Don Kinzler, Published November 30 2012
Hortiscope: Trees that keep leaves into fall may be in troubleReaders: One of my colleagues, Joe Zeleznik, NDSU Extension Service forester, had what I thought was an interesting question posed to him. His response is excellent and I think worthy of a read by anyone who ever has wondered the same thing about trees. Read on!
Q: Why do some of my trees still have leaves, even though it’s early November?
A: Although the question is simple, the answer is more complicated. Quite simply, it depends.
The simplest answer is that some tree species hold onto their leaves longer into the fall or winter than other species. Our native ironwood trees are a classic example. The leaves change to a yellow/orange in the fall, and then to a tan hue later on. They stay on the tree in that color for a long time.
While I was deer hunting in northern Minnesota forests recently, I saw many ironwoods in the understory (lowest height below the forest canopy). Red oak, which also grows in Minnesota but not in North Dakota, also tends to hold its leaves well into winter.
The fruit of the American linden (basswood) stays on the tree late in the fall and has a bract that looks like a leaf.
European buckthorn (common buckthorn), an exotic invasive species, holds its leaves much longer than other species. These trees often remain green late into the fall.
The second possibility relates to the origin of the seeds. Quite simply: Is this tree from an area south of here? To some extent, trees that originate further south tend to hold onto leaves a little longer in the fall. Another way to look at it: They don’t begin the process of becoming dormant as early as the native trees. This is a bit dangerous because an early fall frost can kill twigs or even the whole tree if it hasn’t hardened off sufficiently.
Planting ornamentals in a hardiness zone that they’re not adapted to can result in problems such as this.
Similarly, we need to review the management of the tree during the growing season. If the tree receives excess nitrogen too late in the growing season, it tends to focus its energy on tender, new growth. In the fall, it won’t be able to harden up as quickly as it normally would, so it remains green and tender and holds onto its leaves late into the fall season. For this reason, we recommend avoiding fertilizer applications during July, August and the first half of September.
For watering, we recommend cutting back during August. This “mini” drought stress should kick-start the dormancy processes and result in a tree that is fully hardy once winter arrives. Obviously, withholding too much water can cause problems as well, so a balance is needed.
Lastly, we need to look at the health of the tree or branch that’s holding onto its leaves. If a branch dies suddenly during the growing season, it will retain its leaves and will not drop them in the fall. Certain diseases can result in trees holding onto their leaves late in the fall. In many cases, this is an indication of a broader health problem.
In summary, there are several possible reasons that trees might hold onto their leaves. Usually, it’s not a good sign. It may indicate damage that already has been done or it might be an indicator of future damage.
However, in some cases, it might be nothing at all.
To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or email firstname.lastname@example.org.
For answers to general horticultural questions, go to www.ag.ndsu.edu/horticulture.