In general, afternoon shade is necessary for shade plants. They bleach or burn in the full afternoon sun. However, they can often tolerate morning sun or winter sun, when the sun is not overhead and the air is cool. All plants grow faster, and bloom and yield more fully, with more light if they can tolerate it. Many flowering shade plants flower best when their leaves are slightly bleached by just a little too much sun exposure.
Too Much Sun
Shade plants can have two reactions to too much sun: their leaves can bleach, or they can scorch. Bleaching is caused by excess light, but not excess heat. It occurs in cool places or at cool times of the year. The leaves that are receiving too much light become pale green, yellow, or light gray. This usually happens slowly, over a period of many days, and it happens to the whole side of the plant that is receiving too much light.
Scorching is caused by excess heat, rather than excess light. The parts of leaves that are nearly perpendicular to the sun’s rays during the hottest part of the day turn yellow, then brown, and die. The tissue death is caused by overheating in the sun. Scorching happens in one afternoon, rather than slowly over a period of days. Leaves that have already bleached are more susceptible to scorching than healthy leaves, but the effects are separate symptoms.
Because leaves keep cool by transpiring water, scorching can be caused by drought as well as heat. Plants that might otherwise tolerate a certain level of heat will scorch if they can’t get enough water from the soil to keep themselves cool.
Flowers, as well as leaves, can bleach and scorch. Some flowers, especially white ones, scorch more easily than the leaves.
Too Much Shade
Plants that are not receiving enough light are not getting the energy they need to grow. Plants make energy from light through photosynthesis; too little light starves them just as too little food starves a person. They go through three stages of “starvation,” depending on how light-deficient they are:
1. Fruit and flower production lessens or stops. The leaves may be attractive and the plant appear healthy at this point, but it doesn’t have enough energy for reproduction.
2. The plant becomes more open. The leaves are larger and farther apart than a plant getting enough light. Some plants, such as Japanese maples, are attractive this way.
3. Growth slows and the plant leans toward the light. New growth is produced more slowly, and leaves are smaller. The plant etiolates, or stretches toward the light. Plants in this condition are weakened and more susceptible to stress or diseases than plants getting enough light.
Plants that are not being grown for their flowers or fruit might be perfectly satisfactory at the first or second level. The plant is still healthy and able to care for itself in these conditions. At level three, the plant is weakened.
Time of Day and Shade
Shade varies with time of day and time of year. Not only does the sun move in the sky, causing shadows to move, but plants can tolerate more sun when the air is cool than when it’s hot, so there is a difference between morning and afternoon sun. When locating a shade plant, try to imagine at what time of day it will receive full sun, especially in mid-summer when days are long, the sun is high in the sky, and the weather is hot. All plants “like” as much light as they can tolerate. They grow better with more light, as long as the light level doesn’t bleach or scorch them. A location tucked under the south side of a tree, where they receive full sun in the morning, but are protected from mid-day and afternoon sun, is ideal for many shade plants—especially flowering shade plants like camellias and gardenias.
If a plant isn’t doing well where it is, don’t hesitate to move it. Wait until it’s dormant or growth has slowed, then dig it up and move it to a better location. Some gardeners move plants over and over, until they find just the right spot.