift the clump, so roots are visible
• Gently pull apart - large clumps may need two garden forks, used back-to-back, to prise roots apart
• Plant divisions promptly and keep them well watered

Once temperatures are cooler, reduce watering of container plants. Remove saucers from underneath pots that acted as reservoirs during summer and raise tubs up on pot feet – it’ll help with drainage and prevent compost from becoming waterlogged during winter. Where bedding plants are still blooming in pots, window boxes and baskets, don’t bother feeding to prolong displays – it’s too late. Instead, dead-head regularly to keep fresh blooms coming, and cut away any diseased or dying foliage.

Despite our changing climate, aster flowers remain classic September favourites that, whatever the weather, reliably dazzle with vibrant, daisy-like blooms that inject a profusion of colour into gardens at a time when summer bedding plants are running out of steam.

These stars of late-summer borders play a vital role in wildlife-friendly gardening too. The blooms of asters are laden with pollen and nectar, attracting beneficial insects in their droves. They are invaluable for bees, helping these precious pollinators to top-up their reserves of nectar ahead of the colder months that are around the corner.

Aster flowers are commonly split into three groups:

• New York asters (Aster novae-belgii) are the classic Michaelmas daisies, so called because they’ll be in full bloom on Michaelmas Day (29^th September). Although popular, New York asters, which commonly grow 2-4ft high, can be prone to mildew, so minimise the risk by taking care to avoid splashing foliage when watering, and remove diseased plants promptly.
• New England asters (Aster novae-angliae) come in similar heights, or are slightly taller, with many sporting stiffer stems. 
• Varieties of Aster amellus also bear popular daisy-like flowers, with a good choice for gardeners who like shorter varieties of up to 2ft high.

Which variety to choose? 

Aster ‘Little Carlow’ is a star performer, smothered in vibrant lavender-blue flowers with golden centres that glow in the early autumn sunlight. ‘Winston S Churchill’ bears wonderful red semi-double flowers, while the soft pink blooms of ‘Harrington’s Pink’ come with the bonus of disease-resistant foliage. For something a little different, grow Aster umbellatus – the tall stems of this flat-topped aster hit heights of more than one metre and bear masses of white flowers that lure butterflies and insects.

Whatever variety you go for, plant into well-drained, fertile soil in a sunny position and look forward to an early autumn treat.

There’s no need for kitchen gardens to sit idle once autumn’s harvest has been gathered. Selected varieties of onion sets – that’s baby onions in an arrested state of development – are ideal for planting from September (wait until October to start shallots though). They’ll grow slowly over winter, but gather pace in spring and will be ready to harvest by early or mid-summer.

Which variety to choose?

Onion ‘Electric’ is a great choice for autumn planting: its flavoursome red flesh is perfect for jazzing-up salads and stir fries early next summer. Look out for sets of ‘Shakespeare’, a British-bred winter onion that’s renowned for good-sized, tasty bulbs that infuse casseroles with flavour – or, if you live in a region that is notorious for bitter winters, opt for onion ‘Radar’. This Japanese variety is extra-hardy with exceptional tolerance of winter cold, delivering reliable crops of milder onions with crisp flavour.

Whatever variety you choose, select a site where soil is free-draining, because onions hate sitting in heavy, wet, freezing soil where they’re at highest risk from disease. Plant sets around 10cm apart, pushing them into the soil so that only their tips are on show, and allow 25-30cm between rows.

Autumn is the natural season for planting, and if you only have room in your garden for one more shrub, the beautyberry is the one to go for. The Latin name of our chosen variety (Callicarpa bodinieri var. giraldii ‘Profusion’) is a mouthful, but don’t let it put you off, because this shrub is unbeatable for its clusters of glowing, violet-purple berries that appear in autumn and last long after leaves have delivered a pyrotechnic show of fiery reds and oranges before falling.

Not only does ‘Profusion’ hold a Royal Horticultural Society Award of Garden Merit (AGM), the ultimate endorsement of great garden performance, but it is not uncommon for its berries to cling on to naked stems until Christmas, providing vibrant colour at a time of the year when much of the garden lays bare. To get the most out of this jewel in the crown of autumn gardens, grow it where it can be seen from the house – it’ll sit happily in well-drained soil in sun or a lightly shaded spot, and does best in acidic or neutral soil.