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Why do my trees still have leaves in November?

A simple question with a not-so-simple answer. In brief, it depends ...

Why do some of my trees still have leaves, even though it’s November?  This is a question I receive every nearly every year.  Although the question is simple, the answer is a little more complicated.  Quite simply, it depends …

The simplest answer is that some tree species simply hold onto their leaves longer into the fall or winter than other species do.  Our native ironwood tree is a classic example. The leaves first change to a yellow/orange color in the fall, and then to a tan hue later on. 

ironwood leaves

Ironwood leaves, November 13, 2012.

And they stay on the tree in that color for a long time.  I often see ironwood trees in the forest understory in both North Dakota and Minnesota.  Red oak, which also grows in Minnesota but not in North Dakota, also tends to hold its leaves well into winter. The fruit of American linden (a.k.a. basswood) stays on the tree late in the fall, and it has a bract that looks like a leaf. 

basswood bract

Leaf-like bract from an American linden fruit, November 13, 2012.

European buckthorn (a.k.a. common buckthorn), an exotic invasive species, holds its leaves much longer than other species and they often remain green late into the fall.

buckthorn leaves

European buckthorn leaves remain green late into the fall, November 13, 2012.

The second possibility for trees holding onto their leaves relates to the origin of those trees.  Quite simply, is this tree from an area south of here?  To some extent, trees that originate further south tend to hold onto their leaves a little longer in the fall.  Another way to look at this is that they don’t begin the process for becoming dormant (or winter hardy) as early as the native trees do.  This is a bit dangerous as 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 like 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, holding onto its leaves late in the fall.  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, resulting 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 branch/tree that’s still 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 also 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 has already been done, or it might be an indicator of damage yet-to-come.  But in some cases, it might be nothing at all!

Joe Zeleznik

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