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Understanding How Much Sun a Plant Needs

Different plants require varying light conditions.
June 2, 2017

Most gardeners know they should choose plants that match the conditions in their yards, including the amount of sunlight that is available. However, many people are confused by the terms used to describe light conditions on plant labels and in garden books. What do terms such as “full sun” or “partial shade” mean?

There are standard definitions, says Doris Taylor, Plant Clinic manager at The Morton Arboretum. Unfortunately, these terms aren’t always used consistently, which can be confusing for homeowners. And there’s more to judging your light conditions than just how many hours a day the sun’s rays strike your yard.

“It also matters how intense the light is and which direction it comes from,” Taylor says. Sunlight that comes from the east or south is generally less harsh than afternoon light from the west, Taylor says. Not much light comes from the north. Summer sunlight is more intense than winter light.

A further complication: The sunlight in your yard changes all through the growing season, as the days grow longer or shorter and the angle of the sun shifts. Trees or buildings may cast a longer shadow in your garden at some times of the year.

“You have to really watch and learn where your sun comes from,” Taylor says.

Once you know, you can check plant labels to choose appropriate plants. Here are Taylor’s explanations of the light terms you’ll see:

Full sun: This means the plant is suited to a site that gets a minimum of six hours of direct, unobstructed sunlight every day. “Some plants need even more,” Taylor says. Most vegetables, such as tomatoes, peppers, and cabbage, require at least eight hours of sun a day. “The sunlight doesn’t have to be continuous,” she says. A full-sun site might get four hours of sun in the morning, be shaded by a tree at noon, and be exposed to sun again for four more hours in the afternoon and evening.

Part sun and part shade: “These terms are the most confusing,” Taylor says. They often are used interchangeably, but they don’t mean quite the same thing. Generally, part sun means four to six hours of direct sun every day. Sometimes part shade means a slightly shorter period of sunlight, but it also usually means the plant needs a site that is sheltered from the intense afternoon sun. Many shrubs, such as rhododendrons, hydrangeas, and summersweet, as well as perennials such as astilbe, anemone, and phlox, do best with morning sun and afternoon shade.

Shade: A site that gets four hours or less of direct sunlight a day is in shade. There are different kinds of shade.

Dappled or filtered shade is a mix of sunlight and shade. It may depend on what kind of tree you have. For example, some light can filter through the open branching structure and small leaves of a honey locust; that’s dappled shade. A tree with a dense canopy of large leaves, such as a Norway maple, will block the sun almost entirely.

Dry shade occurs in places where rainfall, as well as sunlight, is blocked from reaching the ground. You will often find dry shade under a tree that has a dense canopy, like that Norway maple, or under a roof overhang next to the house.

On a plant label or in a book, information about the light conditions plants need often is presented through artwork, called icons. Although these icons tend to follow certain conventions, they aren’t officially standardized. If you’re unsure what they mean for a particular plant, look for words, or ask a garden center employee to interpret the label.

Most often, an open circle or an outline of a sun (sometimes filled with yellow) means “full sun.” A circle filled with black means “shade.” An icon with a circle that is half blacked out can mean part sun or part shade, depending on the context.

Icons may be combined to suggest the plant can tolerate a range of conditions. For example, if a plant is labeled with both a sun icon and a part sun icon, it means the plant will do well in either full sun or part sun.