September 8, 2018
I live in a fog belt and my garden is shaded, so it never occurred to me that one of my plants might suffer from sunburn. And yet, I picked a strawberry the other day that clearly had one side facing the sun way too long. Stupid me: I thought if I lifted the unripe berry from beneath its canopy it might ripen sooner. Instead, I got a red strawberry with a brown blotch.
Sunburn is damage to bark, fruit, foliage and other plant parts caused by excessive exposure to solar radiation that heats and dries the tissue. When plants receive more radiation than they need for photosynthesis, their temperature increases and damage to cells and tissues occurs that can lead to cell/tissue death. In other words, sunburn. It often occurs on the south and west sides of trees, the upper side of horizontal branches not adequately shaded, and unshaded fruits and vegetables.
On fruit, sunburn produces a pale yellowish area on the exposed side that eventually may turn black, brown or red. The sunburned area might wither or become leathery, and the cells in that area might die – become necrotic.
Sunburned leaves may appear abnormally shiny, silvery or reddish brown, may progress to necrosis starting at the tips and margins, and have chlorotic (bleached out) blotches between the veins. The leaves can become brown or “burned.”
On trees, sunburned twigs become cracked, discolored, purplish or rough on their exposed side. Sunburned bark cracks and peels, and if the injury is recent, may ooze sap. Sunburned conifer needles turn black or brown or drop prematurely.
Sunburn typically occurs when the environment around a plant or part of a plant changes from shade to sun. This might occur, for example, when a greenhouse plant is brought outdoors, when plants grown together in nurseries move to a solo positions outside, with new plantings that don’t yet have a well-developed root system, or when leaf canopy is reduced whether from normal defoliation, pruning, disease, or insect damage. Transpiration (water movement through a plant and evaporation from leaves, stems, and flowers) helps cool plants. Thus, drought conditions also contribute to sunburn.
Young trees with thin bark are ripe candidates for sunburn as are trees unable to take up enough water because of unhealthy roots or inappropriate irrigation. Sunburn that injures bark makes trees mores susceptible to wood-boring pests and wood-rot fungi. When the cambium underneath the sunburned bark discolors and dies, cankers can girdle and kill limbs.
Although a healthy tree might seal a damaged area over the years, and you may be able to restore color to sunburned leaves with adequate irrigation, shade, and improved soil conditions (if you notice the problem early enough), sunburned fruit and vegetables range from unattractive to inedible and they are not going to get better. Moreover, damaged cells are an entry point for fungus and bacteria.
You can prevent sunburn with proper planting, watering and pruning practices. Let the leaves growing along the trunk of a newly planted tree shade the bark for the first year, or consider painting the trunk with white interior water-based latex paint diluted 50 percent with water. Choose the right plant for the right place. Don’t try to grow shade-loving plants in full sun. Group plants with similar water needs. Make use of shade provided by your garden’s existing canopy, fences and other structures, or consider hanging shade cloth of some kind. Prune in the fall and avoid pruning more than 20 percent of a plant canopy during any one year. And, remember that even sun-loving plants can become sunburned if they grow in dry soil, so water deeply, regularly, and appropriately. Apply mulch to conserve soil moisture and reduce soil temperature during hot days.
As for my strawberries, I removed the black plastic between the plants and am now letting the berries ripen on their own beneath their natural canopy, no matter how long it takes.