Shade varies by habitat

To define degrees of sun and shade, and the needs and tolerances of various plants, I look to natural habitats. In nature, full sun is analogous to meadows, prairies, and other open country. Cultivated plants that require a full day of direct summer sun—10 or more hours—are native to these ecosystems. Light shade occurs along edges of woodlands and in savannas where trees provide up to 25 percent canopy closure and plants receive 5 to 10 hours of direct sun. In partial shade, such as in open woods, and small clearings with up to 50 percent canopy closure, plants get less than five hours of direct sun and are shaded for at least half the day. Full shade occurs in forests and woodlands with complete canopy closure. Plants there may take in less than an hour of direct sun a day, though they may glean filtered or dappled light throughout all or part of the day as the sun tracks across the sky. In deep shade, direct sunlight seldom, if ever, reaches the ground. This occurs in coniferous forests, or in gardens where walls or building overhangs block out the sun.

Equally important as the sun-to-shade continuum is solar intensity—the strength of the sun’s rays. This varies with the time of day, the season, and the sun’s distance from the equator. Early-afternoon sun is more intense than morning or late-afternoon sun. Equatorial regions and mountains experience the most solar intensity. In the United States and Canada, the sun shines most intensely in June through September and is stronger in the South than in the North. Thus, a plant grown in light shade in Minnesota may require partial or full shade in Alabama.

Plant response varies with more or less sun

Plants are adapted to shaded environments in various ways. For instance, in most woodlands there is ample, often direct, sun during springtime while the branches of forest trees are bare. Many woodland species have evolved to handle this temporary abundance of light. They produce rapid growth in spring to spread their leaves and flowers while direct sun is available before trees, shrubs, ferns, and larger wildflowers leaf out. As the light level drops, some more-diminutive woodland wildflowers, such as anemone and spring beauty, simply go dormant. On the other hand, many woodland plants, such as trillium, bloodroot, and epimedium, bloom early but keep their leaves well into summer or even fall. These persistent species use different strategies to ensure ample food production.

Shade leaves are often broader and thinner

Broad and flattened leaves on plants such as umbrella leaf, hosta, and skunk cabbage function like huge solar collectors. Many leaves, such as those on ferns, aralias, and black cohosh, have dissected blades that are equally efficient but also more wind-resistant. A large leaf divided into many small segments is less apt to be tattered or flattened in a summer storm. Leaves adapted to full sun are often smaller than shade-adapted leaves. Sun-adapted leaves have many layers of chlorophyll-rich cells, called palisade cells, piled one upon the other. This piggybacking is possible because the strong, direct sun can penetrate deep into the leaf. In this way, a small leaf can be very efficient. In contrast, shade-adapted leaves have a single layer of photosynthetic palisade cells, so the leaves must be proportionately bigger to accommodate the same number of cells and produce the same amount of food.

Leaves that are efficient in the sun are usually unable to function in the shade, and vice versa. For example, a potted ficus tree placed outside for its summer vacation gets lots of light, so it grows efficient, cell-packed leaves. In the autumn, the ficus suffers a setback. Indoor light levels are low, and the high-efficiency leaves can’t function, so the tree sheds them and produces new, larger leaves better adapted to less light.

Hot sun burns shade leaves

Shade leaves, because they have a thin palisade layer, are subject to burning in the hot sun. This became painfully clear recently when a summer storm brought down half of my largest canopy tree. Overnight, the garden switched from full shade to light shade. Although some leaves have burned, I suspect that next year most of the plants will gracefully make the transition. As new leaves emerge, they will be better adapted to the new light conditions. I may have to move a few plants, such as pulmonarias, which seem unable to absorb enough water to keep from wilting in the direct sun. Large-leaved plants are also subject to scorching in hot, dry weather for the same reason. In my bog garden, the huge, thin leaves of umbrella leaf often burn in July in the hot, afternoon sun. The same amount of sun earlier in the day would not be detrimental. Evergreen plants are the best-adapted to full and deep shade. Since their leaves don’t go dormant, they are full-time solar collectors whenever the temperature is over 45°F.

So how can gardeners make the most of shade? The bottom line is that even the most dyed-in-the-wool shade plants will benefit from bright light, or even some direct morning sun, where duration and intensity are moderate. A plant that tolerates deep shade will grow better in full shade. A plant that grows in full shade will generally grow more lush in partial shade, particularly if it receives direct sun in the morning.