All garden plants have a degree of daily sunlight exposure under which they will thrive, and when you buy trees, shrubs, potted seedlings, or vegetable or flower seeds, the ideal sunlight requirements are almost always printed on the tag, label, or seed package. 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
Since it is a relatively easy matter to simply choose plants based on the specifications on labels, the real challenge becomes in determining just how much sunlight you really get in your yard. This can be more difficult than you think.
While there are gadgets available that measure sunlight exposure, measuring in this way is not an exact science. Climates where a rainless summer day usually includes clouds that come and go may give identical readings to a region where a rainless summer day means piercing blue, cloudless skies from morning to night. Another way to measure average sunlight exposure is to simply observe, checking your planting location every 30 minutes or so throughout the daylight hours over a week or two and determining an average time that the site spends bathed in sunlight uninterrupted by shade. Once you have determined this, it is an easy enough matter to choose plants that match the conditions of the site, as specified by the plant labels.
For a planting location to be considered "full sun," it does not necessarily need to be in direct sunlight for all the hours of daylight. A garden site is considered full sun as long as it gets at least 6 full hours of direct sunlight on most days.
Full sun is probably the trickiest level of exposure to achieve because while many plants need full sun to set buds and flower, some 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 will get more of their sun in the morning when it is cool. As long as the plants get at least 6 hours of direct sunlight, they should grow well. Before choosing a plant, do a bit of research on the species to determine if there are limitations on the "full sun" requirement. Those that are sensitive to heat will usually come with cautions that it requires some shelter in the afternoon in hot climates.
Of course, there are also many plants that will thrive in more than 6 hours of daily sun. These are well suited to handle dry growing conditions once they become established. Whatever full sun plants you choose, a thick layer of mulch will help conserve moisture in the soil and keep the roots cool.
Plants that prefer full sun is by far the largest group you will encounter. The vast majority of flowering annuals and perennials will enjoy 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 shade" are often used interchangeably to mean 3 to 6 hours of sun exposure each day, preferably in the cooler hours of the morning and early afternoon. However, there is a subtle difference:
- If a plant is listed as the partial sun, greater emphasis is put on its receiving at least the minimal sun requirements. 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 may need to experiment to find the ideal spot in your garden for plants listed as the partial sun. Luckily there are not many of them. If the plants you've tucked into a partially shady 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 building where the area is blocked from the direct afternoon sun. Plants for partial shade include impatiens and most begonias.
This is a somewhat rare term, but you may find it used to define the sunlight requirements of a few plants. Dappled sunlight is similar to partial shade. It is the sun that makes its way through the branches of a deciduous tree. Woodland plants, like trillium and Solomon's Seal, as well as understory trees and shrubs, prefer this type of sunlight over even the limited direct exposure they would get from partial shade. Remember that in early spring, the areas under a tree receive much more sunlight than they do in 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 check the moisture requirements of any plants you are planting under a tree since tree roots tend to soak up a lot of groundwater and smaller plants are likely to need supplemental water to become established.
"Full shade" does not mean no sun, since very few plants, other than mushrooms, can tolerate complete darkness. Plants that list their sunlight requirements as full shade are those that can survive on less than 3 hours of direct sunlight each day, with filtered sunlight during the rest of the day. Hosta, astilbe, and heuchera (coral bells) are all considered shade plants.
Many Plants Are Flexible
The 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 do best. While the plant tags or seed labels may make a plant seem suitable for any location, a bit more research into the species may tell you that the plant really does best under a specific sunlight exposure but "tolerates" other conditions. It is always best to do some online research to learn the full story of any plant you are considering.
In the end, the only real gauge is how well your plant is growing. If the leaves look burned or if the flowers are lanky and leaning in search of sunlight, the plant is probably not in an ideal spot. And don't be afraid to move plants around in your garden if you think they are not placed correctly. Most species can be successfully transplanted if you work carefully.
Defining Sun Requirements for Plants. Kansas State University Extension