South Africa is blessed to be graced by rainfalls throughout the year, however heavy rain can cause severe damage to your trees and threaten the survival of your trees. Heavy rain can cause physical damage to trees, soil changes and even make the trees prone to infestations and diseases. The potential of the damage to the trees is dependent on how and when the heavy rains occur. Your trees may require additional attention and care after heavy rains to minimize the long-term damage.
A weak tree can be further negatively impacted if it is stressed before the rainfall. Stressed trees have symptoms such as early discoloration and leaf loss before Autumn. Other symptoms include reduced leaf size, crown dieback, and water sprouts or small shoots emerging from the main stem. Although some might say it may be difficult to prove the direct correlation between heavy rains and the cause of a tree’s death years later, these symptoms may reduce the tree’s ability to recover and cause it to die over the years.
There are several types of damages that heavy rains can cause when pertaining to trees.
Physical damage
Heavy rains, high winds or suspended particles may cause soil around the base of the tree to be washed away, exposing the roots of the tree. Exposed roots can be vulnerable to being uprooted and damaged by fierce winds, this can cause serious stress to the tree. Debris carried by strong water currents can remove the bark and damage tissues leaving the tree wounded causing wood stain and decay organisms. Heavy rains can also cover foliage especially on the lower branches, this can interfere with the process of gas exchange and photosynthesis which can lead to the deteriorating health and even death of the branches.
Soil Damage
Heavy rains change soil conditions, interrupts the flow of oxygen and carbon dioxide between trees and the environment. Heavy rainfall has a negative impacted on trees. Trees need oxygen to survive and grow, water covering the soil can reduce the supply of oxygen to the tree roots. Well-drained soils for trees allows water to percolate at a rate of 25mm. This can be easily tested by digging a hole 635mm deep and filling it with water, letting it drain completely, and filling the hole once again. If the 635mm soil is drained within 24hours, then you have well-drained soil. This is considered the ideal soil conditions for landscape trees. An indication of poorly drained soil is the water taking longer than 24hours to drain the 635mm deep hole. Soils that take approximates 2 to 3 days to drain are not as bad as soils that take 7 to 10 days to percolate.
Sediment can also damage trees. Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, it is usually transported by the action of water, wind, or by the force of gravity acting on the particles. The presence of sediments over the roots reduces the oxygen supply to the tree’s roots. Even a minimal amount of 75mm of sediments can be harmful. Tolerance to sedimentation will vary with the different tree species, but unfortunately all seedlings are susceptible to root injury from sediment.
The pH of the soil is also affected by heavy rains. Flooding will have an impact of the acidity and alkalinity of soils. It will either decrease the pH of alkaline soils or increase the pH of the acid soils. Different tree species react differently to certain pH levels, and their tolerance varies.
Decomposition is also another form of soil damage caused by heavy rains. The level of decomposition of organic matter in flooded soil tends to be half of what is found in unflooded soil. The major end products of decomposition of organic matter in flooded soil is carbon dioxide, methane, and humic substances. Tree roots can be damaged by the high concentrations of ethanol and hydrogen sulphide that are produced in waterlogged.
Soils that experience heavy rain can be prone to chemical damage. Heavy rains contain chemicals from agricultural fields or urban areas that may be harmful to trees if absorbed by the tree roots.
Pest Damage
You need to ensure that your trees are not under attack by insects such as borers. Wood borers can weaken stems, which may lead to breakage during extremely low temperatures and high winds. It is recommended that you evaluate your trees for any pet infections. An indication of a pest infection is the tree bark having numerous small holes. The holes may have pitch, sap, or sawdust secreting from them. The exit holes are cleaner and may be round, oval, or D-shaped. These pests can cause severe damage top trees. It is important that you prevent any further wounding or additional root damage to the trees. Wounds can cause stress, weaken the trees, and even attract other trees. It is recommended that you remove broken limbs and other dead parts of the tree since these areas tend to attract more insects. Tree vigour can be increased by implementing light fertilization treatments or watering if the soil is excessively dry during 2 or 3 years after the heavy rains.
