Late perennials and warm-season grasses will make a garden beautiful from July until the frosts, and sometimes afterwards, even into winter. Subshrubs like lavender and perovskia are also part of the late summer garden.
Fashionable grasses have a long season of interest. What is so lovely about grasses is the way they move in the breeze, giving movement and a pleasant rustling sound to the garden. Some, like Miscanthus cultivars with their lovely arching habit and beautiful late-summer plumes that persist into winter, can be used structurally, placed evenly along a border. Miscanthus and other grasses can be used as specimens, or in drifts in a prairie planting, with late, natural-looking perennials.
Our climate isn’t great for prairie plantings of grasses and perennials (prairies have long, hot, dry summers and icy winters) but many cultivars have adapted to our climate, and if you have fairly light, free-draining soil, they can work well. The best time to establish a prairie planting is March. The idea is to plant grasses (not more than three kinds) in drifts, as a ‘matrix’, and to plant drifts of bright, late perennials to give colour and drama, setting off the grasses. It’s best to use perennials with simple flowers, like heleniums, rudbeckia, echinaceas and asters, and lofty Verbena bonariensis (purple-top) and crocosmias (montbretias) – dahlias and sunflowers would not look well in this kind of planting. As the main show does not come until midsummer, it is a good idea to plant a few bulbs or small, low, early perennials between the prairie plants, which will conceal the dying leaves and gone-over flowers as they grow.
You can establish a prairie planting more cheaply by sowing seed, although weeding is far more difficult when you sow, rather than plant.
Your choice of grasses depends on how much space you have, and also whether they are warm-season (late into growth) or cool-season (early into growth). Warm-season grasses are slow into growth in spring, so if using them in a border, place them in the middle, where they will be concealed by late spring and early summer perennials like spring bulbs and lupins.
I’ll focus on warm-season grasses in this article, as these look brilliant with late perennials. Their seed-heads often persist through the winter, particularly beautiful when touched by frost or snow. Most warm-season grasses like full sun and a well-drained soil, although some like moist soil. Deciduous grasses should be cut back to the base in late winter (February), ideally before new growth starts, or, if in an exposed position, when they start breaking up and flying annoyingly, around the garden. Evergreen grasses only need dead leaves raking out in early spring; they should not be cut back.
Korean feather reed grass (Calamagrostis brachytricha AGM, 3-4’/90-120cm) is a clump-forming, medium-sized, deciduous grass with arching grey-green foliage, a quite upright habit and fluffy, purplish seed-heads which fade to silvery-grey then a rich brown, that persist longer than any other grass, right through the winter. Sterile hybrid Calamagrostis x acutiflora Karl Foerster (3’3”-5’/1-1.5m), also deciduous, has a very upright, architectural habit with pencil-thin flower-heads to 5’/1.5m, opening purplish-bronze and fading to light brown. It is wonderful planted to a grid for a formal or courtyard garden, but can also be used in drifts in a prairie planting; it’s a great grass for a hot, dry spot.
You need a large garden to grow evergreen pampas grass (Cortaderia selloana, 6-10’/2-3m) with its huge, white to pinkish-beige plumes, but it looks wonderful as a specimen against a dark hedge, like yew. Don’t believe what people say about removing dead leaves by setting fire to the clump! Carefully rake out dead leaves, being very careful of the leaves’ sharp edges (these are not plants for gardens where small children play). There are also attractive pink-plumed forms, C.s. Rosea and C.s Pink Feather.
There is a dwarf form, C. s. Pumila, with 18”/45cm arching leaves and 5’/1.5m creamy-white plumes, more suitable for smaller gardens. Cortaderia richardii (8’/2.5m), is another tall form of pampas grass with looser, drooping, silvery-white plumes, and it’s salt-tolerant, so would be a good choice for a coastal garden. There is also Toe-Toe (Cortaderia fulvida, 6’6”/2m) and, like C. richardii, a narrower form) with drooping, golden-brown flower-heads and sharp-edged leaves, but it is not for the coldest areas, and needs winter protection in its first year.
Deciduous Miscanthus sinensis cultivars average 5’/1.5m, and generally have narrow, arching leaves with a white mid-rib and beautiful, plume-like flowers rising above the foliage in late summer, which persist into winter. They form substantial clumps, and make good specimens, or structural plants evenly spaced along a border, but also work in drifts in a prairie planting. They are substantial enough to set off dahlias, violet-blue agapanthus and tall heleniums in a border. Miscanthus like a reasonably moist soil and full sun.
