Abstract: Interactive visualization of virtual trees and forests is an important challenge in computer graphics and virtual reality. Complex geometric representation of virtual vegetation often overwhelms computer processing units and graphic hardware resources. We present here a new cluster based polygon decimation method, Hierarchical Union of Organs, to progressively and hierarchically simplify complex plant organs (foliage, flower, fruits). Using this method, we propose a novel approach to construct multiresolution organ geometrical models under spatial error control. During rendering, the approach generates minimal graphical primitives. The implementation shows its efficiency for geometrical compression on single trees. It also illustrates its fast rendering capabilities on scenes composed of numerous tree and plant species, including broadleaf and coniferous species.