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How to Reduce Heat Stress in Trees

South Arica has recorded one of the highest temperatures in history these past few weeks. Some parts of the Western Cape regions such as Paarl, Franschhoek and Stellenbosch have surpassed temperatures as high as 40°C. Trees need sunlight to grow, but what happens when your trees are exposed to extremely hot weather conditions?

Most trees grow best in temperature ranges of 15 to 30°C. When temperatures above 32°C are obtained for extensive periods of time, the tree’s growth can be slowed down, and some trees can begin to show signs of stress. Many trees will survive above 40°C but will show different signs of heat stress dependent on the tree species, maturity, and external factors such as drought or dry wind. Studies show that extreme air and soil temperatures slow down chemical activity and growth in trees. 

There are many signs of heat stress when it comes to trees. Various trees are known for rolling their leaves or cupping in response to heat. The leaf surface area is minimized, and stomata (microscopic openings in leaves, like pores, that allow movement of moisture and gasses) closes. This helps reduce the loss of moisture in the tree. Wilting is also a form of heat stress. Wilting usually occurs when low moisture in the tree creates a lack of water pressure. Trees with wilting leaves during the hottest part of the day, and recover in the evening and early morning, are most likely suffering from heat stress. Permanent damage increases with the length of time that a tree remains wilted. Some trees abort or drop buds and flowers after prolonged exposure to high temperatures. This is actually a survival strategy that allows the tree to conserve resources for parts of the tree necessary for survival. Other mentionable tree stress includes dry leaf, ozone damage, bolting and sunscald. 

Here are a few ways to reduce heat stress in trees:

Water 

When your temperatures exceed 40°C, it’s important that you keep your trees hydrated especially when they are young trees since they are more susceptible to withering than mature trees. The early morning before the heat sets in is the best time to water your trees, but it is also recommended that you water your trees as soon as possible if the leaves on your tree begin wilting. Young trees may need up to 30 to 35 litres of water. It is best to check to see if your soil is dry or soggy before you water your trees. It is recommended that you always keep your soil moist. Deep watering is always preferred over shallow watering. Shallowly rooted trees usually dry out more quickly and wilt faster. Tap-rooted trees tend to draw water from deeper into the soil. 

Mulch

Mulch is material applied on top of soil. It is primarily made out of organic material and can come in a variety of forms. Most mulch consists of wood chips, pine straw, tree bark, moss, leaves or grass clippings. Some mulch consists of substances like newspaper, manure, compost, and even rubber. Mulch helps conserve moisture in the soil by reducing evaporation and minimizing temperature fluctuation in the soil. Shallowly rooted trees and young trees are mostly impacted when the top millimetres of soil are dry and hot. Mulch is excellent for ensuring that your trees retain water and remain healthy.  

Shade

Heat stresses are magnified when other unfavourable weather conditions like drought, wind, or intense sun are present. Your young trees will require the most protection from the heat. If your trees are still in containers it is best to move them to a place with plenty of shade. You can also use other devices to provide shade for your trees such as a picnic or beach umbrella, however it’s advised that you add it in the late morning, and you remove it towards the end of the day, to allow your trees get some direct sunlight. 

Humidity

Leaves take in small amounts of moisture through the stomata or openings in the leaves. In small-scale gardens and home greenhouses, providing additional humidity can be helpful for many trees. Typically, applying moisture to leaves intentionally is not recommended in a garden, as leaf moisture alongside the presence of a pathogen and the right temperature increases vulnerability to plant disease. However, this is acceptable in cases of prolonged high temperatures since the benefit of a humid atmosphere outweighs the risk of plant disease. An effective strategy for adding humidity to trees consists of a light overhead watering to the tree leaves or canopy in the morning. You can also group your container trees together and spray them with a mist sprayer. A pressurized mist sprayer is highly effective for spraying a large number of young trees and makes the job easier. 

The tips above will help you reduce the threat of heat stresses during times of extremely hot weather conditions. If you are embarking on a new landscape project and are concerned about whether this is the perfect time to plant new trees, visit www.justtrees.co.za and we will help you source the perfect trees for your project and environment. 

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  1. Jaclyn Becker (Post author)