Some are taller, like M. s. Malepartus (6’6”/2m), with narrow, arching leaves and very free-flowering, with feathery, reddish flower-heads which fade to silver and persist into winter. M.s. Ferner Osten AGM (5’/1.5m) has slender arching leaves, which turn coppery in autumn. Stunning deep red plumes turn pinkish, then silver – an excellent form, good along a border or in a prairie planting. M.s. Morning Light AGM (5’/1.5m) has a wonderfully graceful, fountain-like form, and very narrow leaves, narrowly edged with white, but it rarely flowers unless the summer is hot; it’s been grown in Japan for centuries for its great elegance, and makes a wonderful specimen. M. S. Red Chief (5’/1.5m) has feathery maroon-red flower-heads, slowly fading to silver.
There are also smaller forms for the smaller garden, like M.s. Red Cloud (39”/1m) with rich red, feathery plumes. M.s. Little Zebra (3-4’/1-1.2m) is also smaller, and has deep green leaves with horizontal creamy-yellow bands, turning straw-coloured in autumn; reddish-pink plumes, turning silvery, reliably appear in late summer. Smaller forms can even be grown in soil-based compost in a large container.
If your garden is in a very mild area, you can grow Himalayan Fairy Grass (Miscanthus nepalensis, 5’/1.5m) a very desirable deciduous grass with narrow, bright green leaves, often turning bronze in autumn, and from August to October stunning, drooping honey-coloured plumes; it needs a warm, sheltered site even in the mildest areas, and makes a gorgeous specimen.
Deciduous switch or panic grasses are native to the American prairies, and the cultivar Panicum virgatum Prairie Sky (5’/1.5m) is one of the best, upright in habit with narrow blue-green leaves, which turn orange-yellow in autumn, eventually fading to tan; in August and September, drooping branched flower-stems rise above the clump, with panicles of blue-green flowers, giving a misty effect, lovely in drifts with late perennials such as rudbeckia or echinacea.
Deciduous Chinese fountain grass Hameln AGM (Pennisetum alopecuroides Hameln AGM, 4’/1.2m) has narrow green leaves turning yellow in autumn and brown in winter, and gorgeous, long, fuzzy flowers like squirrel-tails, green at first, then creamy-buff with darker hairs, then tinged with pinkish-purple. It’s a lovely grass, but only for milder areas. Good planted en masse but also as a specimen in a small garden, or even in a pot. The flowers are good for cutting, fresh or dried.
Pheasant’s tail grass (Anamanthele lessoniana AGM, 39”/1m) is a striking and very adaptable evergreen grass, with narrow olive-green leaves streaked with yellow, red and orange, particularly in autumn. Very slender, arching flower-stems carry tiny, delicate, crimson flower-heads between June and September, but be warned, it’s a devil for seeding about. There is a smaller cultivar, A. l. Sirocco (2’5”/75cm) evergreen (semi-evergreen in colder areas) that is ideal for smaller gardens or in a pot; it has olive-green leaves that are patched with copper-orange in summer and turns brilliant shades of red, orange, yellow and bronze in autumn, keeping its colour in winter. Tiny brownish-crimson flowers form on delicate, arching stems in summer. These grasses look lovely with heleniums, particularly Moerheim Beauty, or contrasted with shorter violet-blue asters.
Molinia caerulea (purple moor grass) is a native grass, and its cultivars do well here. M.c. subsp. caerulea Moorhexe is a smaller grass, just 18”/45cm with a grey-green leaves which turn a rich rust-colour in autumn. From July to September it carries narrow, purple-brown flower-spikes, and it looks lovely with low perennials like achilleas and asters. There is also a variegated form (2’/60cm), deep green with cream-striped variegation, turning pale buff in autumn, and with similar flowerheads; it looks well with low perennials, including persicaria.
Smaller grasses are good at the front of a border, for lining paths or in smaller gardens. Evergreen sedges, like Carex, prefer a moister, but well-drained soil, so are useful for those with heavier soils. The evergreen leatherleaf sedge (Carex buchananii, 2’6”/75cm) has orange-bronze leaves and brownish flower-spikes, and looks lovely with violet, blue or even magenta flowers, like hardy geraniums. It self-sows mildly, which is an advantage, as it’s a rather short-lived sedge, although it has survived in my bone-dry soil for ages. The orange New Zealand sedge (C. testacea, 16-24”/40-60cm) emerges olive-green, turning orange-brown in sun, with brown flowering spikes; this and Carex testacea Prairie Fire, even more brightly coloured, are great fronting a ‘hot’ border, or are lovely in drifts with ‘hot’ perennials, or even in containers. Carex oshimensis Everillo (2’/60cm) emerges lime green, turning yellow in summer, Irish-bred C.o. Everlime (18”/45cm) has lime-green leaves with a deep green stripe down the centre, and C.o. Eversheen is variegated yellow and green. C.o. Everest (18”/45cm) is variegated, good for a partially shaded part of the garden. Carex comans Frosted Curls (18”/45cm) has slender, curled, silvery-green leaves and tiny green flowers that self-sow mildly so this short-lived grass can be replaced; it is good planted en masse in full sun, especially with low pastel flowers.
