UMBEL: A part of the plant bearing flowers in which all the flower stalks are of similar length and arise from the same point.
UNDERPLANTING: Growing short plants such as a ground cover under taller plants.
UNDERSTORY: The smaller trees that grow below the major forest.
UNDER GLASS: A term in older books referring to "growing under glass", basically growing in a greenhouse.
Understock: The part of a plant to which a graft is attached.
UNISEXUAL: A flower of one sex only.
USA Zone: U.S. Department of Agriculture classifications according to annual minimum temperatures and/or lengths of growing seasons.