Fall may have the slight edge over the spring for fertilizing. But no matter when you fertilize, how can you know if you should fertilize at all?
If your mature trees appear healthy and have never been fertilized, don't start now.
If it ain't broke, don't fix it!
And don't fertilize simply because you think it's the right thing to do.
Instead, look for signs of malnutrition.
Is the foliage light green and the annual shoot growth poor?
Is there an excessive amount of dead wood on the tree? Is the foliage sparse? If so, the plant may benefit from some fertilizer application.
Fertilize the trees in the fall after the leaves drop or in early spring before the buds begin to swell but after the ground has thawed.