Although ashes are sensitive to water shortage, they do not usually require supplemental watering as long as rainfall is regular. Because they partially dehydrate when they go dormant over the winter, they need generous soil moisture in the spring and early summer. Also, when they are first planted, in the late fall just before the ground freezes and during drought periods they need extra water. During prolonged drought, compensate for lack of rainfall by watering the trees at least once a week. If water restrictions permit, run a sprinkler for 30 to 40 minutes so that the water penetrates deeply into the soil. Mulching (see below) reduces water evaporation from soil and keeps it moist longer. This is important for shallow rooted plants like ashes.
Feed ash trees once a year, in the fall. Sprinkle a slow-acting granular all-purpose fertilizer on the soil under each tree out to11/2 times the distance from the trunk to the tips of its branches (its drip line). If the distance is 15 feet, fertilize out to 22 feet from the trunk. Do not let the fertilizer touch the trunk. Use about 1/2 pound of fertilizer for every inch of trunk diameter. Do not fertilize newly planted trees during the first season.
To keep the soil moist as well as to control weeds, spread a 2 to 4 inch layer of some organic material over the root system of the ash tree. Chopped leaves, pine needles, wood chips or a similar mulch, by itself or laid over landscape fabric, will protect the ash tree. Spread the mulch on the ground out about 2 feet all around the trunk.
For more information see file on Using Mulch.
It is not necessary to prune ashes, unless broken branches need removing. These large trees can be trimmed periodically, to control their height somewhat.