The Garden in September 2025 - ‘Into Autumn’

After, let’s just say, an ‘interesting’ summer I am, as always, very pleased to see the arrival of September and I’m hoping for a more ‘normal’ month with a mixture of sun and rain which will allow our precious plants to recover and build up their energy and nutrient stores for the winter to come.

Looking back through past September blogs (to be found under ‘Monthly guides’), I find that in 2020 I was looking at some ‘star’ plants for September including Asters (some of which are now Symphyotrichon), Japanese Anemones, Rudbeckias and Sedums (now Hylotelephium). There was also a section on routine jobs for the month including planting container-grown plants, dividing herbaceous perennials, lifting and moving evergreen shrubs, planting spring-flowering bulbs and autumn lawn care. Also, in September 2020 in a separate blog (to be found under ‘All articles’) I wrote about some of my favourite shrubs. In 2021 the September blog came from the gardens at Aberglasney where, as in most months, there was lots to see and admire. In 2022 I took a more detailed look at Japanese Anemones, Asters, Rudbeckias, ornamental grasses and spring-flowering bulbs. In 2023 the blog was a little different as it looked at ‘Plants for Autumn and Winter Interest’ and included quite a long list of plants which can provide interest through, not only flowers ,but also leaves, fruits and bark as well as shape and form. In 2024 I highlighted the usual September ‘stars’ but also started to take a look at the naming of plants using the binomial system (Genus and Species) devised in the 18^th century by Carl Linnaeus. This blog also contained sections on spring bulbs, taking root cuttings, pruning climbing roses and lawn care.

Three of our Japanese Anemones

At this time of year one plant which clearly enjoys hot and dry weather is definitely the colourful Crocosmia from the Lily family. These late-summer plants were the subject of an article by Naomi Slade in the August edition of the RHS magazine ‘The Garden’. Tall or short, impactful or airy, these attractive plants all have blazing colour which turn up the heat in our summer gardens from July to well into September. I know that Crocosmias have something of a bad name for being invasive and for it being almost impossible to remove all of their corms (some books refer to these as rhisomes) when not wanted in the garden. However, this is down to just one hybrid, Crocosmia x crocosmiiflora, which is still commonly called by the old name of Montbretia. In my opinion this hybrid is best avoided in the garden but some of its cultivars such as ‘Star of the East’ and ‘Debutante’ are still beautiful but much less vigorous plants. It was because of this invasive nature that the original hybrid has spread into the wild as some gardeners have irresponsibly dumped excess plant material beyond their garden boundaries and the relentless corms/rhizomes have done the rest! Since 1981 it has actually been an offence to allow it to escape from gardens and the advice is that excess materials should be disposed of at council waste collection sites so that the corms/rhizomes can be destroyed by the high temperatures generated in industrial composters.

Crocosmia is only a small genus with around eight species, originally native to southern and eastern Africa. They are clump-forming, herbaceous perennials with strappy leaves that emerge in spring, followed by red, orange or yellow flowers on arching stems in the late-summer. They were introduced to Europe in the latter half of the 18^th century and were enthusiastically embraced by the gardening fraternity. Many of the Victorian and Edwardian cultivars have since been lost but more recent breeding has produced many more excellent cultivars such as the well-known Crocosmia ‘Lucifer’ introduced by Blooms of Bressingham and in total today there are over 400 cultivars to choose from. They are unfussy about soil and will thrive in most moist but also well-drained conditions in full sun or dappled shade. However, in areas with drier, free-draining soils Crocosmias in this country do seem to perform better in the shadier parts of the garden. They, especially the smaller cultivars, can be grown in containers but Naomi, the author of the article, believes that they really excel when grown in mixed beds and borders where they blend especially well with grasses, tall Salvias, Heleniums, Dahlias, Agastache, Verbena bonariensis, Ligularia and Hylotelephium ‘Matrona’ (Sedum). The hot colours of Crocosmias really work well with purples and can form striking contrasts with blues, pinks or the high energy reds and yellows of late-summer. In addition, their dramatic, sword-like leaves form a foil to both the flowers and the architectural seedheads which follow. Some gardeners choose to leave these leaves over the winter to protect the corms/rhizomes below, others find them untidy and remove them once they have turned brown. Maomi believes that this is a brilliant genus (literally!) and is well worth serious consideration for inclusion in any garden.

There is no doubt that the heat and dryness of this summer has taken its toll on many plants, stripping away colour and creating a sense of weariness. The temptation is to rush to the garden centre for drought-tolerant plants such as succulents and those from Mediterranean areas. However, we also need to consider the other seasons especially the winter when too much water can be just as much a problem. In our changing climate successful plantings therefore need to be resilient to both extremes. This was the subject of another article in the same magazine by Esther McMillan in which she asked four prominent gardeners for their top plants for these conditions. The first contributor, Jenny Bowden, is actually part of the RHS team working on a 3-year project at Wisley called ‘Gardening for Extremes- Resilient Plants from Rain to Drought’. Her three suggestions are Lythrum salicicaria ‘Robert’, Ceratostigma plumbaginoides and the grass Molinia caerulea subsp. caerulea ‘Moorhexe’. The first is a cultivar of the native Purple Loosestrife that is often found by rivers and lakes. Its vibrant, pink-purple spires of flowers are formed throughout the summer and are great for pollinators. Ceratostigma plumbaginoides produces a carpet of ground cover which seems to be able to cope with really dry, cracked soil in summer as well as temporary flooding in the winter. It has really colourful autumn foliage and lovely blue flowers in late-summer. The Molinia grass is an upright form of Purple Moor Grass that turns orange-brown in autumn. In the sandy, Surrey heathland where the author lives it does particularly well both in the damp hollows and on the free-draining mounded areas where it seeds freely.

