Use shade to produce the best fruits and vegetables

Sunlight is a limiting factor when growing plants. Sunlight helps a plant convert carbon dioxide and water into sugars that the plant uses to make its roots, stems, leaves, flowers and fruit. Without sunlight, plants die. However, like most things, too much sunlight can also cause problems in the garden.

Our hot, dry summers help us produce a variety of fruits and vegetables, but with the climate comes a variety of challenges. Sun-exposed fruits and vegetables can be sunburned. Sunburn injury is common where there is high solar radiation levels and air temperatures, low relative humidity and high elevations. Ultraviolet radiation is greater at higher elevations and is the greatest contributor to damage.

Sunburn injury can be reduced by natural leaf coverage or artificial shade. In a Utah study, as much as 52 percent of unshaded bell peppers were sunburned, while no fruit was damaged when grown under horizontal shade cloth.

There are three types of sunburn. Photo-oxidative sunburn occurs when a plant is suddenly exposed to direct sunlight. The fruit is damaged because they are not acclimated to high light levels. This typically occurs between 90°F to 100°F. The tissue of the fruit is bleached white and covers any sun-exposed fruit.

Sun browning damages the fruit pigments resulting in brownish discoloration. Browning occurs at 100°F to 115°F.

The last type of sunburn is sunburn necrosis. This happens when the cells are superheated to 110°F to 125°F. This causes the tissue to die and turn white or brown in color. Regardless of the type of sunburn, the fruit becomes unmarketable. The damaged area also becomes an entry point for fungus and bacteria, causing the fruit to rot.

Sunburn Management

There are several simple things you can do to prevent sunburn damage to your fruits and vegetables.

WALL OF WATER

Many people use walls of water to prevent frost damage early in the season with garden vegetables. They do an effective job of keeping up the temperature inside the structure, but sometimes do too good of a job.

The wall of water can become a miniature greenhouse with no ventilation to move air. Temperatures can get as high as 140°F on a warm sunny day. As soon as the threat of freezing temperatures passes, remove the wall of water so the air next to the plant can move.

PLANT CARE

Select cultivars that produce more foliage or that have a fruit color that reflects more sunlight. Use good agricultural practices to grow healthy plants. Healthy plants produce more leaf cover that will naturally shade the fruit. Supply adequate water and fertilizer. and monitor plants closely for insects and diseases that can cause leaf curling.

In the case of fruit, proper training, pruning and fruit thinning will reduce the risk of sunburn. Finally, harvesting fruit increases the risk of sunburn. Be careful to avoid removing any leaves, vines or branches during harvesting.

SHADE CLOTH

Shade cloth is a common method to avoid sunburn damage to fruit. Shade cloth can reduce the temperature by as much as 20°F. Shade cloth can provide 10 percent to 90 percent shade, but the best shade for preventing sunburn with no decrease in production is 20 percent to 40 percent.

A structure is needed to support the shade cloth above the plants. Shade cloth can be oriented horizontally above the plants, or if you are trying to shade a row crop-oriented north-south, a vertical shade cloth can be suspended directly over the crop so that it provides shade in the heat of the day.

Shade cloth does carry an additional cost of $0.10 to $0.30 per square foot, depending on the percent shade, fabric size and manufacturer source. Framing, suspension wire, clips for securing the fabric and poles to anchor the fabric add to the costs.