All plants require a certain number of hours of daily exposure to sunlight in order to survive and thrive. When you purchase trees, shrubs, flowering annuals and perennials, vegetable plants, houseplants, or packets of seeds, their ideal sunlight requirements are almost always printed on the tag, label, or seed packet.
The terms used to describe sunlight requirements quickly become familiar to anyone who works with plants:
- Full sun
- Full sun to partial shade
- Partial shade (or part shade)
- Dappled sun/shade
- Full shade
Determining Sunlight Exposure in Your Yard
Because it is a relatively easy matter to choose plants based on the specifications shown on labels, the real challenge becomes in determining exactly how much sunlight your yard receives. This can be more difficult than you think. No matter how good a gardener you are, the tendency is to grossly overestimate how much and what kind of
sun an area receives. In North America, the best months to assess sunlight amounts are May through July when deciduous trees have leafed out and the angle of the sun is high in the sky.
While there are gadgets available that measure sunlight exposure, using gadgets is not an exact science. In climates where a rainless summer day usually includes clouds that come and go might result in identical readings to a region where a rainless summer day means piercing blue, cloudless skies from morning to night.
The best way to measure average sunlight exposure is to simply observe your planting area every 30 minutes or so throughout the daylight hours over a week or two. Use those observations to determine the average amount of time the area spends bathed in sunlight, dappled sunlight, or shade. When you have determined the average amount of sunlight an area receives, it is easy enough to choose plants that match the conditions of the site, as noted on the plant labels.
For a planting area to be considered a full sun location, on most days the area must receive six to eight hours of direct sunlight mostly between the hours of 10 a.m. and 4 p.m.
Full sun is probably the trickiest level of exposure to achieve because while many plants need full sun to set buds and flower, some plants cannot handle the intense heat and/or dry conditions that often come with that much sunshine.
One way around this is to site these sensitive plants where they receive most of their sunlight in the morning or very late afternoon when temperatures might be cooler. If the plants receive at least six to eight hours of direct sunlight, they should grow well.
Before choosing a plant, do some research on the species to determine if there are limitations on its full sun requirement. Those that are sensitive to heat will usually come with a caution that it requires some shelter from direct sunlight in mid-afternoon in hot climates.
Of course, there are also many plants that will thrive in more than six to eight hours of daily sun. These are well suited to handle dry growing conditions once they become established. Regardless of the type of full sun plants you choose, a two- to three-inch layer of mulch will help conserve soil moisture and keep the roots cool.
Plants that prefer full sun are by far the largest group of plants you will encounter. The vast majority of flowering annuals and perennials need full sun provided their moisture requirements are met. Vegetable gardens also are generally best positioned in the very sunniest location you can find, although a few vegetables and herbs (mostly the leafy types) will tolerate some shade.
Partial Shade or Partial Sun?
The terms partial (or part) sun and partial (or part) shade are often used interchangeably to mean four to six hours of sun exposure each day, preferably in the cooler hours of the morning. However, there is a subtle difference:
- If a plant is listed as requiring partial sun, greater emphasis is put on its receiving at least the minimal sun requirements of four to six hours. These plants need several hours of sun to set flowers and fruits but are not as fussy as the sun worshippers that need a full day of sun. You might need to experiment to find the ideal spot in your garden for plants listed as the partial sun. If the plants you've tucked into a part sun garden aren't flowering or growing up to expectations, it is probably because they need more direct sunlight.
- If a plant is listed as partial shade, the plant will need some relief from the intense heat of late afternoon sun. You can easily accomplish this either by planting where a nearby tree will cast afternoon shade or by planting on the east side of a structure where the area is blocked from the direct afternoon sun. Plants for partial shade include impatiens, crossandra, the yesterday-today-and-tomorrow plant, and most begonias.
This is a somewhat rare term, but you might find it used to define the sunlight requirements of a few plants. Dappled sunlight is similar to partial shade where the sunlight filters through the branches and foliage of deciduous trees. Woodland plants, like trillium and solomon's seal as well as understory trees and shrubs, prefer dappled sunlight. Remember that in early spring, the areas under a tree receive much more sunlight than they do in late spring and early summer after the tree canopies have leafed out. This is one reason why spring sun-loving bulbs can be successfully planted beneath trees.
It is wise to monitor the moisture requirements of any plants you are planting beneath a tree because tree roots absorb groundwater and smaller plants are likely to need supplemental water to become established.
Full shade does not equate to no sun because very few plants, other than mushrooms, can tolerate a complete lack of sunlight. Plants that require full shade are those that can survive with four hours of full sunlight mostly in the morning or late afternoon or a full day of dappled sunlight. Hosta, astilbe, and heuchera (coral bells) are all considered shade plants.
Many Plants Are Flexible
Sunlight requirements for many plants will include terms like "Full Sun to Partial Shade" or "Partial Shade to Full Shade." This indicates that the plant will do fairly well in a range of sunlight exposures, which gives you more flexibility in determining where you can plant it.
However, be aware that many such plants still have a preferred sunlight requirement under which they grow best. While the plant tags or seed packets might imply that a plant is suitable for any location, doing a bit more research into the species might reveal that the plant really does best under a specific sunlight exposure but tolerates other conditions. It is always best to do your research to learn specific nuances and the preferred growing environment of any plant you are considering planting.
Ultimately, the only real gauge is how well your plants are growing. If the foliage is scorched or burned or if the flowers are lanky and leaning in search of sunlight, the plant is probably not in an ideal spot. Don't be afraid to dig and move plants in your garden if you think they are not planted in the right location. Most species can be successfully transplanted. If possible, do so on a cloudy day and remember to water well in its new location until it is established.
Defining Sun Requirements for Plants. Kansas State University Extension