In order to do their peak job of photosynthesis, each species of plant has a certain ideal level of light that they need. Those that come from the wide open plains and prairies generally prefer to have sunlight shining on them the entire day long. Others that traditionally live in forests or under the canopies of taller plants can complete their task with less sunlight. But, they all need light.

A. Full Sun - Most people think that full sun means that a plant should be exposed to sunlight for 6 to 8 hours per day. Well, in fact, that is just a MINIMUM number of hours. These are plants that would prefer to be bathed in sunlight from sunup in the morning until sundown in the evening. The 6 to 8 hours is just enough to allow them to get by without any serious deficiencies.

Plants that don't get their required minimum amount of light will soon tell you of their displeasure. They will grow artificially tall with unusually long spaces between their branches i.e. internodes. This is called etiolation and, in its extreme, will cause the plant to have weakened stems that bend over under the weight of their foliage or flowers. Also, these plants will not flower fully and may have lighter colored foliage.

B. Shade - If full sun plants need at least 6 to 8 hours of sunlight, then by definition, shade plants can do their thing with less than 6 hours. There is an unfortunate phrase which is often used in the gardening trade. Some plants are called “shade loving” which would imply that they would grow even if you locked them in a dark closet. We know that is not true since all plants require a certain level of light for photosynthesis.

So, a better term for such plants is “shade tolerant.” A few shade tolerant plants such as highly variegated hostas with very thin, tissue-like foliage may be considered "full sun intolerant" due to damage they sustain in high light conditions but they too need a certain amount of light to survive and thrive.

The standard way of talking about shade is as if it were one thing. In reality, there are at least three categories of shade that we encounter in our gardens.

1. Light Shade - In light shade, plants may actually receive levels approaching those of full sun i.e. six hours or so. However, that light is spread out over the entire day where full sun plants need to receive it in contiguous hours. As the sun crosses the horizon, plants in this situation may receive an hour of sunlight here and a half hour there as the day progresses. This condition might occur under the lacy leaves of a honeylocust tree or if you just have a few small trees scattered around your garden.
    

2. Moderate Shade - The second category would be moderate or high shade that you often find beneath very large deciduous trees such as oaks. It is a bright environment but only rarely due direct sunrays land on the plants beneath them. You could probably sit and read a book beneath these trees without any trouble but would only occasionally be hit by direct sun rays during the course of the day.
    

3. Heavy Shade - The last category is dark or heavy shade. This would occur close to the north side of buildings or under such trees as pines (Pinus),  spruces (Picea) or Norway maples (Acer platanoides) which block most of the light. In these cases, the plants below only receive light that is reflected by the surrounding environment.

Needless to say, even those shade tolerant species like hostas, will respond differently depending on the level of shade. Generally, they will thrive in the first two categories but will only be able to maintain themselves in dark shade situations.

For example, a single division of a hosta plant placed in light shade would thrive and add new divisions while growing at the expected rate over time. In medium shade, it would also thrive but may add to its clump size at a slightly slower rate. That same division planted in dense shade would survive and maintain itself but would only add new divisions at an extremely slow rate.

We will be addressing the challenges and joys of shade gardening in a future eBook. The anticipated publishing date for "A Rookie's Guide to Shade Gardening" would be autumn in 2015. Another eBook of interest to shade gardeners would be our, "A Rookie's Guide to Hostas, Hostas, Hostas".