Garden Tips: Extreme temperatures take toll on plants too

I was trying to be optimistic and avoided talking about how this extremely hot weather influences our landscape and garden plants. However, continued record high temperatures have compelled me to discuss it.

Perhaps you recall learning about how plants take the sun’s energy via the process of photosynthesis to create carbohydrates. The carbohydrates provide the energy for plant growth. The rate of photosynthesis increases with increasing temperatures up to a point. Once temperatures reach about 95 degrees, the rate of photosynthesis decreases.

At the same time, higher night temperatures increase a plant’s rate of respiration. Respiration is the process that breaks down carbohydrates to provide the plant with energy. As a result of decreased photosynthesis and increased respiration, the plant has to start using its energy reserves. Sugars and other carbohydrates that would ordinarily be used for plant growth and the development of fruit are used to keep the plant alive. Plant growth slows to a stop along with the production of flowers and fruit.

However, there is more to the story. Plants use the process of transpiration to cool themselves, similar to our bodies producing sweat. Transpiration involves the absorption of water by the roots. The water then moves up through a plant into the leaves where it changes to water vapor and exits through pores, called stomates, in the leaves.

If a plant does not have an adequate root system or there is little available water, transpiration stops and the stomates close. When this happens, a plant has no way to cool itself. As a result, damage can occur to plant tissues in the form of sunburn or sunscald (large brown blotches), especially on a plant’s south and west sides. Other symptoms of heat damage include stalled growth, leaf drop and even death.

In addition to the extraordinarily hot weather, many gardeners face a limited supply of irrigation water. Plus, the hot weather arrived so early in the season that some transplanted trees, shrubs, flowers and vegetables may not have had the chance to grow a root system capable of absorbing adequate amounts of water to keep transpiration going.

The problems brought on by high heat are exacerbated when plants are surrounded by materials that absorb heat, such as dark rock mulches or brick walls, or situated next to surfaces that reflect light onto the plant, such as light-colored building walls or white rock mulches.

What does this mean? If hot weather persists, it means we may see sunscald on vegetable plant leaves and fruit, especially if gardeners are not able to keep the soil moist. Woody plant leaves may exhibit sunscald or leaf scorch (browning of leaf edges and tissues between the veins.) It also means plants are stressed. Woody plants will be vulnerable to attack by borers and other insects. Tree and shrub roots may succumb to desiccation leading to dieback of branches from the top of the tree downward.

What to do? If water is available, keep garden soil and container mixes evenly moist. Deep water trees. Apply a mulch of wood chips or bark in landscape beds if you don’t already have mulch in place. Also, mulch veggie gardens with compost or newspaper. Where practical, shade recent transplants during the heat of the day.

This story was originally published July 1, 2015 at 3:08 PM with the headline "Garden Tips: Extreme temperatures take toll on plants too."