Evaluate light in your landscape

Richard Bogren, Gill, Daniel J.

For Release On Or After 03/01/13

By Dan Gill
LSU AgCenter Horticulturist

When it comes to gardening, knowing the sunlight conditions in different areas of your landscape is critical to success. If you should plant a shrub you just bought in partial shade, do you know where partial shade exists in your landscape?

To interpret sunlight conditions, it’s helpful to know the compass directions. Knowing the compass directions is not necessary to find your way around your yard. Instead, it has everything to do with providing the appropriate light for plants.

It doesn’t take many years of trial and error to discover that you must learn the light preferences for a plant and then provide that that amount of light as closely as possible. Certainly other factors, such as soil, drainage and climate, are important, but nothing else matters if you don’t get the light right. It’s that critical.

As a gardener, you must understand a fundamental principle of plant life. Plants absorb light to provide the energy they need to power and build their bodies. Plants are solar-powered organisms. They use light energy to manufacture sugar out of carbon dioxide and water. The sugar molecules created by photosynthesis store the energy of the sun in their molecular bonds. This sugar, along with tiny amounts of minerals absorbed from the soil, is used to power the biological processes and build the body of the plant.

So now you get an inkling of the relationship of plants to light. And it’s not surprising why we consider getting the light right is so important to successful gardening.

But here’s the rub. You can read up on a plant and find that it is best grown in part shade. But to make this information work you need to know both what “part shade” means and where in your yard part shade occurs.

First, we need to look at what these terms mean. Then we will see how the compass directions influence where different light conditions occur in the landscape.

Here are some good working definitions of the terms we commonly use to denote light conditions in our landscapes:

– Full sun indicates locations that receive eight hours or more of direct sun daily.

– Part sun areas receive about four to six hours of afternoon sun.

– Areas that are part shade receive about four hours of morning sun.

– Shady areas receive about two hours of direct sun in the morning or bright dappled light through the day.

To put a plant in a location that provides the proper light conditions, you need to know where those various conditions occur in your landscape. And for that, it really helps to know the compass directions.

The path of the sun across the sky follows the compass – it rises in the east and sets in the west. Because we live in the northern hemisphere, the tilt of the earth’s axis puts the sun in the southern sky as it crosses from east to west. The movement of the sun interacting with various structures (such as your home), trees, walls and larger shrubs creates light conditions that may vary through the day. Knowing the compass directions is important in understanding these changes.

An eastern exposure receives several hours of direct sun in the cooler morning hours and provides part shade later in the day.

Because the sun crosses the sky south of directly overhead, the southern exposure gets sun most of the day and is considered full sun.

A western exposure, like the eastern exposure, receives several hours of direct sun. But the sun occurs in the more stressful heat of the afternoon. The western exposure is part sun.

Because of the sun’s position in the southern sky, shadows are cast to the north. The northern exposure, then, is shade.

See how knowing the compass directions would be extremely helpful when evaluating the various light conditions in your landscape?

Unfortunately, you can’t simply go out and evaluate the light conditions in different parts of your landscape just before you head out to the nursery. To evaluate light conditions, you have to pay attention to where the sun shines and where it is shady whenever you look at your yard throughout the day. You will notice the light patterns change dramatically.

To complicate matters, the sun moves differently in the different seasons, and light conditions may change. So evaluating the light conditions in a landscape is an ongoing process of observation over time.

Given enough time, light conditions can even change slowly but significantly as young shade trees grow large and mature. Or they may change suddenly when mature trees are lost to storms or other events.

The point is, properly evaluating the many different light conditions in a landscape is one of the most important things we do to ensure success with what we plant. It requires paying attention to light conditions in your landscape whenever you look at it – noting where it is sunny and where it is shady throughout the day and throughout the year. And knowing the compass directions of your property can help greatly as you do this.

Rick Bogren
2/26/2013 9:33:32 PM
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