After a dry early summer and some nice rains in August, a three week dry spell has pulled soil moisture levels back down to levels that are stressful for many plants. A good soaking with sprinklers or irrigation systems will help reduce stress on landscape plants as they begin the process of going dormant, and increase the likelihood of handling the long winter without damage.
All over town trees are turning color and/or dropping leaves early, while others are still all green. The reasons are many. Some varieties of trees such as American Lindens, Boxelder, Ash and Walnut are programmed to shut down their leaves earlier than other kinds of trees, and this early turn gets accelerated even earlier when drought stresses of several consecutive years add up.
Some trees are turning color early because of poor sap flow from the roots to the top of the tree. Roots that wrap around the trunk of the tree just below ground level press more tightly with each passing year as they grow larger in diameter. Sap flow to the crown gets more difficult each growing season, and the tree turns color earlier each year. Eventually certain branches die, and then most of the tree. Correcting or eliminating these step girdling roots is difficult, but not impossible. You may want to consult a certified arborist for help.
Some trees lose leaves early because they are on sandy/gravely soils that dry out very quickly during dry periods. If the dry periods are long, and/or several years in a row as we have had this last three years, the cumulative stress causes some trees to die. Replant with drought resistant varieties such as Oaks, disease resistant Elm, Hackberry, Catalpa, Kentucky Coffeetree and Autumn Blaze Maple.
Other trees loosing leaves early are victims of disease such as Dutch Elm disease, Oak Wilt, and Verticiliium Wilt. Treatments for these diseases tend to be difficult and expensive. If you loose trees to these diseases, choose replacements from disease resistant varieties. Fortunately, there are plenty of choices. Planting one of the many varieties of disease resistant elms now available is growing in popularity, as is the case with other little known new varieties.
Another major cause of decline in urban trees is city infrastructure work that cuts important tree roots. There’s not much to be done about this other than providing adequate water and fertilizer to help the tree recover from its trauma.