Labor Day and Fall
With the Labor Day holiday behind us, the approach of fall surely can’t be far off. However, just because it’s September, we know the weather isn’t going to suddenly turn cold. In fact, there is almost always one final heat wave during late September and early October. By mid- to late-october though we should see nighttime temperatures dropping into the mid-thirties.
One of the signs of fall I always impatiently watch for is the appearance of yellow sycamore or cottonwood leaves. The trees in the river bottom area spend all summer with their roots surrounded by river water flowing past the trunks and through the sandy soil around the roots. Ninety percent of the feeder roots are in the top twelve to fifteen inches of soil.
Sycamore and cottonwoods are both heavy water users with towering canopies of bright green leaves. Their ability to pull water from the river and release it through the leaves into the dry summer air is significant. Unlike the trees in our yards needing regular watering, these trees are able to use the water flowing in the river channel. These thirsty trees growing along the bank of a flowing river have water-soaked sand and soil surrounding the roots all day every day.
A rough rule of thumb for keeping our lawn trees appropriately irrigated, is to provide ten gallons of water for each inch of tree diameter. Depending on the type of tree, this may need to be applied at least once a week, and for some trees less often.
It is not at all unusual to see towering sycamore and cottonwood trees lining the riverbank with big strong trunks of twelve to fifteen inches and more. Using the same rule of thumb, each tree would require a minimum of 1,200 to 1,500 gallons of water per week. But how often would they require that much moisture?
Even if you conservatively allow each tree approximately 175 to 215 gallons per day, for every tree along several miles of river bank, it amounts to a large amount of water removed from the river flow. When the trees cycle into their fall colors, the yellow leaves do not use nearly as much water as the bright green ones have used all summer. It takes a small slowdown in the demand for water by each tree, to show a noticeable increase in the flow of the river.
The irrigation ditches with rights to withdraw water from the river to distribute to their owners, watch carefully this time of year for the first increase in river flow. As a long hot summer progresses, the flow-rate of the rivers gradually decline, sometimes limiting the amount of water the ditch can withdraw. Only when the heavy water-using trees along the river begin to demand less water, does the river flow stop decreasing.
Do the yellow leaves turn color because the tree allows them less moisture from its sap? Or do the leaves turn yellow from weather changes, thus demanding less moisture from the tree? Probably a little of both, I suspect.
This is a slow process, by hundreds of growing plants, along several miles of river. Since trees often shed leaves very slowly at first, one here or one there, the changes to the river’s water flow is even more gradual. By the time the leaves are falling steadily, the first rain is usually behind us, often with a few nights of freezing temperatures.
Our temperatures have recently dropped below the century mark, though not by very much. There is every possibility they will soon jump back into triple digits for a few days. But the early morning hours have a different feel than they did in July and August.
Have these changes in temperature caused changes to our trees? Without a doubt. It is possible these are not visible changes. Will those changes mean the river is going to run full to the banks? Not hardly. However, give Mother Nature time to work her magic, and the trees will eventually shed their leaves. Then the river flow will increase.
So far, I’ve not seen any yellow leaves in the crowns of the sycamores or cottonwoods. Several trees are beginning to show a dusty green tint, a condition preceding the appearance of the first yellow leaf. In the next days and weeks, as I pass through the river bottom on my journeys to and from home, I’ll be carefully watching the tops of the trees for the first sign of yellow leaves.
If I watch the treetops too closely Sharon is sure to remind me to keep my eyes on the road and let the leaves turn by themselves. Well, maybe she’s right. Oh wait, is that a yellow leaf?