Finally, two superb cool-season grasses whose flowers come in early but last well into autumn. Stipa gigantea AGM, the golden oat grass, forms a low, lax, mid-green, evergreen clump, spreading to 4’/1.2m, over which rise in early summer beautiful panicles of purplish, later golden, oat-like flowers to 8’/2.5m. You do need space for this grass, but it is lovely as a specimen on a corner, on either side of steps, or in a large prairie planting; it’s good with verbascums or agapanthus. Stipa (now Nassella) tenuissima AGM, Mexican feather grass or ponytail grass, is far smaller, at 2’/60cm; this is an deciduous grass, and new leaves emerge jade-green in spring, gradually turning straw-coloured, and it carries silvery-green seed-heads, fading to straw, each seed with a long, silky filament. It is best on poor, dry soils, and goes well with small daisy-flowered and spire-shaped perennials. It self-sows mildly, but once you have this grass you will want to plant it everywhere. Cut back dead leaves in early spring.
Late summer perennials often flower repeatedly (if deadheaded) or for a long period, like Helenium, Rudbeckia, Monarda and Echinacea, all of which are very good with grasses.
Echinacea purpurea is a wonderful plant for the late summer garden, and is particularly good with grasses. E.p Magnus with pink-purple daisy flowers and a deep orange central cone. E.p White Swan is white, with a deep yellow conical centre. Both are 3’/90cm. Don't cut back in autumn, as the seed-heads are ornamental and provide food for birds.
Ornamental achilleas (related to yarrow, that annoying weed of lawns) are brilliant plants for late summer, as their flat, plate-like flower-heads contrast so well with the spires and daisies of other late perennials, and their feathery, grey-green leaves are lovely in themselves. They are excellent with grasses in a prairie planting and are also attractive to hoverflies, whose larvae are famously useful as aphid-eaters. Achillea millefolium Moonshine AGM, a smaller cultivar at 2’/60cm, has soft yellow heads. A.m.Terracotta AGM to 39”/1m) has terracotta-coloured flowers, slowly fading to creamy yellow, and can be cut back to prolong flowering. For a blast of rich colour, try A.m Pomegranate (2’/60cm) with unfading red flowers. They love full sun and a well-drained soil.
Verbena bonariensis (purple-top) is a 5’, drought-tolerant perennial, good for the front of a sunny border, as it is so transparent – you can see right through the stems. It’s also good in a gravel garden or prairie planting, and it self-sows mildly. There is a smaller (2’/60cm) version, V.b. Lollipop, good with achilleas and low grasses; it does not seed about so much but is less hardy, so give it a protective mulch in winter.
For rich golden colour, perennial rudbeckias rule supreme in the late summer garden, and are brilliant with grasses. Rudbeckia fulgida sullivantii Goldsturm AGM, the shorter Gold Rush, and even shorter Little Gold Star are particularly good, and there is a taller rudbeckia, R. laciniata Herbstsonne (Autumn Sun) AGM which grows to 2m, and is lovely in a prairie planting or at the back of a border. Rudbeckias need a moist but free-draining soil in full sun. Don't cut this rudbeckia back; the seed-heads are ornamental and a magnet for goldfinches.
Eryngiums or sea-hollies are also good with grasses or in a prairie planting, and are a magnet for bees. The native sea holly, Eryngium maritimum, is short (2’/60cm) and silvery-blue, with a cone of fertile flowers surrounded by long-lasting, jagged bracts. It’s also wonderful in a coastal planting. E. variifolium is slightly smaller, with silver bracts and blue, cone-like flowers. E. gigantea (Miss Willmott’s Ghost) is taller at 3’/90cm with silver-grey bracts and tiny blue flowers in a tall cone, and is excellent in a prairie planting, but it is a biennial or short-lived perennial.
Monardas (bergamots), with their shaggy flowers from July to September, are good in prairie plantings, but they do like a reliably moist soil, or they will succumb to mildew. Monarda Cambridge Scarlet (35”/90cm) is one of the best; keep it well watered, and leave the seed-heads for winter structure.
Sanguisorba officinalis cultivars, like Red Thunder (up to 2m, often less) or the shorter Tanna (2’6”/75cm) have little burgundy-red bobbly flowers and look wonderful against grasses. They like full sun and a moist soil. (There are other sanguisorbas, often white or pink with caterpillar-like flowers, which are not so good in a prairie planting, and are better in a border.)
Perovskia (now Salvia) atripicifolia Blue Spire AGM (4’/1.2m) is a delightful low sub-shrub with year round interest. It has small, aromatic grey-green leaves on white stems, which look beautiful in low winter sunshine. In August and September little violet-blue flowers like velvet bobbles appear, a lovely contrast to the hot colours of most late-season perennials and the colours of warm-season grasses. Wonderful planted in drifts, and good for pollinators. Cut back in late February.