The most important influence on your garden’s microclimates is the amount of light a spot receives. “Sun” and “shade” sound like absolute terms, with only one meaning, but they’re not. Each has a range of light levels, and these levels change throughout the day.
Horticulturists usually measure light in footcandles, the level of brightness thrown by a candle on an object one foot away. Full sun on a clear day is from 10,000 to 12,000 footcandles. 25 footcandles is enough light to read by. Supermarkets are brightly lit, at about 500 footcandles. As you can see, the range of brightness between dim light and bright light is wide.
Sun and Shade
“Full sun” is usually defined as direct sun for at least 6 hours a day. Most sun plants, such as pine trees and corn, will grow pretty well in this many hours of sunlight, but they will grow faster or bear more fully with more hours.
“Shade” is more difficult to define. The broken sunlight under a birch tree, with its constantly moving pattern of sun and shade, is one type of shade. The deep shade under a redwood tree—especially a redwood tree against a house—is something else entirely.
The word “shade” is best used with some sort of adjective to qualify it: “broken shade,” “partial shade,” “deep shade,” or “afternoon shade” all help to define the type of shade.
Estimating Light Levels
In analyzing shade patterns in your garden, imagine where the shade will be at different times of the day, and at different times of year. In the spring and fall, the sun is lower in the sky all day, and the shadows are longer. A shady spot in April might get full afternoon sun in June. Remember that deciduous trees drop their leaves in winter and let more light to the garden floor.
The light in shady areas is reflected there either from the sky or from light-colored surfaces. A good way to judge the light level in a shady spot is to put your head in that spot and estimate the amount of sky or reflective surfaces you can see. If you can see a large portion of sky, that spot is medium shade or bright shade.