Even though redbuds are understory plants, and they benefit from the protection of taller plants in the summer, they need full sun in the early spring to bloom. If you plant a redbud beneath an evergreen tree, it may never bloom or never bloom well. Another reason redbuds may not bloom is excessive nitrogen fertilizer. Redbuds need moisture more than they need compost. As legumes, they can make their own nitrogen fertilizer from the air.