One of our Molinia grasses- Molinia caerula ssp. arundinacea ‘Windspiel’

Caron Billington is a professional horticulturalist who has created a garden on a new-build site with heavy clay soil that sits wet in the winter and bakes hard in the summer. Her three recommendations are Thalictrum ‘Elin’, Eurybia divaricata and Succisella inflexa ‘Frosted Pearls’. The Thalictrum is one of the ferny Meadow Rues with clouds of cream and lavender flowers on tall, purple-tinged stems, perfect for the back of a border. For her, Thalictrum ‘Black Stockings’ also performs very well. The Eurybia is a white wood Aster that she uses around ferns, Rogersias and Hostas which she grows in areas of afternoon shade. The Succisella which she grew from seed has made itself at home in the shadier end of the garden where it survives winter drowning but also at the sunny end where it has to battle for water with next-door’s trees in the summer. It is a vigorous, mat-forming perennial that sends up bobbles of lavender flowers in late summer which remind me a little of a low growing Scabious.

Christine Liddle owns Birkheads Secret Gardens near Gateshead and recommends Lysimachia clethroides, Euonymus europaeus ‘Red Cascade’ and Darmera peltata.  The Lysimachia holds slightly drooping (she calls them ‘goose-necked’) spikes of white flowers on top of sturdy stems. This particular type of Loosestrife is known to be a vigorous spreader in shadier, damp areas but for Christine it also does well in full sun with a gravel mulch. The Euonymus is a good shrub or small tree for autumn colour and has shiny green, spring leaves and bright pink fruits that open to reveal vivid orange seeds. She grows it with Japanese Acers and in a hedge with Dogwood (Cornus) and Willow (Salix). Darmera peltata has beautiful, umbrella-like leaves and rose-pink flowers held on red stalks which come before the leaves. It is best known as a bog garden plant but does well on Christine’s boulder clay soil even in the height of summer. The flowers remind me of those of Bergenia and the leaves of a small version of Gunnera.

Maurice Parkinson, is owner of an RHS partner garden in Northern Ireland and his three choices are Agapanthus ‘Midnight Star’, Cornus alba ‘Sibirica’ and Hemerocallis ‘Sammy Russell’. Agapanthus may seem to be an unusual choice but for Maurice, despite its African origin, it grows extremely well and survives not only the winter wet but temperatures as low as minus 15 Celsius. The Cornus is a robust shrub, great for damp places but equally good at tolerating summer drought and is still able to produce its wonderful red stems in the winter. Rather than cut all of these back in spring Maurice just prunes a quarter of the stems annually to retain the bulk of the plants but also to produce new stems each year with their stronger colour. Hemerocallis ‘Sammy Russell’ is a well-behaved, clump-forming Day Lily that flowers in August, later than many other cultivars. Each flower may only last a day or so but it produces its bright red, star-shaped flowers over at least a six-week period, so it is of great value in the summer garden. Maurice says that it goes well with just about everything but is particularly wonderful when paired with Miscanthus grasses.

Finally for this month, this is a great time of year for collecting and storing seed ready for sowing this autumn or next spring- all for free! In terms of collection look out for brown or straw-coloured seedheads which will often rattle when gently shaken. Choose a dry day and wait until any dew has evaporated then snip off individual pods or whole seedheads with their stalks, spread them out on a tray and leave them to dry out for a few days on a windowsill or in a greenhouse. Once they are fully dry the seeds can be separated from the pods and husks with a combination of gentle squeezing and blowing. For storage, paper envelopes or air-tight containers are best which need to be kept in a dry, reasonably cool place- the fridge is not a bad place! Obviously, any of your plants which have produced seed can be collected from including Agrostemma (Corncockle), Ammi majus, Calendula officinalis Pot Marigold), Centaurea cyanus (Cornflower), Cerinthe (Honeywort), Lathyrus (Sweet Pea), Nigella damascena (Love-in-a-mist), Papaver rhoeas and somniferum (Poppy) and Tropaeolum (Nasturtium). As for autumn or spring sowing, I think that the best idea to maximise success is to do both!

That’s all for this month but I will be at the Old Railway Line for the free September talk on Saturday the 6^th at 10am where I will be joined again by plant area manager Lucy for some gardening chat and some lovely plants and the blog will return in October as we enter the real part of autumn with all its glories. Until then enjoy your gardening in September.

Keith.