Full sun, partial shade, well-drained . . . people and plant tags use these terms like we know what they mean. Well, we do know what they mean, don’t we? Full sun – that describes a plant that can stand in blistering sun all day long, right? Full shade – that means no sun whatsoever and no blooms. And partial shade, that’s everything in between!
All kidding aside, you can put some definition to these terms and, in doing so, feel more comfortable in choosing the right places for the plants you admire (or acquire) or the right plants for the places you have. Plants are healthier and happier (and bloom more) when you can provide the site conditions they need. Understanding what they need and what you can provide will make it more likely that your plants perform their best. Learn to recognize what plants require certain light and moisture conditions and what plants tolerate them. Azaleas require good drainage. Cinnamon ferns tolerate wet soils.
Light Definitions
Authorities differ in their exact definitions of such terms as “full sun,” “part sun,” “full shade,” etc. However, below are some guidelines that you may find helpful.
The definition of “full sun” is six or more hours of direct sun during the growing season. Direct sun is like the sun you get at the beach; there is nothing overhead to filter the sun’s rays. However, not all direct sun exposure is equal. Here in the southern states, morning sun is a much kinder full sun than afternoon sun. Native azaleas, for example, thrive in full morning sun, producing great amounts of flower buds. Afternoon sun, however, is more stressful, and the plant uses energy trying to cope, resulting in fewer flowers.
“Partial shade” is defined as having four to six hours of sun, some of which may be filtered (filtering is often provided by canopy trees). Plants that thrive in full sun in more northern climes might require partial shade in southern areas.
“Full shade” does not mean no sun at all. It refers to an area with no more than two to four hours of direct sun during the growing season, and these are not during the hot, intense afternoon hours. Full shade conditions can be found under dense tree canopy or on the north or northeast side of buildings, walls, or fences.
Moisture Definitions
Many plants love well-drained soil, but do you have it? Well-drained soil allows water to absorb at least a half inch of water per hour, or water is removed from the soil readily and soils are not wet for significant periods of time. Hopefully, most of your garden meets this definition. Sometimes one has “imperfectly drained” soil such that water is removed from the soil slowly enough to keep it wet for significant periods.
In extreme cases, drainage issues can be addressed by grading the area and/or installing a system of drains. You can improve light to moderate drainage problems by creating raised beds, incorporating organic matter such as composted pine bark, and by mulching. Mulch helps slow water runoff and keeps the soil surface soft and permeable, allowing water to penetrate to the layers below. At the very least, one needs to recognize when the soil is retaining water such that the choice of plants is appropriate to that condition. There are trees, shrubs, perennials, and ferns that can be quite tolerant of those conditions. For those plants that are not tolerant, the condition can cause the plant to die.
Areas that drain too well or do not receive enough moisture require special consideration as well. Some plants are tolerant of being dry for longer periods. Dry shade is a condition that indicates low light and low moisture. Some plants, like Christmas fern, are tolerant of dry shade.
Working With What You Have
First, determine the amount of sun that an area receives. I came across a tip for doing this: choose a simple marker – survey flags, white plastic knives or forks – and observe the area that you are evaluating every hour over the course of the day. For each hour, if the area is in direct sun, place a marker. Continue checking and marking until the sun has moved on. Count up how many markers you placed, and that will be the number of hours of direct sun. Of course, you’ll want to choose a sunny day for this evaluation. Once you know your light conditions, what can you do if you want more or less sun?
Do you want more sun for blooming shrubs, perennials, and annuals? Removing trees, while it sounds drastic, is an option. Another option is to limb up existing trees (remove lower branches) so that when the sun is rising and setting it can reach the area.
Do you want more shade for woodland perennials and ferns? Planting trees and large shrubs is one way to bring more shade to an area. Deciduous trees will allow for the late winter sun that spring ephemerals need, yet still provide shade during the hot summer months. If you want filtered sun, then be sure to choose a tree whose canopy is a bit more open (serviceberry is one choice). Be careful about planting maples, as they are known for creating “dry shade” because they create a lot of surface roots that take in ground moisture and create a difficult planting area. You don’t want to trade one problem for another! Manmade structures can also provide shade: arbors and pergolas provide filtered shade. Training a vine across the structure can provide even more shade. Also, keep in mind that plentiful moisture conditions can sometimes compensate for more hours of sun. Cinnamon fern and royal fern are two plants that can take full sun when they are in a naturally moist, even boggy, area.
Can you plant in partial shade if the tag says full sun? Yes, you can. Think of plant tags as a guideline that describes ideal conditions. You can give a plant less sun, but keep in mind that it may affect the plant’s ability to produce plentiful blooms. Move the plant if after one or two years you don’t have the bloom production that you envisioned.
[NOTE: This article originally appeared in the July 2007 issue of NativeSCAPE, the newsletter of the Georgia Native Plant Society. It is presented here with permission of the author.]
