'Limelight' hydrangea (Hydrangea paniculata ‘Limelight’) typically grows as a shrub with multiple trunks. A fast grower, it typically adds more than 24 inches a year until it reaches maturity. Growing 6 to 10 feet tall and spreading from 6 to 10 feet wide, it thrives in U.S. Department of Agriculture plant hardiness zones 4 through 8. To prune it as a tree, you have to train and develop it beginning with a nursery sapling, eliminating all the stems but one trunk-like stem.
Timing Your Pruning
‘Limelight’ blooms on new spring growth. Although you can prune one any time after it finishes blooming in early autumn, the best time is in late winter or early spring before it begins its spring growth. There's no need to put any form of dressing on pruning wounds.
Pruning to a Single Stem
Imagine a ‘Limelight’ tree as being shaped like an umbrella with a straight handle. The handle is the stem or trunk. The canopy, shaped like the top of an umbrella, is composed of about six branches that arch out and hang down. The stem of your tree hydrangea will not be completely bare. It will have short stubs that will yield flowers. It is in the rough shape of a tree, yes, but a tree with flowers on its sides.
As a tree-form ‘Limelight’ should have just one main stem, eliminate multiple or competing stems growing around its base, cutting all but one of them to the ground. Also remove any suckers or shoots growing around the base.
Prune side branches of the single stem until they contain two or three sets of nodes, small bumps that will grow into side shoots. Buds on these short side shoots will yield flowers in the summer. Unpruned side branches will grow long, arching down from the weight of flowers on their tips. When you shorten them their flowers will grow closer to the trunk of your hydrangea tree.
Shaping the Top
Choose the best-looking, healthiest six branches for the umbrella-shaped top. For a well-proportioned, even canopy, select branches spaced evenly around the top.
Prune the other branches in the canopy, including those that grow inward or straight up or down, as close to the main stem as possible.
Selecting and Sterilizing Pruning Tools
What pruning tools to use depends on the size of the ‘Limelight’ you’re trying to shape into a tree.
Hand shears are good for branches up to 1/4 inch wide. Scissor pruners have blades that work like scissors. The blades of anvil pruners cut by pinning branches against a flat surface.
Lopping shears are useful for branches up to 1 1/2 inches wide.
Pruning saws have coarse teeth, good for branches more than 1 inch wide.
Sterilize the cutting blades of pruning tools before and after pruning by soaking them for five minutes in a solution of 1 part household bleach to 3 parts of water, then letting them air dry.