Monday, September 7, 2015

Distressing Colors: Early Leaf Change An Unhealthy Sign

Early leaf colorIf trees held a race to see which would be among the first to have their leaves turn color, the winners would be losers. Premature leaf color change is a reliable indicator of failing health, and the worse a tree’s condition, the sooner it begins to turn.

Precious few places in the world have a fall color show like ours, and the display that northern hardwoods produce each autumn never fails to fill me with awe and appreciation. But when it starts in July, as was the case again this year on some roadside maples, I know those trees aren’t long for this world. In early August even some forest hardwoods growing on thin rocky soils began to show color, which is also unusual.

The summer of 2012 is ancient history to some, but to trees it was practically yesterday. In 2012 soil moisture dropped to record low levels, partly due to scant rainfall, but also because of low humidity, high temperatures, a very high UV index, and frequent and persistent winds. In such conditions, tree roots actually die back, beginning with the fine absorbing roots a few inches below the surface.

Most people, myself included, are surprised when they first hear that ninety percent of tree roots reside in the top ten inches of soil, and that very few roots penetrate beyond eighteen inches deep. Knowing this, it’s easy to understand how tree roots can run out of moisture in a drought.

A sort of Catch-22 situation happens as a result of root dieback. All the starches stored in the affected portions of the root system root are lost, so the tree has less energy the following year. But that’s when it needs extra energy to re-grow its roots. And because its roots are compromised, the tree can’t get adequate water and nutrients.

Dr. George Hudler, recently retired from the Plant Pathology Department at Cornell University, says it takes a healthy tree about three years to recover from a significant drought. But what about trees whose health was less than stellar to begin with? Most, though not all, of the leaf color change I observed in late July was on older trees on roadsides. Those are subject to road salt, root zone restriction, reflected heat from pavement, and in many cases turf grass, which gobbles up all but the heaviest rains. It’s no shock that these are the ones that turn color first, but it was jarring to see it in July.

Understanding the significance of early color can be distressing, but it’s also an opportunity to gauge how the trees around your home and in your community are doing. Trees that have almost completely turned color by late August are probably in irreversible decline, and it’s time to consider their removal and replacement in the next few years.

Color change of twenty percent or less indicates moderate decline but severe stress. Watering the root zone about an inch per week over the next few years may help keep them around longer. Mulching the root zone (twice the branch length) two to four inches deep in lieu of grass will also help.

Quite frequently, poor planting results in a tree that should live for a hundred or more years dying after 20 or 30 years. It seems hard to believe, but the sudden decline and death of a 25-year old hard maple is often due to bad planting practices like failing to remove burlap and wire cages, planting too deeply or making an inadequate planting hole. These things lead to poorly formed root systems that develop girdling roots and other problems years later.

If a branch or section of a tree has turned while the rest is green, probably one or more major root flares have been damaged, either mechanically or by disease (of course one often follows the other).

Let’s do what we can to make our trees come in last in the leaf-change competition so we can enjoy fall’s colors without fretting that they mean trouble.

Photo of early leaf change courtesy Gone Hiking.


Paul Hetzler

Paul Hetzler is the Horticulture and Natural Resources Educator at Cornell Cooperative Extension of St. Lawrence County.

You can reach Paul at the Cornell Cooperative Extension office in Canton at (315) 379-9192.




2 Responses

  1. Dave Gibson says:

    Thanks, as always for a great post, Paul.
    New residential subdivisions are often required to “save mature trees” by ordinance or site plan, but the way the soil around those trees are treated during all phases of construction make unintended violations, and tree stress and death after just a few years inevitable – to say nothing of other conditions. Your post serves as a useful reminder to towns near me, and I will circulate it.

  2. Charlie S says:

    I did not know premature coloring in leaves means the doom of the those trees. That makes sense. Thank you for educating me.