Sun or Shade: How Sunlight Affects Plants
Some plants are not fussy about whether they are planted in sun or shade, but most have a preference. When a plant doesn’t thrive, the problem may be either too much or too little light. In this article, you’ll learn how sunlight affects plant growth, and how to match the quality of sunlight in your yard with the right types of plants.
How Intense is the Sun?
Plants experience sunlight in much the same way we do. Imagine sitting on a park bench with no trees nearby. If it was 9am on a sunny day in May and the park was in Boston, it would be a nice place to sit for an hour. But if the bench was in Houston, and it was a sunny afternoon in August, you’d be running for the shade within a couple minutes.
The point is, it’s important to think about more than just how many hours of sun a plant gets. The quality – or intensity — of that sunlight is just as critical. Latitude, season, and time of day all affect light intensity.
Time of Day
In the morning and late afternoon, the sun’s rays strike the earth at a relatively low angle. At these times the light may be bright, but it’s not very intense. The sun is at its strongest when it’s high in the sky, from 11am to 3pm. As a general rule, sun-sensitive plants like begonias, prefer morning or late afternoon light and need protection from harsh midday sun.
Sunlight gets more intense the closer you are to the equator. This is why shade-loving plants like hostas, will tolerate more sun in the North than they will in the South. Planting and care labels rarely differentiate between growing regions, so keep in mind how your geographical location will affect the plants in your garden.
The angle of the sun also changes with the time of year. This affects the intensity of the light as well as how shadows are cast. The arc of the sun is at its highest (most directly overhead) and at its most intense around the summer solstice in late June. Later in the summer, the arc is lower in the sky. This both decreases the intensity of the light and lengthens shadows. Observe your garden through the seasons so you get to know how the light moves around. This will help you match the right plants with the right location.
Standard Definitions for Sun, Part Sun and Shade
Full Sun – an area that gets unobstructed sunlight from dawn to dusk; as much as 15 hours of sun per day.
Sun – an area that gets 6 or more hours of direct sunlight over the course of the day.
Partial Sun or Partial Shade – an area that gets 3 to 6 hours of sunlight each day.
Shade – an area that receives less than 3 hours of direct sunlight per day.
Full Shade – an area that gets no direct sunlight, but receives enough bright, indirect light to support healthy growth.
Dappled Shade – an area beneath deciduous trees. Foliage filters incoming light, creating shifting patterns of sun and shade.
How Do You Know When the Light Isn’t Right?
All plants require sunlight to produce energy for growth and flower production. But some plants need more of it than others. Here is how to recognize when a plant is not receiving the right amount of light.
Too Much Sun
Fair-skinned people are easily sunburned. The same is true for sun-sensitive plants. Too many hours of sun, or light that’s too intense, can bleach out the color of a plant’s flowers and foliage. Parts of the leaves may be scorched and the edges may brown and curl. Sun sensitive plants may also wilt because their foliage is trying to conserve moisture.
Not Enough Sun
When plants are not getting enough sunlight, they may be shorter than normal and new growth is often weak or spindly. The foliage might be pale and limp rather than robust. Flowering can decrease or completely stop. Sun-starved plants are also more susceptible to disease problems such as mildew.
What to Do If the Light Isn’t Right
Gardens are never static. Over time, trees and shrubs get bigger and cast more shade. Others die and let in more light. A plant that was once well matched to the light conditions in your garden, may now need to be moved.
If you suspect a plant is not getting the right amount of light, there are several things you can do. The most obvious one is to relocate the plant. If you are new to gardening, you may be reluctant to do this. But most plants are incredibly forgiving about being moved. Just avoid doing it in midsummer and/or when a plant is flowering.
You could add an arbor or free-standing trellis to provide some additional shade. Or position a taller plant nearby to help block the sun. To bring in more sun, you could prune, relocate or tie back nearby plants that are casting too much shade.
Understanding how the qualities of sun or shade affect your plants will help you have a healthier, better looking garden. Learn more here: