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 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 U.S. and Canada, the sun shines most intensely 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.