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How to Cope with Changing Light Levels in the Garden

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A key mantra of garden design is “right plant, right place.” This holds true to a point. How many of us begin gardening in full sun only to several years later find ourselves enjoying a shady afternoon while our sun-loving plants are straining for light? Landscapes are flexible, evolving canvasses. Transitioning from a vibrant, full-sun garden to the more mellow tones of shade requires a challenging change in perspective.

Clethra alnifolia, or summersweet, is a summer-blooming shrub that takes part shade.

Clethra alnifolia, or summersweet, is a summer-blooming shrub that takes part shade.

Sunlight shifts throughout the year, as well as over longer periods of time. A spring garden reveling in full sun tells an entirely different story when trees leaf out. There may also be a great deal of variation in sunlight throughout the garden. Look closely and you’ll notice intermediate zones of part shade and confusing areas of part sun. There are areas that receive filtered light, and those that sit in cool darkness even at midday. How can we begin to understand our gardens’ changing needs? Studying the degree of sunlight reaching different areas of the garden at different times of day and year is a start.

Taking Notes on Light
On a clear spring day, beginning early in the morning, step outside and jot down the time and the amount of sun reaching the planting areas. You may wish to create a chart with the time in one column and the relative amount of sunlight in another. If there are trees or other structures casting shade over about half the area, indicate “partial sun.” If the area is completely shady, indicate “full shade.” If the area is entirely lit, with no obstruction, it is in “full sun.” Repeat this process every hour until sunset.

If it’s difficult to determine the amount of sun just by glancing at the area, a sketch may help. Draw a rough outline of the area in question, including any trees and existing structures. When you step outside, shade in areas that aren’t receiving as much light. Record the time in the shaded area. Think of it as quality time with your garden. Better yet, recruit friends and family to help. At the end of the day, you’ll have a pretty good idea of how much sun you can count on.

To get a sense of the changing needs of your landscape, take measure of the sun at the beginning of each new season. By recording changes over time, you’ll likely get a sense of transitional zones. Identifying plants that grow well in these areas will tie the landscape together. Local nurseries are often good sources of information on light requirements and growing conditions, and they tend to carry plants that grow well in a particular climate zone. For tricky areas, I’ve found that indigenous plants tend to be most successful. They are well adapted to the growing conditions of a particular region, grow easily in native soil and seem to tolerate a greater range of light and moisture conditions.

If you live in a region with distinct seasons, as I do in northern Virginia, it’s possible to achieve a garden with year-round interest. Succession planting in areas that change from winter sun to summer shade ties the seasons together. Evergreens fill in gaps in the dead of winter, but by late February to early March, ample sun and modest warmth invite tender greenery to emerge. The happiest winter-to-early-spring combination includes spring ephemerals, which bloom ever so briefly before trees leaf out.

Favorites for Shifting Light
Chionodoxa, better known as glory-of-the-snow, pokes through late winter frost, reassuring us that spring is just around the bend. The tiny blooms and discrete, grassy foliage naturalize slowly beneath deciduous trees. Grow them alongside crocus and daffodil bulbs for continuous blooms straight through spring.

First to appear in early spring is the aptly named spring beauty (Claytonia virginica), followed by the somewhat fussy but equally lovely trout lilies (Erythronium) and trilliums. Bloodroot (Sanguinaria canadensis) is native to a wide swath of eastern North America, from the Great Lakes up to Canada and down to Florida. Though the blooms last only a day or two, the foliage remains until summer heat takes over, forming a subtle ground cover. Bleeding hearts (Dicentra eximia), Virginia bluebells (Mertensia virginica) and wood poppy (Stylophorum diphyllum) steal the show in early spring. They naturalize easily in moist woodlands and attract native pollinators. The foliage dies back early, paving the way for shade-loving ferns and astilbe beneath the tree canopy.

Working with Trees and Shrubs
Finding plants that grow in dry shade beneath trees is a challenge. Wood aster (Eurybia divaricata) blooms in late summer and early fall and is an important late-season nectar source for native pollinators. Despite common belief, some ferns handle dry shade quite well. Christmas fern (Polystichum acrostichoides) is a great, evergreen choice. Bishops hat (Epimedium) also handles dry shade, preferring dappled light beneath trees or between garden structures.

Repetition is key in tying garden zones together to create a cohesive palate. Filling gaps where larger flowering shrubs once thrived is challenging, but a little shade doesn’t mean we need to forgo blooms altogether. Bluestar (Amsonia hubrichtii) is a particularly appealing grass-like shrub that grows happily in part shade to sun. Lacy foliage precedes stunning blue spikes in summer then erupts in autumn gold. Intersperse bluestar with lower-growing part-shade plants, such as lady’s mantle (Alchemilla mollis), or tie it into a back drop for sunnier zones featuring black-eyed Susan (Rudbeckia) and coneflower (Echinacea).

Many flowering shrubs, such as summersweet (Clethra alnifolia), oakleaf hydrangea (Hydrangea quercifolia) and viburnum grow in part shade and appreciate the respite from midday heat. These midsize shrubs fit neatly between taller trees and provide multiseason interest. Smaller native trees, including redbud, serviceberry and dogwood, are great for spring color, then give way to wildlife-friendly fruit. They’re tidy enough to repeat throughout the understory, happily embracing early summer sun, as well as late season shade.

Tie it all together with a stone path weaving through the garden, a strategically placed bench or a trellis. Garden structures help create natural transitions between shade and sun. Taking note of the shifting light, throughout the seasons and over time, can open our eyes to a host of gardening possibilities.

Image credit: The Plantography Project/CC BY 2.0