Fast-growing with drooping branches lined with feathery foliage, mimosa (Albizia julibrissin) also bears puffy pink flowers in early summer. The branches of this tree are relatively weak-wooded, breaking easily in strong thunderstorm winds. Strategic pruning of narrow-angled branch junctions, as well as cuttting away low-reaching branches that block walkways or street thoroughfares, are the extent of pruning needed annually.
Pruning Saplings for Improved Structure
Look at the branch and trunk structure of a young mimosa tree. Evaluate it, examining the quality of its branches. Determine if there is one, straight central trunk. Look for rubbing branches or if the tree is leaning or lop-sided in habit.
Prune off a co-dominant leader stem, the less vigorous or more crooked upward extension of the trunk. Make the pruning cut with a hand pruners or loppers 1/4 inch above the junction between the two co-dominant leader stems. Take care not to injured or slice into the thin bark on the nearby stem that you wish to retain as the primary leader stem.
Look for any branches that are unusually long or are rubbing against another healthy branch. Also look for any branches that grow inwards across the central structure of the tree. Remove these branches as needed, making the pruning cut 1/4 inch above a lower branch joint or a leaf or dormant bud. Support the limb being removed with your free hand so as the pruning cut is made, the bark does not tear downward, injuring the retained length of branch.
Cut away any small branches on the lower portion of the slender trunk so that it is free of twigs and foliage. Also cut off any suckers at the trunk base, those leggy twigs that sprout upwards creating an untidy thicket.
Pruning of Mature Trees
Examine the structure of the tree each spring, looking for dead, damaged or broken limbs. Make an extra quick evaluation of the tree after a severe thunderstorm or winter ice storm, as the weak-wooded branches are likely to break from these common weather events.
Remove undesirable limbs with a hand pruners if twig diameters are less than 1/2-inch, or a loppers or hand pruning saw if they are larger. Make the pruning cut 1/4 to 1/2-inch above a junction with a living branch junction or nearly flush with the trunk of the tree. Support branches with your free hand to prevent the bark from tearing into desirable branches and trunk as the cut finishes and it falls away.
Clip off low-hanging branches throughout the growing season. Make crisp, quick pruning cuts with the pruners to reduce the length of the branches so that they do not block sidewalks or protrude into the clearance of driveways or roads. Make the pruning cuts 1/2 inch above a connecting branch joint, leaf or dormant bud further back on the branch to obtain the desirable length.
Remove any branches that are growing in a tight angle crotch from the main trunk or primary branches. Since mimosa grows so quickly in the warmth of summer, suckers or sprouting twigs can quickly grow into large branches. Branch connections or crotches that have an angle less than 30 degrees should be pruned so the straight, naturally extending limb remains. Make the cut with a loppers or pruning saw 1/2 inch above the joint, taking care not to lunge the blades into the bark of the desirable nearby branch.
About this Author
James Burghardt has written for The Public Garden, Docent Educator, numerous non-profit newsletters and for Learn2Grow.com's comprehensive plant database. He holds a Master's degree in Public Horticulture from the University of Delaware and studied horticulture and biology in Australia at Murdoch University and the University of Melbourne's Burnley College.