Why tree growth slows and stops in August
It seems odd that a tree would stop growing so far ahead of the cold winter months, but there are other factors at play which are not obvious. What is essential for the long-term health of a tree is to be able to gather resources to enable it to survive until the following spring when new growth renews. Water availability is key to this potential.
A tree transpires large amounts of water (100+ litres) on hot days simply to cool its leaves which are exposed to baking sunlight and drying winds. This is where new growth occurs – on the exterior of the canopy. But these new cells are soft and vulnerable to the midday sun, thus it is essential that availability of water during the heat of the day is consistent, even though this is often the season when water tables are low.
As a result, the tree, which is a realist, will cease new canopy growth but continue extending roots which, when shaded by other nearby canopies, are protected by the cool of the soil. This is why it is important to plant trees in cities at much closer spacings (just as they would naturally grow in woodlands) so that they can cool one another’s soil mass.
A healthy tree has many more leaves than it requires (170,000 or more for a mature 60-80 year old tree) to produce the carbohydrates it needs. For example, an apple tree uses 10 (and as many as 50) leaves to produce an apple.
Therefore, by June, the new growth will have achieved the leaf surface area required for carbohydrate production for the season.
Come the days of August, the tree now recognizes the shortening day length and transfers its focus to storing carbohydrates throughout the root system and, to some extent, in the trunk and branches. As a result, new growth ceases, the cells begin ‘hardening’ their structure, and even when water becomes available again and the cool of autumn gives ‘friendlier’ temperatures they still do not begin new growth.
Native trees in particular respond to the length of day as to whether it will speed up or slow down their growth cycles and not be ‘fooled’ by short term seasonal temperatures. This is why we have to choose plants which come from a familiar hardiness zone as they have the genetic wisdom to prepare for the extremes of seasonal change in both summer and winter. 🌳