Cherry trees have white flowers when they bloom in the springtime. This is why places like Washington, DC celebrate Cherry Blossom Festivals in the springtime. If allowed to mature, the flowers are fertilized and become cherry fruits in the fall.
The poet is talking about trees being observed at "Eastertide," which means it is in the season of spring. The trees are wearing their white cherry blossoms as part of their yearly life cycle, as a symbol of purity at Easter, and as a mark of the changing seasons - they're wearing white cherry blossoms in the spring, as contrasted with the white snow of winter that they also may wear. This could also be considered symbolic of approaching death, since the poet is thinking about the declining number of years remaining in his life.