Unfortunately, insecticide barely assist in controlling stem borers. It is advised that insecticides only be used in high value trees after an evaluation by a professional entomologist, arborist, or forester. Stem borer insecticides can be applied to the tree bark as a defensive measure before the infestation.
Disease Damage
There are several diseases that can cause serious damages to trees, these diseases will weaken trees during heavy rains, and can also lead to the death of trees. Symptoms of Armillaria infection include leaf chlorosis (yellow then brown), defoliation, reduced leaf and shoot growth, dieback, and death. They affect a tree’s lower stem, root collar and roots. Armillaria is one of the most common tree diseases. It is also known as shoestring root rot. Armillaria usually infects and kills trees that are already weakened by other factors; however, it can cause wood decay, malnutrition, and the demise of many tree species. Heavy rains are considered one of the causes of Armillaria root rot.
Some recommendation for reducing damage from Armillaria include reforesting stands with a mixture of species ecologically suited to the areas that are not infected with Armillaria. You should also refrain from wounding your trees and ensure that tree stress is minimized. You can also uproot vulnerable root systems and those infected roots since that’s where Armillaria thrives and spreads the most. It is recommended that you clear at least 10 meters away from any disease infected trees.
How to manage rain damage to trees
Whether your trees will survive the heavy rains or not will be dependent on whether your trees were healthy before the floods. The survival of your trees after heavy rains will depend on how you take care of your trees after the heavy rains. It’s always best to contact your local forester or arborist for professional advice.
Improve the trees health
After the heavy rains you will need to rejuvenate and improve the tree’s health. This will include removing the damaged and dead branches, it is crucial that you prune the trees only when bark surfaces are dry or during the dormant season to minimize infection by opportunistic pathogens. You can also apply a low nitrogen fertilizer by aerating the soil and applying mulch around the base of small trees to eliminate weeds and conserve moisture. Only water your trees if soil conditions become excessively dry. You will need to remove sediment deposited over the roots by the heavy rains. It’s always best to protect your trees from leaf spot diseases, such as anthracnose, and from early-season insect defoliators and sucking insects, such as aphids or scales. Unfortunately, large trees that have been uprooted may need to be removed by a professional arborist. These trees are usually challenging to set upright and may not recover from the root damage.
Replant the damaged trees
If you are able to salvage some of the damage trees you can replant especially the small, easy-to-manage trees. When dealing with fallen or partially uprooted trees it’s best to keep the roots covered and moist until resetting. The trees need to be planted at their original planting depth otherwise they can undergo further shock if the roots are exposed or if there’s excessive soil. You will need to place the soil firmly around the roots to eliminate air pockets and provide them with support. You might also need to stake the tree for a year or two until its roots become re-established. It’s best to water reset your trees frequently to promote root growth. Pruning branches may also be necessary, you will need to cut close to forks or the main stem but minimize the surface area of an exposed cut.
Clean up the tree waste
Our landfills have been overburdened for years so we suggest finding other uses for tree waste to help reduce the burden. Hardwood from the damaged trees can be used for firewood. You can accelerate the decay process by cutting grooves in the stump, covering it with soil or leaves, and occasionally adding nitrogen fertilizer to the top of the stump. This will keep the stump moist.
Use the waste for compost
Materials such as leaves, weeds, and other non-woody garden debris can be used for compost. You can also mix these materials with grass to prevent over-packing that leads to unbearable odours. Wood chips and sawdust usually require extra nitrogen fertilizer or fresh lawn clippings. Compost piles need to be protected from drying winds. You will need to locate it in partial sunlight to help heat the pile. The compost pile can be prepared in layers to facilitate decomposition. A compost pile ideally should be about 1.5 meters high. It’s recommended that you moisten the materials as they are placed into the pile or bin. Water and oxygen are important requirements in the composting process, you need to ensure that the pile is moist, but not soaked. It’s also advisable to avoid over-chopping composting materials or packing the pile in order to encourage oxygen infiltration. Your compost will be ready to use once it has shrunk to about one-half of its original volume, and has a pleasant, earthy smell.
These are just a few tips on how you can prevent and manage trees that undergo rain damage. If you are looking to plant trees, do not hesitate to contact us, visit www.justtrees.co.za and we’ll help you select the best trees for your project.