Shade is a major design consideration in most gardens in the Southern U.S. Given the opportunity, we nestle our homes under the spreading boughs of forest giants and are forced from the outset to develop a garden that will never know the full intensity of the sun. Or, if our subdivision was a cotton field or cow pasture in a previous life, we grow our own shade – never quite believing that those small switches we plant will one day become sylvan giants and rob sunlight like a thief in the night. Shade is a good thing, though. It makes our outdoor living spaces habitable during the muggy months and permits the summer-long enjoyment of our gardens.
Shade changes the environment by affecting the temperature, the amount and quality of light, and the soil moisture. That shaded gardens are cooler than those with more sunlight is hardly surprising, but what kinds of differences exist? In my garden, I used a pair of recording thermometers and found that my sunny front lawn was, when averaged over a 24-hour period, twelve degrees hotter than my shaded back yard. Using an instrument that measures total radiant energy, I find my shade garden has about one-fifth (an 80 percent light reduction) as much light energy as found in full sun. By comparison, most nurserymen use 35 to 50 percent saran shade cloth when they produce their shade plants.
The summertime temperature reduction occurs because the tree canopy absorbs much of the red and far-red energy – the hot portion of the spectrum. The light that finally filters through the leaves and reaches the ground is altered by the experience. More of the red and far-red rays are absorbed than the rays at the blue end of the spectrum; hence the quality of the light is modified. Not only do shade plants have to grow with less total light energy, they have to be able to use light that has been depleted of part of its essence.
Plant-care tags refer to plants as doing best when given either full sun, part shade, or shade, but how can you delineate between these gradations in your own garden? Southern gardeners have a simple method of making this distinction – they can let their lawn tell them. Bermudagrass is a good indicator plant for full sun locations. Unless Bermudagrass receives at least six hours of direct sunlight, it will not grow. Full shade locations are those where no grass will grow and only denizens of deep shade such as ferns, periwinkle and springtime ephemerals flourish. But fortunately there is a middle ground – those part-shade locations where cool-season grasses such as fescue or perennial ryegrass and warm-season species such as St. Augustine or even zoysia will grow. It is this part-shade location where most Southern gardeners do their thing, and it is in this kind of location where most shade plants flourish.
Part shade can be achieved in a number of ways. The most obvious is the filtered light situation where patterns of shade and sun move across the garden throughout the day as the sun makes its arc across the sky. In areas with larger, widely spaced trees, the patches of sun may last for an hour or two. If the canopy of the trees has closed, then the flicker of sun or shade may last minutes before changing. My favorite kind of shade though is a location exposed to the sun during all or part of the morning and then shaded the rest of the day. Plants benefit from the full power of the sun during the cooler morning hours and then are protected from the full blast later during the day.
I know there are places with moist shade, but I can’t think of a gardener friend who actually has any. Trees use a tremendous amount of water and, even if you irrigate frequently and have the deep pockets to pay exorbitant summertime water bills, it’s difficult to give a large tree and a small herbaceous plant growing beneath it all of the water they both want.
The wildflowers of the woodlands have adapted a strategy to deal with this summertime competition – they go dormant. Many of our favorite woodland wildflowers are spring ephemerals – plants such as bloodroot, trilliums, mayapple, rue anemone and Dutchman’s breeches – that begin growing early before the trees leaf out and complete their life cycle before the crush of summertime competition for water becomes too severe. Surprisingly, one of the moistest locations in the woodland garden is right next to a large tree trunk. Tree branches collect a lot of moisture with every small shower, and as the bark becomes saturated, gravity pulls it towards the base of the plant. Few of the pesky feeder roots persist right adjacent to the tree trunk, making these areas ideal for most of the delicate spring ephemerals.
Shade limits the choice of plants we can grow and the deeper the shade or the drier the garden, the more our choices are limited. About 80 percent of all garden plants do best when given at least six hour of sunlight, leaving the remaining 20 percent for the shade garden. The challenge of developing a shade garden is selecting an array of understory trees, shrubs, ground covers and herbaceous plants that create an interesting, functional and beautiful display throughout the four seasons.
