It's not a matter of whether you should irrigate trees in autumn or not. It is more complicated than that. We need to pinpoint the right time within the fall season to water. Let's look, in detail, into when to do it -- and when not to do it.
When to Water Trees in Fall
- Stop watering trees, both evergreen and deciduous, throughout early autumn, until the time when the leaves of the deciduous trees fall (this remarkable change on the deciduous trees serves as a useful indicator, whereas their evergreen counterparts, being relatively unchanging, offer little in the way of guidance). This stoppage in watering will allow both evergreen and deciduous trees to enter a transitional phase, not unlike the "hardening off" undergone by nursery plants in spring. What you are trying to avoid here is causing spurts of new growth that will not be winter-hardy. Such non-hardy growth is more likely to be damaged if you have cold weather suddenly sweep into your region.
- In late autumn, after the deciduous trees have dropped their leaves, give both evergreen and deciduous trees a deep watering. This should be done before the ground freezes. If you were to wait until after the ground freezes, the frozen-solid soil would act as a barrier. This barrier would prevent water from seeping down properly to the root zones of the trees in a timely manner.
Examples of Deciduous and Evergreen Trees
For the sake of beginners, I will mention examples of deciduous versus evergreen trees here. If you are at a more advanced level and wish to skip on to information about exactly how much water to give your trees in fall, scroll down to the next section.
The following are examples of deciduous trees:
Evergreen trees fall into two different categories:
- Those that have needle-like leaves, awl-shaped leaves, or flattened sprays of scale-like leaves
- Those with leaves shaped more like those on a "regular" (that is, deciduous) tree.
For purposes of watering trees in fall, these two types of evergreen trees can be treated the same way. In the North, there are far more examples of the first group. Perhaps the representative of the second group that will be most recognizable for the average landscaping enthusiast in North America is American holly. Examples of the first group include:
Measuring the Correct Amount of Water
You now know not only why, but also when to water trees in fall. But, in a sense, that is the easy part. The question of how much to water trees is harder, because it involves a measurement.
Note also that there is more than one way to measure the correct amount of water to supply. Some people prefer to measure in terms of the number of gallons of water required. But I am going to give you a different guideline to go by because it will also serve as a teaching tool for exactly where to water your trees.
Applying Water to Trees Around the "Dripline"
You should be watering your trees around what arborists call the "dripline." What is this dripline? Stand under your tree and look up into its canopy. Move so as to position yourself directly under the outer edges of that canopy. You are now standing on a portion of the circle that makes up the dripline.
Most of the water your tree's roots are going to draw out of the ground will be drawn from this area and from the area just outside it (further away from the tree). In other words, people who water a tree right up near its trunk are betraying a fundamental misunderstanding of how tree roots take up water. The smaller, so-called "feeder" roots are the ones that will draw up most of the water from the soil, and these feeder roots tend to emanate out from the dripline.
Soil Should Be Moist, Not Soggy
So much for the "where?" But how much water should you apply to this dripline area in fall (or any time of year, really)? The all-important feeder roots reside mainly in the uppermost 1 foot of the soil. So your goal in watering a tree is to moisten the top 1 foot of soil in the dripline area. Note that you want the soil to end up moist, not soggy (yes, there is such a thing as over-watering a tree). This depth-of-1-foot guideline is more useful than speaking in terms of gallons because the number of gallons required will depend on factors such as how well your soil retains water.
But you may wonder, "How can I tell if I have managed to moisten the soil down to a depth of 1 foot?" Well, there are products specifically designed to help you make such determinations. For example, you can buy a soil probe on Amazon. This product consists primarily of a metal rod. The idea behind it is that, after watering your tree, you push the rod down into the soil as far as you can.
Wet Soil Is Easier to Penetrate Than Dry Soil
Wet soil is easier to penetrate than dry soil, so the rod should slip pretty easily down through whatever soil has been watered sufficiently. If you can push the rod down 1 foot deep but then meet resistance (signifying dry soil), you have probably achieved your goal of watering the tree to the correct depth. Water that percolates down lower than that will go unused and is therefore wasted.