While it's important to irrigate trees in autumn, it might not be clear when to do so and you need to pinpoint the right time to water. Here is a look at when to water trees in the fall and when not to.
Stop Watering In Early Fall
Stop watering trees, both evergreen and deciduous, throughout early autumn until 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.
Water Deeply In Late Fall
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.
Determine How Much Water to Use
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 there is another method, described here, that will also illustrate exactly where to water your trees.
Apply Water to the "Dripline"
You should be watering your trees around what arborists call the dripline. The dripline is the perimeter of the widest part of the canopy. To find the 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 acting on a fundamental misunderstanding of how tree roots take up water. The smaller, "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.
Create Moist Soil
The all-important feeder roots reside mainly in the uppermost one foot of the soil. So your goal in watering a tree is to moisten this top foot of soil in the dripline area. You want to water enough that the soil ends up moist, not soggy so as not to over-water the tree. This 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.
You can use products specifically designed to test your soil for moistness. For example, a soil probe is a metal rod to test how deeply your water has seeped into the soil. After watering your tree, push the rod down into the soil as far as you can.
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 a 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.