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Late-flowering perennials

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Perennials are great for adding masses of colour while at the same time being substantial space-filling plants that demand relatively little care. However, in late summer and autumn the perennial border can be something of a problem. It's too early to be doing much digging up and dividing — the weather's often still too warm and the plants too active — but even so, most of the summer-flowering perennials are looking untidy and unsightly fungal diseases only compound the problem.

Happily, there is a solution. It's partly a matter of choosing the right plants and partly a matter of how you use them. Caring for them properly is important too, so that the flowering season is extended for as long as possible.

Choosing the right plants isn't difficult because there are a surprisingly large number that flower well into autumn, and many that carry on into early winter provided they aren't frosted. The accompanying table — which also includes annuals, tubers and bulbs to expand your colour choice — should give you some idea of the varieties to start with.

Using the plants to the best advantage requires somewhat more forethought and attention to detail. There is of course nothing to stop you simply planting late-flowering perennials among others in a mixed border or combining them with shrubs or annuals. As the weather cools, however, many of those other plants will die back quite rapidly and in doing so they may develop fungal diseases. In a mixed-season border this can lead to a rather untidy patchwork of declining and still active plants, and the last thing we want is for the declining plants to spread disease that still have some time to go.

The best way to avoid these problems is to group at least some of your perennials by season. Keeping together those that flower at roughly the same time should ensure that they all need much the same care and attention. And when the season does finally conclude, they can all be cut back at the same time. Certainly, mix some later and earlier plants together too, so that you don't have a garden that flowers in blocks, but do try some late-flowering groups. After all, that's the great beauty of perennials: the ease with which they can be mixed and matched and shifted around.

Many perennials, such as Rudbeckia, Sedum, obedient plant (Physostegia virginiana), perennial Verbena and shasta daisies (Leucanthemum maximum) are so easy to propagate that you don't even need to worry about moving them — just start new plants. With the hardier evergreens, like perennial wallflowers (Erysimum) and verbenas, you can remove self-rooted layers, sideshoots (slips), cutting and divisions at just about any time.

Don't forget foliage: Joe-Pye weed (Eupatorium purpureum), pokeweed (Phytolacca americana), bishop's hat (Epimedium) and some of the bergamots (Monarda), for example, develop fiery red tones in their leaves during autumn. Some hostas, particularly those with yellow foliage, develop their most intense tones at this time.

There's also often beauty, albeit a poignant one, in dying leaves. The eryngiums, particularly Eryngium giganteum remain attractive even when dried. Because of the ethereal silvery white of its dried leaves and flower heads, this species is commonly known as Miss Willmott's ghost (after the famous British gardener).

For variety in foliage colour and form add a few ornamental grasses. Many of these are evergreen, and apart from their beauty when carrying plumes of flowers, their foliage adds its billowy presence throughout the year. Using these foliage variations to accent flowers and break up the lines of the garden is important because the heavier-flowering perennials often have the least interesting leaves. In damper gardens, horsetails (Equisetum) and rushes can serve a similar purpose.

If you're really interested in adding foliage variety you could try introducing a few easily trimmed shrubs with vivid leaves. Cotinus coggygria, Fuchsia magellanica 'Versicolor' and the various forms of Berberis thunbergii come to mind, though I'm digressing rather from perennials.

Attention to cultivation details early in the season will help ensure a good flower display later on. Many perennials that start to bloom around mid-December, such as the bergamot, Helianthemum, Helianthus, Plume poppy (Macleaya cordata), Rudbeckia, and even Tanacetum can be made to flower later by cutting them back as the young foliage develops. Even naturally later flowering perennials, such as monkshood (Aconitum napellus) and the toad lilies (Tricyrtis), can be made more compact with shorter spaces between the flowers by reducing some of the rampant early season foliage growth.

Of course, any trimming back has to be done well before the flower buds start to develop, otherwise you'll simply lose the blooming season altogether. I prefer to simply pinch back the first strong shoots and completely remove any weak stems. Exactly when to pinch back depends on the plant in question, though it's usually too late once the foliage is so dense that it's hard to easily discern the individual stems.

Late perennials tend to be less hardy than those that flower earlier, which is only logical really — flowering later indicates less concern about upcoming cold. Plants like Salvia involucrata and other sages, Cosmos atrosanguineus and the dahlias will bloom well into May in mild areas but the first hint of frost will kill them off. In really cold winter they can die rather than just die back. Consequently it can be worth finding a sheltered place for these plants so that you make the best use of their later blooms.

Among the best of the later-flowering perennials are asters, dahlias and chrysanthemums (now more properly known as Dendranthema), which are available in such a huge range of colours and cultivars that they could make up a garden by themselves. However, great though they are for colour and abundance of bloom, they are among the plants most subject to attack by fungal diseases: the curse of late perennials.

If allowed to run their normal course these diseases won't do any great harm. After all, they're simply part of the natural process of autumn die-back. What they will do, however, is drastically shorten the flowering season and make the plants unsightly well before their time.

Although the time of their arrival varies with the climate, from late summer onwards fungal diseases are almost inevitable. Once they take hold they can be very difficult to control, so you need to be thinking of prevention rather than cure. There are many relatively low toxicity sprays to control mildew and botrytis. Even the old household cure of baking powder used at the rate of 1 teaspoon per litre of water with an oil or detergent sticker will work provided you repeat it every week, or more frequently if rains are frequent.

So there's no need to despair about the perennial garden once autumn rolls around. With just a little attention to cultivation detail and some basic planting planning you can have a great display right up until the first hard frost or driving winter rains. Have a look at your selection now and see if it can't be improved on for next autumn.

Copyright Geoff Bryant