University of Illinois Extension
Sunlight - Site Assessment - Stepping Stones to Perennial Garden Design - University of Illinois Extension

Sunlight

The amount of direct sunlight your garden receives daily directly influences the types of plants that will grow successfully. Plants need sunlight to produce food through photosynthesis. Different plants need different amounts of sunlight to produce a sufficient amount of food to grow and maintain health and vigor.

What time of day does your garden site receive sunlight? Observe the sun pattern. You may be surprised by what you discover.

The terms sunny and shady unfortunately are open to interpretation. Again, many people want a recipe to follow and clear definitions of sunny and shady are, well, rather shady. Clear definitions do not exist. After researching numerous garden catalogues and books, and watching the Home & Garden Network (Hey! research is research!) the following definitions have evolved:

Full Sun: Most sources agree that full sun is six or more hours of direct sunlight per day. This doesn't have to be continuous; you could have four hours in the morning, shade mid day, and four hours in the afternoon. As long as it is direct full, sun.

Partial Sun or Partial Shade: A fine line differentiates the two. Partial sun means the amount of sun isn't full sun, yet not partial shade. Most references put this between 4-6 hours of sun a day. Partial shade means the amount of sun is less than partial sun, but more than shade, so we will define it as 2-4 hours of sun per day.

Shade: Shade by definition is lack of sunlight, but in gardening terms this means less than two hours of sunlight a day.