I often am asked about the best time to plant or transplant trees and shrubs. And it depends. Midfall, after trees and shrubs are dormant, can be a good time to plant or move trees and shrubs.
There are several reasons why I recommend fall planting. One is the soil temperature is warmer. This is an advantage because roots of woody plants will grow when the soil temperature is above 40 degrees. This allows fall planted trees and shrubs to become established before the stress of summer heat next year.
Another reason for fall planting is that as plants go dormant, there is less competition for the carbohydrates needed for growth. In the spring, carbohydrate reserves are needed for top growth and root growth. This results in a fight between the roots and the top of the plant for those reserves.
If you are considering moving a tree or shrub that is already established, fall is the best time to do this. The stress caused by the root damage and the shock of moving the plant will be less. The most important thing when moving a plant is to get as many of the roots as possible. It is estimated that when woody plants are dug up in a nursery, as much as 95 percent or more of the root system is lost. It is likely that even more roots are lost when landscape plants are moved.
Those were the pros for fall planting, but there is one significant con: water. Fall planting is great for the regions of the Northwest that can depend on natural fall and winter rains to keep the soil moist. We cannot. Fall transplanted trees and shrubs should be thoroughly watered at planting time, and then the soil should be kept moderately moist to help the roots grow.
If you plant or move a woody plant in the fall, you must find some way to provide it with water through the fall and winter months. The advantages gained by fall planting will disappear if the soil is not kept moist. Check the soil regularly to monitor the moisture situation even when foggy and overcast weather predominates.
I also recommend that you mulch your newly transplanted tree or shrub with a 3- to 4-inch layer of shredded bark or wood chips to help retain soil moisture and discourage weed growth next year.
--Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.