Partial Sun vs. Partial Shade: How Much Sun Do Garden Plants Need?

Planet Earth may only have one sun, but in the world of gardening, there are several: full sun, partial sun, afternoon sun, morning sun, and dappled sun. Then there is shade: full shade, partial shade, and dappled shade. Determining the light requirements for plants is never an exact science; there are too many other factors at play (cloudy days, extreme heat). Still, if you have ever pondered the difference between partial sun and partial shade, here are some generally accepted definitions to help serve as a guide.

• Full Sun: Generally speaking, full sun means a plant requires 6 hours of direct, unfiltered sunlight each day. In warmer climates, this may refer to the hours of sunrise to 11 am and 3 pm to sunset. In cooler climates, this may include noontime sun.

• Afternoon Sun: Generally refers to sun from 3 pm to sunset.

• Morning Sun: Generally refers to sun from sunrise to 11 am.

• Partial Sun vs. Partial Shade: Partial sun and partial shade are similar in that these plants do best when receiving 3 to 6 hours of sun per day. If a plant is listed as partial sun, the emphasis is placed on the plant receiving these minimum sun requirements, preferably in the morning and early afternoon. Plants recommended for partial shade are slightly different. They will take full morning sun just fine, but require the protection of a nearby tree or building from the intense mid-day sun.
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• Dappled Sun/Dappled Shade: Similar to partial sun, the term dappled refers to sunlight that is filtered overhead by the leaves and branches of a tree or other tall planting. Most woodland plantings will prefer this type of sunlight rather than receiving even a limited amount of direct sun each day. With dappled sun, the emphasis is placed on sun, where as with dappled shade, the emphasis is more on shade.

• Full Shade: All plants, even those that do well in the shade, require sunlight. Plants recommended for full shade require less than 3 hours of sunlight each day (either low intensity morning or late afternoon sun), with filtered sunlight throughout most of the afternoon.

Plant Categories Based on Reaction to Light

The amount of sun and its intensity is only part of the equation when growing plants. Physiologically, some plants have preset internal light meters that trigger flowering not according to sun, but according to overall daylight conditions. As a result, botanists have created three categories based on how a plant reacts to light. Some plants are "short day" plants, others are "day neutral", and some are considered "long day" plants.

Short day plants: Plants that must receive less than 12 hours of light each to in order to flower. Examples include Christmas cactus, chrysanthemums, and poinsettias. Most spring crops, such as lettuces, chard, snow peas, broccoli, cauliflower and herbs are not considered "short day" plants, but perform best with 12 to 14 hours of light.

Day neutral plants: These plants are labeled neutral because they will flower regardless of the number of hours of light received each day. (Of course, this does not mean they will flower in total darkness). They perform best if given a relatively long day consisting of 18 hours or more of light but will also flower if less is available. Examples include tomatoes, corn, and cucumbers.

Long day plants: Lettuce, spinach, and potatoes, as well as many summer flowers and garden vegetables fall into this category. These plants perform best when days are long (18 hours) and night cycles are short.

Light Meters

A light meter is an easy way to find out how much light a part of your garden receives. Using a photocell, most light meters measure the available light in foot candles (the amount of light one candle throws from a distance of 1 ft) or give a simple intensity reading using "low", "medium", or "high". Today's light meters are relatively inexpensive (around $20) and come with built in extras like moisture meters and pH testers. Make sure the meter you buy comes with information that includes recommendations for determining how the readings translate into sun exposure.