September in the Garden - Tidy, Sow and Support Planting for the Best Five Perennials

September 1, 2025 Rachael Jones

September in the Garden - Tidy, Sow and Support Planting whilst enjoying the late summer colours

While we have certainly enjoyed the long warm days, the intensity has been rather unforgiving on our gardens. Despite all the preparation and planning through the winter and spring months, many of our plants, and especially the lawn, have struggled to keep up. It’s been a challenging season for gardeners across the country.

But now, September brings a welcome shift. The air is cooler, the evenings are drawing in, and the soil still holds the warmth of summer, which are ideal conditions for getting back to work in the garden. It’s the perfect time to roll up our sleeves, tidy the borders, refresh tired beds, and make the most of the late-season colour.

After a summer drought, September is a good time to water newly planted items. If you haven't mulched over the summer months, I would advise doing so now to help your plants conserve moisture. My jobs are tidying up the beds, clearing dead foliage, and collecting leaves for composting, and not forgetting fruit and veg picking. The real joy and deep satisfaction are picking the fruits and veggies I have nurtured throughout the year. I think we all agree this is part of what gardening is all about.

We can start to sow seeds for overwintering vegetables, such as spinach, broad beans, turnips, to name but a few. You still have time to sow seeds of quick-growing salads and radishes for picking in a few weeks, but they will grow even better if you cover the young seedlings with a cloche or fleece to help trap warmth from early chills.

Planting spring-flowering bulbs like daffodils, tulips, crocuses, and hyacinths, and all other spring-flowering bulbs you favour, can be planted in September all the way through to November, to get a head start on next year's garden.

Watering and Mulching Container-grown plants and newly sown seeds need consistent watering, so hold off on planting until you have had some good rainfall. Laying a generous 2-3 inch layer of organic mulch, such as compost or leaf mould, will help retain moisture and suppress weeds, like a natural woodland floor, it will keep the soil in good nick as autumn settles in.

Tidy beds and Leaf Collection Now is the time to clear away and remove dead or decaying plant material and leaves to reduce pests and diseases. This will rot down nicely into a leaf mould, which is brilliant for improving soil structure later on.

Harvest Fruits and Clear With the shift in seasons, it’s time to roll up our sleeves and make the most of the garden’s late offerings. Fruits are either ripening or ready to pick, so keep an eye out for windfalls and heavy boughs to save damaging the branches and bruised fruits. Clearing up the excess fruits will also minimise the risk of diseases spreading, but leave a few as our wildlife friends may love to have something to enjoy through the winter months.

Finish tying in shoots on fan-trained and other wall-trained trees. Cut back old canes of blackberries and hybrid berries after fruiting and tie in the new canes.

Tidy up strawberry plants by cutting off the old leaves and leaving just the youngest in the middle. Weed the plant, looking specifically for fruit debris, which can harbour fungal disease spores and damage the next year’s crop.

Vegetables Continue to sow vegetables for overwintering, to mature next spring. To name a few: turnips, spinach, winter salad leaves, and oriental vegetables. Plant out your overwintering onion sets and your spring cabbages that were sown last month; these are probably ready for planting out now, too. Cover with re-used netting to stop the pigeons shredding them. Other late vegetable crops can also be covered by cloches or horticultural fleece, such as carrots and salads. Take a look at our Fruit & Veg Stepovers

If you have been eating your homegrown potatoes since growing in the springtime, you probably have some left in the ground. Lift your maincrop potatoes now while the soil is still workable, and leave them to dry off before storing somewhere cool and dark.

Top Tip: Make life easier for yourself before the autumn rains and frosts come along. If you have some areas in the veg garden or a small veg patch, it’s time to dig this over thoroughly and add some generous amounts of enriching manure.

Divide and replant perennials Once the rain returns, it's the ideal time to divide clumps of perennials and replant the most vigorous sections. Take your cuttings from the new shoots of the plant that have not produced any flowers, which are called non-flowering stems.

Division of plants is most successful when the plants have stopped flowering and are not in active growth.

🌱 Lawn Care

A brown lawn in summer can look alarming, but it's often just the grass going dormant to survive the heat. With the right care, you can help it bounce back faster and even build resilience for future dry spells until the rain returns. The autumn after a dry summer is the ideal time to carry out renovation and repair. This will help your lawn recover and also make it more drought-resistant the following year.

Patch things up
Take a walk around and spot any bare or thinning areas. Re-seed these patches with a drought-tolerant mix; these hardy grasses are better equipped to handle future dry spells. The soil is warm, and hopefully expect some rain to help germinate the seeds.

Renovate smart
Aerate compacted soil to help water and nutrients reach the roots. Top-dress with compost to give your lawn a nutrient boost and improve soil structure.

Water wisely
Keep the soil moist while new seeds establish, but don’t overdo it. Autumn rain usually lends a hand, but if it’s dry, a gentle soak every few days will do.

Feed for strength
Use an autumn lawn feed that’s rich in potassium. It’ll help strengthen roots and prepare your grass for winter, so it comes back swinging in spring.

Top Tips on Saving and Collecting Your Own Seeds

The colours are starting to change in the borders. As plants go to seed, the intense green of spring and early summer starts to fade, replaced by shades of gold and burnished bronze. At this time of year, I start collecting seeds from some of the annuals and perennials that I want to keep spreading around the garden. The best plants to choose are the species, as you know that the seed will come true; for hybrids and named forms, the resulting plants may well be different from the parent plants.

