Understanding Sunlight for Plants
Plants produce energy for growing and sustaining life via a process called photosynthesis where light is absorbed into the leaves via a substance called chlorophyll. As a result, light is one of the most important conditions for any plant and it is important to select the correct amount of light based on the needs of the plant.
Anytime you buy a plant, the tag or description should list a range of lighting conditions that the plant will tolerate. Understanding these terms is the key to selecting the appropriate location for your plant. Lighting conditions are described as a range, so any conditions that fall into that range will be acceptable for the plant. For example, the range full sun to part shade also includes part sun.
Some plants are adaptable to a wide range of conditions, but have a preferred set of conditions. The white dogwood for example is adaptable to full sun, but prefers part sun to dappled light. A white dogwood will struggle with a full day of sun, leaves may take on a wilted appearance and growth will be slower, however with proper care it can survive in 6+ hours of sunlight. When planting, keep in mind that moving a plant out of its preferred light conditions is a trade-off. You will need to provide more care and good quality soil when locating trees out of their preferred conditions.
Finally, not all light is created equal. Morning sun is tends to be superior to afternoon sun because it will dry dew from leaves of the plant, which reduces disease, and it is cooler. Most plants, even those that tolerate full sun, will appreciate even momentary respite from the afternoon sun.
Full Sunlight (6+ hours of sun per earth day)
Full sunlight. It should be obvious, Right? Well maybe not - what's important to realize about full sunlight is that plants on this list can thrive in the sun when it is at its most intense and that these plants need a minimum of 6 hours of sun each day. This the sunlight doesn’t have to be continuous, for example 5 hours in the morning and 2 hours in the evening qualifies as full sun.
Partial Sunlight (less than 4-6 hours sun)
A lot of times plants will be marked for both full sun and partial sun. These plants are typically those that evolved in the boundary areas of forests. They are opportunists who can deal with shade (and even benefit from it), but still need a good helping of sunlight to grow and thrive. These plants grow best with morning sun, but can also grow well with only afternoon sun.
Partial Shade (less than 2-6 hours with afternoon relief)
The key difference between plants in partial shade and partial sun is that plants in partial shade prefer more shade, and more gentle morning light. Plants in this category should be planted so that they receive direct light, but only for a short time and preferably not during the middle of the afternoon.
Dappled Sunlight (AKA light shade)
This category is dominated by sunlight opportunists. These plants typically evolved in the under-story of shaded forests, or forests with a less dense canopy. Dappled sunlight is so called because the leaves of the canopy partially block out the sun, resulting in dancing and shimmering patches of light. If you've walked in the forest with sunlight coming through to the under-story you have experienced dappled light. It is non-direct light and, in me at least, it stimulates feelings of tranquility.
Dogwoods and red-buds are naturally evolved to dappled shade, but can also do well in partial shade and full sun.
Full Shade or Deep Shade
Deep shade/Full Shade means that the area receives no direct sunlight or just some occasional dappled sunlight. Plants that do well in deep shade are those that evolved to grow on the forest floor. Rather than try to out-compete the other trees for size and height they just evolved to photosynthesize in non-direct sunlight.
Many ferns do well in full shade, as do many rhododendrons, mountain laurel, and quite a few members of the viburnum family. These plants are good choices when you have other larger trees that block direct sunlight, but you want to fill in your landscape.