Plan the position of your perennials based on their need of sun or shade.
Just as they have evolved to thrive in different soil, plants have also managed to find niches in different light conditions. Exotic desert perennials bake in the full glare of the sun throughout the day, while small woodland perennials may receive only an hour or two of dappled sunlight filtering through trees.
Light is a more complicated problem than soil, however, because it is an extremely variable factor. Light can be direct or indirect. Indirect light describes conditions in which the sun's rays first bounce off another object, such as the white wall of a building, or when they light up an area, such as a clearing in a woodland, without directly touching down on it. A woodland clearing would typically receive some hours of direct light and then a good deal more of indirect light.
Light conditions also vary by season and by time of day. Winter light in the northern hemisphere is a "low" light, because the sun is lower in the sky. The sun's rays slant under branches of evergreen first and brighten up areas cloaked in dense shade during the summer months. Similarly, a house corner that faces northwest and is shaded throughout the winter months can be warmed by several hours of sunshine during summer when the sun is high in the sky.
Plants react differently to different types of sunshine, depending on the time of day. Morning sunshine, for example, tends to be cool and refreshing. It seems to waken plants from the quiet restfulness of the previous evening. Hot afternoon sun, on the other hand, contributes even more warmth to an area that might already have been parched by hot, dry weather.
Shade plants, such as hostas, can often tolerate three to four hours of early morning sun but will "burn" (the leaves will brown at the edges) if exposed to that much sun in the height of afternoon. On the other hand, sun lovers, such as New England asters (A. novae-angliae), will flower quite freely with four hours of midday sun but will only bloom sporadically with a similar amount in the first hours of the day.
Light brings warmth with it. A building basking in summer sunlight often retains much of the sun's heat, and sometimes a corner will act as a heat trap. Temperatures can rise quite high in the afternoon, then decline by 10 degrees F or more within an hour after sunlight ceases to shine on the area. Take care when planting perennials in these situations because they must cope not only with light, but also with intense heat in summer.
Dappled light, the kind that filters through trees, is an even more complex factor. Dappled or filtered light not only changes with the season and time of day, but also with changes in leaf growth of the trees overhead.
Since shade is so variable, then, experiment with your own light conditions. Although you might read that a plant is suitable for shade, you may discover that in your particular shaded spot it needs just a bit more light in order to thrive.
You can always help a plant adapt better to your garden by altering the light. In wooded areas, you can increase light by lopping off the lower limbs of trees or by thinning out shrubs. You can lighten the dark corners of a building by painting the building white or by adding reflectors in the form of brilliant white pebbles in an adjacent terrace or path.
In sunny areas, you can reduce the amount of light your plants receive. Trellises, arbors, and lath houses are ideal for shading plants and thus reducing the heat. These structures are often used in southern gardens to prevent heat-induced dormancy. Nature can also be used to reduce the amount of sunlight reaching your perennials, although this takes a little longer. Just plant a hedge or border of tall shrubs or small trees along the south side of the garden.
These perennials are among the hundreds that need shade to look their loveliest.
Lady's-Mantle (Alchemilla vulgaris) Sprays of tiny chartreuse flowers from spring into summer.
Goatsbeard (Aruncus dioicus) Soaring plumes of white flowers in late spring.
Pink Turtlehead (Chelone lyonii) Pink flowers in late summer.
Black Snakeroot (Cimicifuga racemosa) Towering spires of white flowers in summer.
Christmas Rose (Helleborus niger) Brilliant white flowers in late winter.
Virginia Bluebells (Mertensia virginica) Rich blue flowers in early spring woodlands.
Primrose (Primula spp.) Sprightly pink, red, white, or yellow flowers in spring.