Defining Full Sun through Full Shade
One of the first questions we ask when a customer is seeking help selecting plants for their home is, “How much sun does that side of the house get?” Followed by, “Is it direct or filtered (aka: dappled)?” Followed by, “Is it morning or afternoon sun?” The latter here involving generally warmer air temperatures as the day progresses. The goal here of course is to help the customer choose a plant that will thrive with that amount of sun exposure.
Now other factors are involved in helping a plant do well in the space it is placed, but let’s just focus on the sunlight. Plants usually come labeled with their sun exposure requirements. You can also find their preferences online, in printed magazines, catalogs and gardening books. However, the only real gauge is how well your plant is growing in that spot. If the leaves look burned or if the flowers are lanky and leaning toward the sun, the plant is probably not in an ideal spot. Before that happens, use the following definitions, to get a better idea of the generally accepted standards for determining sun exposure in your landscape or garden area.
For an area to be considered full sun, it does not necessarily need to be in direct sunlight for all the hours of daylight. An area is considered the full sun as long as it gets at least 6 full hours of direct sunlight.
Full sun is probably the trickiest level of exposure because while many plants need full sun to set buds and flower, some cannot handle the intense heat and/or dry conditions that often come with that much sunshine. One way around this is to site these sensitive plants where they will get more morning sun, than an afternoon. It's cooler in the morning and as long as the plants get at least 6 hours of direct sunlight, they should grow well.
There are also many plants that will thrive in more than 6 hours of sun, that can handle dry growing conditions, once they become established. Whatever full sun plants you choose, a thick layer of mulch will help conserve moisture in the soil and keep their roots cool. The majority of flowering annuals and perennials will enjoy full sun, provided their moisture requirements are met.
Partial Sun / Partial Shade
These two terms are often used interchangeably to mean 3 - 6 hours of sun exposure each day, preferably in the cooler hours of the morning and early afternoon, however, there is a subtle difference:
If a plant is listed as partial sun, greater emphasis is put on its receiving at least the minimal sun requirements. These plants need several hours of sun to set flowers and fruits but are not as fussy as sun worshippers that need a full day of sun. You may need to experiment to find the ideal spot in your garden for plants listed as partial sun. Luckily there are not many of them. If the plants you've tucked into a partially shady garden are not flowering or growing up to expectations, it is probably because they need more direct sunlight.
If a plant is listed as partial shade, the plant will need some relief from the intense heat of late afternoon sun. You can easily accomplish this either by planting where a nearby tree will cast afternoon shade or by planting on the east side of a building, which will be blocked from direct afternoon sun.
Filtered/Dappled sunlight is similar to partial shade. It is the sun that makes its way through the branches of a deciduous tree. Woodland plants and understory trees and shrubs prefer this type of sunlight over even the limited direct exposure they would get from partial shade. It would still be wise to check the moisture requirements of any plants you are planting under a tree, since tree roots tend to soak up a lot of groundwater and smaller plants will need supplemental water, to become established.
Full shade does not mean any sun. Full shade plants can survive on less than 3 hours of direct sunlight each day, with filtered sunlight during the rest of the day.