You’ve seen the plant tags protruding from the soil in pots of perennials and annuals with a darkened circle or half circle. It’s a quick reference indicating that plant needs shade or half shade, respectively. So, you ask, what constitutes shade?
There are four categories of shade, determined by the amount of time without direct sun plus the density of the shade. They are light shade, partial shade, full shade and dense shade.
Light shade is where trees account for 25 percent of the canopy, and the plants receive 5 to 10 hours of sun. In the home landscape this is generally just under the drip line of trees (the outermost extension of the canopy).
Partial shade occurs when the canopy is 50 percent, the plants receive less than 5 hours of sun and are shaded for at least half a day. Typically this occurs in a yard with trees and where a house shades the garden for part of the day.
Full shade means the garden should receive less than 1 hour of direct sun, although filtered light may be present occasionally. This could be under a dense deciduous tree, close to the trunk.
Finally, deep shade occurs where the sun doesn’t reach the ground, such as in a coniferous forest or in a yard where the sun is blocked out by structures such as walls and overhangs.
Another factor is the strength of the sun’s rays. This differs with time of day and time of year. A couple hours of daybreak sun is not as intense as early afternoon sun. Often plants, usually shrubs, that require partial shade need it in the afternoon, which can be achieved by planting them on the east side of a house or established trees.
Leaf shape and size differ on shade plants compared to full-sun plants. Shade leaves are generally broad and thin like a hosta’s. Ferns, another shade lover, have broad leaves designed to allow for less wind resistance. Broad, thin leaves have only one layer of palisade cells, which are photosynthetic and rich in chlorophyll. In contrast, full-sun leaves tend to be small and thick because they have multiple layers of palisade cells. This allows the stronger sun to go deep into the plant without burning it. The smaller surface area loses less water but still can produce the needed energy that broad-leafed shade plants do.
The thin palisade layer can easily fail, as gardeners discover when they suddenly lose a tree sheltering a shade garden. The now-exposed plants quickly burn in full sun. Over time, some may adapt, but many will have to be moved back into a shady environment.