Joburg and Cape Town’s grand old trees are in the spotlight this Arbor Week (September 1 to 7). Like respected senior citizens, trees planted decades or even centuries ago need continual care to flourish.
With so many trees reaching a ripe old age, a new generation of highly qualified arborists is emerging to offer cutting-edge advice on how to prolong the life of old trees. Arborists specialise in the care of individual trees and are trained in the latest science regarding the needs of trees, as well as being equipped to provide effective care.
“Trees are complex systems,” says arborist John Parker.
“A tree has about 45 000 protein lipid pairs in its DNA strand. Humans have about 23 500,” he adds.
Recommended by the Tree Society of South Africa, Parker qualified in arboriculture in Australia and has worked on trees across the world. He now operates as the Tree Guru and works on projects around the country.
“Trees have been fighting disease and injury for 160 million years, using a strategy called compartmentalisation,” Parker explains. Understanding how trees defend themselves is an important foundation for understanding how to care for old trees.
“Compartmentalisation of Disease in Trees (CODIT) is a system first described by tree scientist Dr Alex Shigo. It refers to a process whereby trees use walls or chemical barriers to enclose a wound site to limit the spread of pathogens and associated rot,” says Parker.
“At a wound site, chemical compounds are deposited and form a defence barrier to stop and limit the progression of the rot, fungus or pathogens into the rest of the healthy tree.
“There are nine positions in the tree where a barrier response, to protect the tree against injury or disease, occurs. Chemical in nature, these barriers are found in various places in a tree such as roots, or at leaf nodes where they can quickly seal wounds left by leaves that fall off a deciduous tree in autumn.
“One of the most important barriers is found at the ‘branch collar’, the place where the branch of a tree grows out of the main stem, or trunk. It is described as a collar as it looks like a shirt collar which is narrow on the branch (neck) and thickens out on the trunk wood (shoulders),” Parker says.
“Another way of understanding what the branch collar looks like is to take a look at your hand. Extend your thumb outwards. The collar looks just like that flared part joining your thumb to your hand. In fact, the crease one sees between the thumb and hand is exactly like the branch bark ridge found on branch collars.”
This branch collar is also known as the Branch Defence Zone (BDZ).
“The chemical barrier gets more concentrated over time, which means that barriers can increase and strengthen as the tree ages,” explains Parker.
Why is it important to know about the BDZ? If you prune a branch to a spot in front of the branch collar, your tree will recover. If you cut into the branch collar, you compromise the survival of the tree.
What advice does Parker offer gardeners for dealing with old trees?
Consider these tips this weekend:
* Never remove structurally sound healthy branches from old trees. Trees store sugars, made in the leaves, in the wood. Removing large branches “steals” years of stored energy from the tree, and reduces the life of the tree.
* Never use tree sealants. Trees have their own chemical barrier mechanisms for sealing off a newly cut branch, as discussed above. It is far better to correctly cut the branch to in front of the branch collar than to use a sealant to mask a huge unnecessary wound.
* When pruning, never remove the branch collar, and always try to cut branches perpendicular across the branch. When we prune trees in a scientific way, we seek minimum wounding of the tree. Never flush-cut branches.
* If a tree drops all its leaves due to stress, such as flooding, avoid pruning the tree for a whole season. It is more energy efficient for the tree to grow new leaves than wood. Only if no new leaves emerge after one season should you remove the dead wood.
* Staking young trees only weakens them. In a research experiment at Berkeley in California, tunnel-grown trees were moderately shaken for 30 seconds each day for 21 days (10.5 minutes in total) to simulate wind action. The trees reduced canopy volume by 25 percent and started to measurably thicken their trunks, thus improving taper, and strength, like on a fishing rod. Wind is the gymnasium in which trees become strong, and this process is known as wind hardening.
* Maintain a 5cm thick mulch layer across the root zone at all times, also called a competition-free zone. Keep mulch off the bark and flare, to prevent bark collar diseases.
* Water stress through either drought or flooding severely stresses a tree. Remember to water all trees at least every two weeks throughout the hot dry summer. Seventy percent of the processes in a tree are water-dependent, just like in humans.
* Never top a tree. Topping is the most destructive tree-harming practice known. Yet, despite more than 50 years of literature and knowledge explaining its harmful effects, topping remains a common but outdated practice.
* Mature trees are a valuable asset. and professional tree care is an investment that can improve the health of your tree and raise the value of your property.
* Pruning or removing trees, especially large trees, is dangerous work. Tree work should be done only by those trained and equipped to work safely in trees.
* Poorly maintained trees can be a significant liability, to your family and pets.
* Always use tree companies who have workman’s compensation insurance when subcontracting tree work. For info contact [email protected]
KAY MONTGOMERY, Weekend Argus and Saturday Star