“Preferring U.S. Department of Agriculture plant hardiness zones 6 through 10, mimosas need to be large enough to cultivate flowers and subsequent seed pods -- younger trees do not have the energy reserves for reproductive activity. In general, a mimosa will not bloom until it is approximately 10 feet tall. Each tree grows to this height at different rates, based on soil nutrients and moisture availability. Additionally, the mimosa must be old enough to have extensive branches for blossom development,” (Homeguides.SFGate.com).
Since height and extensive branches are important to the flowering of the tree, it would be a good idea to have this in mind when trimming the tree so as not to stunt the development of the tree itself, or the blossoms.