Wait for a dry day to collect seeds from your favourite flowers. When collecting, only pick when the seedheads are not dewy or moist. Dry seed heads are often indicated by a colour change from green to brown, black or red, but must be before they open and shed their contents. Look out for the papery seed heads of aquilegia, the rattling pods of lupins, and the dusky umbels of astrantia. Delphiniums, cosmos, verbena bonariensis, and echinacea, also produce viable seeds if left to mature. Even hardy geraniums and foxgloves will reward a patient gardener with seeds that can be sown for next year’s display.

Store in labelled envelopes or paper bags. Avoid plastic, which can trap moisture and cause mould. Store your seeds somewhere cool and dry, and come spring, you’ll have a treasure trove of homegrown potential, each one a little memory of the season just passed. It's a satisfying job, and you will thank yourself next spring when you’ve got a stash ready to sow.

Early Sweet Peas Gardeners are competitive and love being one step ahead, so if you have a cool greenhouse or somewhere else suitable, try sowing some sweet pea seeds this autumn and growing the plants on through the winter. You will be able to boast your first blooms early next summer, while most people are still looking at tiny buds.

Essential Plant Supports for Perennials During and After a Dry Summer

Don't forget to get your plant supports sorted now, to make sure they receive adequate support, which is crucial for them to flourish.

Perennials are the backbone of many gardens, providing colour and structure year after year. However, a dry summer can be challenging for them. Making sure they receive adequate support is crucial for them to flourish. Here are some essential plant supports that can help your perennials survive our dry summers, along with a look at some of the most popular mid to late summer flowering perennials.

Stakes: Gardening gospel! If there’s one lesson the late-summer border teaches you, it’s this: stake early, stake generously, and then stake again. You think you’ve done enough? Do a lap, and do it again. From top-heavy perennials to newly revived post-drought plants, everything benefits from a little extra support. It’s not just about keeping things upright, it’s about preserving the shape, the show, and your sanity.

Like all of us, I don’t get them in soon enough…or enough of them…but it’s never too late to wade into the border, lift the drooping phlox or galega and tie it up with 3 or 5 stakes .

Phlox clumps are held up with multi-ball stakes

Cages and Rings: Peony supports were designed with peonies in mind, but are very useful for other bushy perennials such as salvias. Single or double rings are best placed over the plant early in the season, allowing stems to grow inside the ring for natural support: sedums and the many varieties of nepeta come to mind.

Salvias are held upright with a one-ring support

Grids: these are the Rolls Royce of perennial supports, and I use them over tall perennials in the middle or back of the border to stop them from leaning forward and cutting out light and water from the smaller plants in front of them. Two of the tall clumps that come to mind are Campanula lactiflora and Galega officinalis, both of which shoot upwards suddenly, come into spectacular flower and then flop. They have strong stems and will be held by a grid at quite a low level; the stems and leaves will grow through the grid and be held upright.

Galega officinalis grows through a grid

Five of the Best Mid to Late Summer Flowering Perennials

Echinacea: There are some lovely varieties of echinacea that have come onto the market in recent years. The plants are hardy and drought-tolerant, once established, and attract pollinators. They have strong stems and often don’t need supports, but our wraparound stakes will hold the clump together

Phlox paniculata: I mention phlox at every opportunity – I love them! They need rich, fertile soil and moisture; enrich the soil with a good supply of organic matter in winter and water throughout the summer. Divide them every three to four years in late winter and keep them watered as they come into growth. Although I don’t normally fall for chorus girl colours, I do recommend ’Starfire’ in carmine pink and, my favourite, ‘Blue Paradise’ in deep blue-violet shades with a magenta eye – as a bonus, it has a lovely, delicate scent. Taller cultivars will either need a ring of stakes and string or an L3 hydrangea grid well pushed into the bed
Asters: There are so many to choose from, and many of the newer cultivars are resistant to powdery mildew. My favourite is Aster frikartii ‘Monch’, which flowers from mid-summer with bright lavender flowers. The stems are sturdy, but it does have a tendency to splay and will benefit from a one-ring support. For semi-shade, consider Aster divericatus - I use it in my woodland border and support its wiry stems with a short one-ring support
Agastache: I have always placed great emphasis on pollinators: Agastaches give us a long season, aromatic flowers, and they appeal to butterflies. Agastache ‘Blue Fortune’ is one of the finest with peppermint-scented foliage and upright spikes of blue-violet; it loves good drainage and sunshine – ideal for the dry border and lovely planted among grasses. A few stakes woven among the stems should keep them upright
Rudbeckia: also known as ‘cone flowers’, the starry flowers of this late summer perennial shine in late summer borders alongside asters and grasses. Although Rudbeckia ‘Goldsturm’ needs very little support, many of the taller varieties, such as Rudbeckia ‘Henry Eilers’ or Rudbeckia laciniata, benefit from a few stakes to keep the clump together

Echinaceas ‘Lucy’ and Tequila Sunrise’ put on a late summer display

Aster frikartii ‘Monch’ comes into flower in late July

All of the rudbekias produce masses of flowers in late summer

Time to get in the Garden

Take a look at our full range of garden supports, or keep reading our Muntons Garden Journals to see how we can support you in your gardening adventures!

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