The early change doesn’t signal an early fall. But if it’s just one tree, or part of a tree, it might be a sign of stress from drought or damage.
Was that it? Is summer over already?
With low temperatures dipping into the 50s, it’s felt more like September than August recently. Now come sightings around the metro area of trees with their leaves starting to change color, raising concerns that our ultra-late spring is going to be sandwiched with an ultra-early fall.
Don’t start looking for your snow shovel. Yes, the trees are trying to send a message, but not that message.
“Any time you see trees changing color early it means that they’re stressed,” said Gary Johnson, a professor of urban forestry at the University of Minnesota. “It means that the tree has started to shut down.”
Not all the bright leaves are early. It’s normal for some foliage to have started changing color, said Jim Gilbert, a Twin Cities phenologist.
“Butternut trees, which are native to Minnesota, will start turning as early as July 4th,” he said. “Sumac, which is actually a shrub, should start turning any day now, if it isn’t already. And there’s a vine, the Virginia creeper, that’s also changing now — and is right on time.”
But with other species, the leaves still should be dark green, said Kent Honl, master arborist at Rainbow Treecare.
The temperatures haven’t been cool enough to cause a change in color, he said. “That’s more a function of the length of daylight, anyway. And even though the days are getting shorter, they haven’t gotten that short.”
There are all sorts of things that can put trees under stress, the experts said: disease, insects, improper planting, damage from the storms that have moved through this summer and even residual effects from the drought last summer.
Because of the shortage of rainfall last year, some trees were not able to store up the water they needed before winter, Johnson said. In addition, “droughty soil gets colder and freezes deeper, which can damage roots. The roots near the surface are the least hardy.”
Even though we’ve had plenty of rain this summer, some trees still aren’t getting enough water, he said. “Trees on small spaces like boulevards or in parking lots don’t have enough ground around them to absorb the rain,” he said. “Trees on hills can be affected because the water runs down the hill” before it can be absorbed.
That’s assuming that the roots are functioning, said Peter Moe, operations director at the Minnesota Landscape Arboretum. Many trees that turn color early suffer from a condition known as girdling roots. Typically associated with the replanting of container-grown trees, it occurs when the roots grow around the main stem of the tree and cut off the flow of water.
Experts also suggest checking the tree for insects, damage or disease. One disease that often results in early leaf changes is chlorosis, which Honl compared to anemia in people.
“It’s the same sort of thing where the body — in this case, the tree — can’t absorb the nutrients it needs,” he said.
How can you tell if the changing leaves are the result of stress? Look at where the changes are happening, said Ralph Sievert, forestry director for the Minneapolis Park and Recreation Board. If lots of trees in the area are changing color, it’s normal. But if it’s happening in just one tree — or just one part of a tree — it’s not.
“Stress is site-specific,” he said. “If you see it in just one place, that’s stress, and stress on a tree isn’t good.”
Even healthy trees need to be watched for signs of stress, Johnson said.
“It’s absolutely critical to keep watering trees, especially small trees, and we’ve had a lot of new trees planted all over the metro this summer,” he said. “And keep watering them until the first hard freeze. When the hose finally is so stiff that you can’t use it, then you can quit.”
Now that we know that the trees aren’t forecasting an early fall, what does meteorologist Paul Douglas expect?
“We’re sort of careening from one extreme to the other,” he said. “But we seem to be moving into a period of longer, warmer autumns. Temperatures should warm up again by the last week of August, and, if I had to wager a guess, I’d expect a warmer-than-usual September.”
Keep those T-shirts and shorts handy. Summer isn’t over yet.
Jeff Strickler • 612-673-7392