Fertilizer salts may burn roots when there is insufficient soil moisture. Fertilizers applied in September may also stimulate top growth that could result in excess leaf area for the tree or shrub to maintain during periods of limited soil moisture. If adequate water is available, certain trees and shrubs that only put on one growth flush per year (pines, spruces and firs) could benefit from a fall fertilization. With limited water supplies, do not fertilize these particular plants.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).