Travis' Tip: these terms define the categories of sunlight that we use on our site, so that you can use this information to select trees that will grow well in the different shade densities of your landscape.
Shade and sun play an important role in selecting the plants you want in your home landscape. As you consider locations for planting, it is important to do some organized thinking about how a few key things will affect the amount of shade/sun you have to work with. It is obvious that your home, patio, or mature trees will create varying degrees of sun and shade, but it is also important to realize that non-mature trees and shrubs will grow, and thus change your shade/sun ratio over time.
In addition, the cardinal direction an obstruction faces is also important. For example, in the northern hemisphere of earth a north-facing slope, wall, or tree yields less sunlight than a east, west, or south-facing slope or obstruction. This effect gets stronger as you move north because the earth is naturally tilted, and the sun hits at a less direct angle.
In order to make it easier to match a plant to your level of sunlight, gardeners use a few key categories. Not all of these categories are easily quantified or made mutually exclusive from one another, but I'll do my best to explain. Its important to remember that all species of plants evolved in a habitat or niche and that some can deal with variation to that niche better than others. Also these conditions vary by zone. Remember that due to the tilt of the earth, as the zone number increases so does the year-round intensity of the sun. As a result, light requirements are just guidelines and not always hard fast rules.
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 thrive in sunlight when the sun is at its most intense. This means plants listed for full sunlight can tolerate the afternoon summer sun. There might be some shade in the morning or evening, but in full sunlight, the plant gets a full taste of the afternoon sun for at least 6 hours.
Travis' Tip: If a plant is listed as full sun and partial sun then it does well in either, but will require more regular watering in full sunlight (e.g. dogwood or red bud).
Partial Sunlight (less than 6 hours with some relief)
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 well with morning sun or afternoon sun. They can often withstand full sun, but when exposed they require a little extra TLC. Typically this involves extra watering and close attention to soil drainage thanks to the extra H20.
Maples, oaks, willows, magnolias, and most plants in the hydrangea family do well in partial sunlight. Remember the farther north you travel the more sunlight a plant tolerates, or even needs.
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.
Boxwood, service berry, Japanese maple, and the more delicate members of the hydrangea family do well in partial shade.
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.
Dappled Sunlight (AKA light shade)
Once again 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. Its non-direct 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.