Understanding the sun exposure needs of plants

As we enter the hottest parts of our San Joaquin summer, sun is a timely topic. 

Fall planting season will be here before we know it, and this is an excellent time for both experienced and budding landscapers to make plans for large-scale garden makeovers or the addition of a few new plants here and there. No matter what your future garden goals, it’s important to know about sun exposure and how it should affect your plant selection process.

Most gardeners are familiar with nursery plant tags and gardening reference books that use phrases such as “full sun” or “partial shade”—but what do those descriptions really mean? Let’s shed a little light on this important topic!

Sun versus shade

Over the eons, different plant species have evolved to adapt to the various environmental conditions specific to their location. Sun and light exposure is one important element of this adaptation, and we need to have an understanding of it to ensure that our plants thrive. General categories (which vary from source to source) are:

  • Full sun = 6 or more hours of direct sun. Vegetable plants and fruit trees need 8 to 10 hours of sun for best production. 
  • Light shade = 4 to 6 hours of direct sun. Many sun-loving plants can tolerate this condition, and plants preferring some shade can handle a bit of exposure to not-too-intense sunlight. 
  • Part(ial) shade/sun = 2 to 4 hours of direct sun. Some conditions that qualify: locations near or under trees with a light-filtering canopy; areas that received reflected light off buildings or fences; and areas that get several hours of morning sun but little to no afternoon sun.
  • Full shade= 2 hours or less of direct sun. Such areas include those covered mostly in shadow or that have dappled shade throughout the day.
  • Dense shade= No direct sun and little indirect sun. This includes areas under dense evergreen trees or shrubs, under overhangs, or in narrow, sheltered pathways. Growing conditions in such locations are difficult because they can also be very dry or because there is lots of competition for root space. Attractive mulches, pavers, or garden art are good solutions for such plant-unfriendly spaces.

Keep in mind that even dense shade does not mean a completely dark location, because all plants need some light to grow.

Plant tags will sometimes list more than one of these ratings. What does that mean? The first term listed is the plant’s preferred condition, while the secondary term is a condition the plant can also tolerate. For example, a plant with a tag saying “sun to part shade” would grow best in a sunny location but could still handle some shade. 

These ratings are sometimes qualified even further. One example from the Sunset Western Garden Book is the plant known as beach strawberry (Fragaria chiloensis).The sun exposure notation for this plant says, “afternoon shade in hottest climates,” and that description is very accurate. This ornamental groundcover is growing in one part of my yard to provide small, seedy snacks for chickens, and over the years it has spread by runners from the lightly shaded location where it was planted to an area that receives full afternoon sun in the summer. The adventurous, spreading parts of the plant look severely stressed at this time of year, whereas the parts shielded from afternoon sun are still lush and happy.

Sun-scorched leaves on a beach strawberry plant, Fragaria chiloensis (Photo by Kathy Ikeda)

That’s intense

Light intensity is another important consideration. It’s a relatively simple concept: morning sunlight is gentler and less intense than the hot, harsh afternoon sun. Plants that need some shade will do better if they’re exposed to sun earlier in the day and protected from sun in the latter hours of the day.

Direct sunlight, where the sun’s rays fall directly on a plant’s leaves, is also very different than indirect sunlight, where a plant receives light reflected from other sources. Some plants, particularly houseplants native to shady tropical forest environments—Philodendron, Calathea, Plectranthus, Schefflera, and more—can be quickly and severely damaged by direct sun exposure on their sensitive leaves.

Location, location, location

Climate zones (either those defined by the USDA or Sunset) are an important consideration when evaluating sun exposure. If you purchase a plant from an out-of-area nursery or mail-order source, carefully investigate how it will respond to our local growing conditions. A plant might be rated as suitable for “full sun” by a grower located in the Bay Area or states in the northern U.S., but that rating often won’t translate well to our harsher summer environment.

Microclimates—the often-profound differences in climate conditions that can exist in a landscape—are also a key factor. Large trees, fences, walls, house orientations, and the seasons all affect the sun exposure that each area of a garden receives. A part of the yard that’s shady in the winter might not be in the summer as the angle of the sun changes. Plants in locations with a southern or western exposure or those near heat-reflecting walls or paving are more susceptible to light- or heat-related damage. Plants under a deciduous tree will receive more direct sun in the winter when the tree is bare than they do in the summer when the tree is leafed out. Pay close attention to these factors and make note of them when developing a planting plan.

I’ll use one difficult part of my own backyard to illustrate this concept. Due to the ever-changing angle of the sun throughout the year, the footprint of my house, and the location of overhangs, one small dry garden bed receives lots of full summer sun during the hottest time of year—including intense afternoon heat reflected off an adjacent stucco wall—but it’s in full shade all day during the coldest winter months. I haven’t found a perennial plant that’s happy with such extremes of sun exposure and growing conditions, so this little patch of soil is where I’ve resigned myself to “crop rotation,” growing heat-and-sun-loving herbs in the summer and shade-loving annuals in the winter.

As always, gardening is a matter of experimentation and trial-and-error! If a plant is scorching/bleaching or failing to thrive because of improper sun or shade exposure (or any other site-related condition), try transplanting it to another location that might be more suitable, and watch to see how it responds to its new home.

Some helpful resources that include sun exposure guidance/considerations are:

For gardening-related questions, call the UC Master Gardener office at 209-953-6112, or visit our website.

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