Hi John:

Fall is a great time for planting trees, so your dogwood should be off to a good start. Once trees are established, they shouldn’t need supplemental watering except during unusually hot and dry periods. Depending on the size of your tree, it is probably a good idea to water it for 1-2 years. This article from the U. of Florida discusses how long it takes for a trees to establish, and it recommends a year or more per inch of trunk diameter for our hardiness zone. This article from the U. of Minnesota (lower hardiness zone) says 1.5 years for a 1-inch trunk and 3 years for a 2-inch trunk. You can estimate based on the size of your tree.

With regard to how to water, the key is deep, infrequently waterings, which promotes growth of deep roots rather than shallow ones. This article from the U. of Nebraska says that means watering to a depth of 12-18” (for a tree) and waiting until the upper 6” is dry before watering again. Note that the type of soil will affect how quickly the soil will dry out, with clay soil holding moisture longer than well-draining soil. You do not want to over-water a tree, as that can kill a tree as effectively as under-watering. You can assess how wet or dry the soil is by using a screwdriver or a similar tool and seeing how easily it penetrates and wether it feels cool and moist when removed. Of course, you can also use a soil moisture probe if you happen to have access to one.

The best and most efficient way to water is by using a method that slowly releases water over a long period of time. That allows it to be absorbed by the soil and the tree roots, and very little is lost to evaporation. The U. of Minnesota recommends a Treegator bag, which slowly releases water over a long period of time. This can be very effective for small trees, but as a tree grows, you should spread the watering out further from the trunk (typically as broad as the crown). You can make your own slow watering device by drilling a small hole in the base of a 5-gallon bucket, which can be moved around the base of the tree. Soaker hoses are also effective.

Hopefully you have already mulched your tree, but if not, you should certainly do that. That will help retain the water you apply, and it reduces competition with weeds and turf. Make sure you have a thick, even (i.e., flat) layer of mulch that extends out 2-3 feet from the base of the tree. It should not touch the trunk itself; it should not be a mulch volcano. The photos in the U. of Minnesota and U. of Nebraska pages show trees that are mulched correctly.