Willows are easy to grow plants that love water but do not need a lot of fertilizer. Most trees and shrubs do fine without additional plant food once established. I would add Osmocote around the base of the willows in the spring once you see signs of new growth. Follow the recommendation on the package for the correct dosage but when it comes to slow-release plant foods like Osmocote, you won't' harm the plant with a bit too much or too little.   Then observe the growth of the trees and consider adding fertilizer in the spring the following year if the plants are not performing well. Keep in mind for more growth willows respond to more water rather than more fertilizer. Keep growing, Marianne Binetti