Well-paired species make a space look right
Some of the best-looking plant partnerships that happen in my garden have very little to do with me. A recent happy accident involved some Salvia pratensis 'Rose Rhapsody' (a pink-spired meadow clary) and some opportunistic bronze fennel. The latter had sown itself among the salvia, and although I considered removing it as soon as I saw the first feathery leaf, I let it be. (I have a soft spot for self-sown seedlings, and find it hard to rip them from the earth.) When the pale, rose-tinted salvia flowered and the metallic fennel foliage frothed around it like a dark thunder cloud, there was an ethereal feel to the combination - as if it had been planted by angels.
There were no celestial beings involved, of course: it was Mother Nature who had organised this planting scheme. She is an accomplished gardener, planting species only in the most suitable soil and conditions. Both the fennel and the salvia like fairly dry soil in full sun, so they form a contented - as well as a good-looking - relationship.
The way that plants collaborate with each other is often the thing that dictates whether a garden is special or just ordinary. When species are well-matched it can make a space appear right; that is, we recognise the way they look with each other, from having seen similar associations in natural or nearly-natural landscapes, and it ticks a "yes!" box in our brain.
In the wild, or in naturalistic man-made creations, such as meadows or woodlands, there are plenty of leaves and comparatively few flowers. Lots of flower in a garden can sometimes be too much of a good thing - for instance, when a gardener relies greatly on bedding plants. To borrow a thought from Anna Pavord, in her book Plant Partners (Dorling Kindersley, £12.99): "A garden that is all flowers is like a cake that is all icing."
In cities and built-up areas, foliage is especially valuable, as the green provides somewhere soft and calm for the eye to rest, particularly if there is lots of brick, concrete and paving. I'm not against masses of bloom in urban situations (and am a great fan of overflowing window boxes and cascading hanging baskets), but unrelieved flowers can be a bit jangly. Many town and city gardens are subject to a fair bit of overshadowing, so jolly summer-flower-growing doesn't work terribly well - although busy lizzies will survive with little light.
True shade-lovers, such as hostas and ferns, will thrive in an area that receives hardly any sun, and the big, ribbed leaves of the first are the perfect mate for the busy, filigree fronds of the second. Both have swung in and out of fashion for more than a century: each is such an outstanding genus, with so many splendid species and varieties, that plant-lovers keep coming back to them. Together, hostas and ferns form one of the great and timeless alliances. Ferns are immune to most pests and diseases, but not so the poor hosta. Although there are some supposedly resistant kinds, most are martyrs to the mollusc - and the resulting slug-holed or snail-ripped leaves are not a pretty sight. If yours is a sluggy or snaily garden, then grow hostas in big pots or not at all.
Bergenia, which has hefty, glossy foliage, can be used as a substitute if you have a hosta-hostile garden. Its common name, elephant's ears, is appropriate, as its jumbo leaves are as tough as old leather. It's also fairly impervious to drought, a condition that often prevails in shady, urban gardens. Team it with Japanese anemones - which have pretty, matt, dissected leaves - if the soil is too dry for ferns. The white-flowered 'Honorine Jobert' produces bright blooms on tall, wiry stems, from late summer and into autumn.
Another joint venture for dry shade is Mrs Robb's bonnet (which, unfortunately, is more commonly known as the tongue-twisting Euphorbia amygdaloides var. robbiae) with Spanish bluebells. Add some lady's mantle (Alchemilla mollis) with its scalloped leaves that attract silvery beads of dew, and its foamy lime-green flowers (although you'll get less of these in the shade). All three of these plants are thugs, so don't expect to grow much else. But dry soil in the dim zone is such a challenge that it can be a relief to cover it with anything that's green and alive.
If you have moist soil in the shade, you can grow heaps of plants; just be sure to contrast broad foliage with finer foliage, or busy leaves with simpler leaves. These all have large or uncomplicated leaves: Astilboides tabularis, Brunnera macrophylla, Darmera peltata, epimedium, Ligularia and skunk cabbage (Lysichiton); while these have more complicated foliage: astilbe, dicentra, meadowsweet (Filipendula), Kirengeshoma palmata, Pachysandra terminalis and Rodgersia.
But let's get back into the sunshine before we run out of page space. There are hundreds of plant coalitions that suit bright and warm positions. For instance, grey foliage sets off blue flowers, while bluish foliage (as on some grasses, and the rue, Ruta 'Jackman's Blue') is delightfully icy when paired with purple or lilac blooms. Purple-leaved shrubs, such as Cotinus coggygria 'Royal Purple' and purple hazel, make a soothing backdrop for the hot reds of dahlias, potentillas and Crocosmia 'Lucifer'.
Ornamental grasses, especially the airy or wispy kinds that go brown or buff with age, such as the Carex, Pennisetum and Stipa genera, look good when woven in with simple prairie-type or meadowesque flowers. These include many umbellifers, and most members of the daisy family, including the yarrows (Achillea) and decorative thistles such as the pinky-purple Cirsium rivulare 'Atropurpureum'. The tallish, hard-headed Allium sphaerocephalon is also an engaging grass companion, as it bounces its dark little bobbles in the breeze.
A final thought on summer affiliations, and the summeriest plant of all: the rose. Grow it with hardy geraniums at its feet (especially if it has unlovely, thorny legs). And if it's a rambler or climber, train a clematis to scramble through it. Rose is beautiful on its own, but married to the geranium or the clematis - or polygamously to both - it shines out like a happy summer bride.