Question
when is the best time to cut down an ash or walnut tree so the stump/roots continue to grow

Answer
The early spring is when the root systems have the most stored food and will be able to product the new sprouts and have them grow enough before winter sets in. Ashes, hickories, sycamore,cottonwood, alder, willow, and elm are good stump sprouters at young ages but lose much of this potential as they age. Walnut and hackberry sprout well only as seedlings and saplings. The success of sprouts will depend on the size of the stumps. The smaller the stumps the better.