Known for their showy flowers and long blooming season, hydrangeas make great tree filler for some of the same reasons they're great filler plants in gardens and landscape design. As trees and shrubs, hydrangeas are dense and produce large blooms in a variety of colors - many of which work well with a traditional holiday palette (white, light green, pink, and even red). The conical and round shapes of these flowers make them easy to tuck into the gaps that Christmas tree branches have.