Record the growth of the tree. If new shoots have grown 6 inches or more in the last year, fertilization is probably not needed. If verticillium wilt is present, fertilize the tree with nitrogen fertilizer. Verticillium wilt manifests through sunken cankers on limbs.
Take a soil test in the area where the redbud is growing. An analysis of the soil will explain which elements are lacking to feed the redbud tree, and you can adjust the type of fertilizer required based on the results of the soil test.
Apply the proper fertilizer, based on the results of the soil test, to the area around the tree in the late winter, before the flowers appear on the redbud. Flowers on this specimen appear before the leaves emerge.
Broadcast, or scatter, fertilizer pellets under the canopy of the tree to the drip line and beyond if roots are visible.
Water the area thoroughly after scattering the fertilizer. If rain is forecast around this time, apply the fertilizer before the showers occur and let the fertilizer get watered in naturally.