We have a very mature Linden which turns yellow in the Fall. Sometimes, in the past, parts have even turned yellow prematurely. But this year there is no yellow; it is just dropping it’s leaves which are still green. This is the first time it hasn’t turned in the past 14 years. What’s up?
The tree is in our backyard in downtown Toronto. It is about 30 years old, maybe 40 feet tall, with a spread of about 30 feet (it is considerably wider than our yard). We had an arborist prune it this past February, taking off some boughs that were growing into the adjoining properties and removing some of the lower branches — but they did not remove even 10% of the tree. It seems otherwise quite healthy.
Thank you for contacting Toronto Master Gardeners regarding your linden tree.
The main factors that influence the degree and duration of fall leaf colour are day length, temperature and water supply.
The shorter days and lower temperatures in fall arrest the production of chlorophyll, the pigment that is responsible for the green colour in leaves. Chlorophyll breaks down faster than it is replaced, allowing yellow pigments in your linden’s leaves to become visible.
Drought during the growing season can cause leaves to drop before they change color. The drier than usual May, June (only 50 cm where 71.5 cm is the average) and July this year likely contributed to the linden leaves dropping without changing colour.
A warm period during the fall, such as the very warm September in Toronto this year, can also trigger early leaf drop before the colours have had a chance to develop.
Here are two references for more background on fall leaf colours:
Changing colors of leaves
How weather affects fall colors