Goodbye summer, hello fall: What the changing seasons mean for watering your trees

Picture of Paul Cappiello Ph.D.

Paul Cappiello Ph.D.

Yew Dell Botanical Gardens Executive Director

Given enough time, Mother Nature always seems to even the score.

An unusually cold winter isn’t always immediately followed by a correspondingly warm summer, but over the long haul, things do tend to balance out. I guess that’s where the whole concept of average originates. Not that we ever actually experience an “average” growing season and I’m not sure we would know what to do with it if it ever did happen.

But this year, after a relatively mild summer with reasonably reliable (might we even call it average?) rainfall and temperatures, a few weeks ago the other foot finally came home to roost. Consistent 90s and nary a drop of rain. Seems lately even my succulents are looking a little cranky!

And a quick look around the yard and across the neighborhood this week might even have you finding a few signs of early fall color. The dogwood trees across the street from my house are always good indicators. As I look out the dining room window, they’re showing a few signs of gold and red that, with any meteorological luck, will shower the area with brilliant color in no time at all.

This early tip that fall is on the doorstep might just start you thinking that you can throw in the irrigation towel ― that the tree’s leaves are doing just that, and you might just as well save on the water bill and avoid one more shot in the face by the uncooperative (and, probably by this point of the season, malfunctioning) sprinkler, and give up on any more watering.

But you’d be wrong. The late summer/early fall season is absolutely key to getting your trees off to a good start next spring. Here’s why.

What is the growth cycle of trees?

Most trees from the temperate parts of the world experience two growth stages that sit at opposite ends of the growing season.

Spring growth is focused on two things ― roots and shoots. Each spring brings the production of tons (sometimes, literally!) of fine, little roots that will do most of the water and nutrient absorption during the rest of the growing season. Most people don’t realize that the majority of our deciduous trees slough off most of those fine roots during fall and winter, only to regrow them the following spring.

The other part of spring tree growth is focused on shoot or stem growth. This is the part of the story that has trees trying to outcompete each other for sunlight and space. Sure, you read lots of bits out there about trees communicating and singing songs to each other and such. But when it comes right down to it, it’s still a tree-eat-tree world out there. If you don’t get your branches out there before the other guy, you’re not gonna be around very long to sing to your neighbor.

Fall tree growth, on the other hand, is all about structure and storage. All the nutrients that are harvested from the senescing leaves and dissolving fine roots are packed up and sent to the trunk, large branches and major roots to build wood and store reserves for next spring. Without a healthy fall, there’s not enough in the tank for next spring. And irrigation this time of year is key.

Why autumn watering is key for tree roots

Now as I’ve preached before, once we get to this point of the summer/fall season, it’s awfully hard to get water deep enough in the parched, clay soil, to have much of an impact on big old, mature trees. Not that it isn’t worth the effort. A good, long, slow drip irrigation can’t hurt. And a little moisture in the ground, even if it’s at the surface, is at least something the rains will add to.

This article was originally published in the Courier Journal on September 15, 2023.

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