By Trish Grenfell, UC Master Gardeners of Placer County
Q: One of the characteristics I try to consider when purchasing a plant is its light requirements but the terms used to describe the necessary light can be confusing. In particular, what is the difference between “part shade” and “part sun” or do they mean the same thing?
A: You are correct to check out the light requirements! Yes, different plants need different amounts of sunlight to grow and maintain health and vigor. But unfortunately, accredited standard definitions of sunny and shady do not exist, but that doesn’t stop the growers and nurseries from using the terms freely. The University of Illinois Extension suggests the following to help you decipher what they mean:
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.
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.
Ellen Brown, an environmental writer and owner of Sustainable Media, has a different take on the “partial” definitions. Partial Sun vs. Partial Shade: these are similar terms in that both do best when receiving 3-6 hours of sun per day. If a plant is listed as partial sun, the emphasis is placed on receiving these 3-6 hours, preferably in the morning and early afternoon but not required in those hours. Plants recommended for partial shade require the protection of a nearby tree or building from the intense mid-day sun.
Whatever definition you use, I would definitely keep the “partial shade” plants out of the hot afternoon sun.