Selecting Trees And Shrubs
As a group, the Japanese maples make some of the best understory trees for the shade garden. Their graceful form blends well with the more massive trunks of the canopy of trees. Most of the upright growing selections will reach 20 to 25 feet tall and effectively fill in the background spaces between the trees, creating lacy walls to break up the expanse.
Favorites of mine are the green-leafed, dissected types such as ‘Seiryu’ or ‘Viridis’, but you can’t go wrong with non-dissectum selections such as ‘Glowing Embers’, ‘Osakazuki’, or ‘Sango Kaku’. Provided the first hard freeze doesn’t occur too early, almost all Japanese maples have good fall color even when grown in fairly heavy shade.
Azalea, boxwood and holly make up the quintessential understory shrubs for the shade garden, and rightly so. These evergreens, along with a few others such as osmanthus, camellias and mahonia, form the wintertime skeleton of the garden by designating spaces and presenting an eye-level perspective for the viewer.
Hydrangeas are among the best for providing this needed contrast. The bigleaf hydrangea (Hydrangea macrophylla) has been a workhorse in gardens for decades, and many of us are familiar with the head-high ‘Nikko Blue’ when it comes forth with its display in early summer. The large, dark green leaves provide a wonderfully bold contrast to most other plants in the garden. But, if you haven’t noticed, there has been a revolution occurring in the hydrangea world of late and the old standbys are now joined by a host of others. The Teller series of hydrangeas – about 20 cultivars named in German after various birds – have introduced reds, pinks, blues and striking bicolors into the lineup of cultivars. Selections with white and yellow variegated foliage are also available to brighten up dark corners.
Hostas make even a mediocre gardener look like they know what they are doing. An assortment of old and new selections scattered about your beds provides bold, colorful contrast to whatever surrounds it. Gardeners often tend to crowd hostas. Most of the larger types should be planted with 30-inch spacing as a minimum. Giants such as ‘Sum and Substance’ may need 4-foot spacing. The smaller selections such as ‘Golden Tiara’ can be used for edging walkways. Some such as the old standby ‘Royal Standard’ have beautiful white flowers in summer that are much more attractive than those of many hostas.
Of the 35 or so cultivars I grow, I have noticed a couple points worth considering. It’s easy for a hosta to look good in June, but the challenge comes in August with the summer heat. Stop by a garden center in September and look over their offering. The sissies will stand out. Plants battered too severely in the summer heat disappear early, depriving the garden of their beauty during the fall – the best time of the year in many shade gardens. Selections with H. plantaginea in their background – such as ‘Fragrant Bouquet’, ‘Royal Standard’ and the ‘Iron Gate’ hybrids – usually hold up better and give a good performance in the fall. ‘Frances Williams’ and its siblings that are selections of H. sieboldiana seem to melt away in our heat.
Also consider heucheras. This group of native woodland plants has lurked in the borderland of popularity for years, but when breeders such as Dan Heims began lavishing attention on the group and selecting for foliage characteristics, their popularity exploded. Many of the parents of the hybrids used in his breeding program are native to the eastern woodland, so not surprisingly, his hybrids have done well throughout the southern U.S. Many may have tried ‘Palace Purple’ and been disappointed with both the effect of purple foliage in the shade and the plant’s lack of heat tolerance. It was a first generation hybrid and has been improved upon greatly. Try ‘Pewter Moon’ or ‘Ruffled Petticoat’ if you like the purple types; ‘Dale’s Strain’ and ‘Garnet’ are selections with foliage marked with green and silver. ‘Summer Snowflake’ is a white variegated selection that I find to be well suited to my garden.
And toad lilies, epimediums, polygonatums, disporums, Lenten and Christmas roses, etc., etc. The list of interesting shade plants could go on for a long time. The fun of gardening though is not reading about the plants, but actually trying some new ones and seeing how they perform in your garden.
From State-by-State Gardening June 2003