Hydrangea Tree Pruning and Care
A panicle hydrangea on a single trunk: when to cut the head, what to rub off the trunk, and how to get it through winter.
Hydrangea tree pruning and care is panicle hydrangea care with one extra rule. The hydrangea trees sold in Canadian garden centres are usually panicle hydrangeas (Hydrangea paniculata) trained to a single trunk, and they flower on this year’s growth, just like the shrub form.
The habit is natural to the species. N.C. State describes panicle hydrangea as a large shrub or small tree, and the University of Massachusetts notes the old-fashioned Pee Gee hydrangea, 'Grandiflora', often grows with one main trunk in a tree form. A nursery simply makes that choice for you, and your job is to keep it.
Hydrangea tree care at a glance: Prune the head in late winter or early spring, cutting each stem back to a fat bud to keep a rounded shape. Rub off shoots on the trunk and remove suckers at the base. Stake it for the first few years. Guard the trunk from rabbits in winter.
When to Prune a Hydrangea Tree
The University of Minnesota’s timing for panicle hydrangeas applies: fall, late winter or spring, with late winter or spring preferred so the dried heads stand through the snow. N.C. State adds that panicle hydrangea can be pruned severely in late winter or spring because it blooms on new growth.
On a tree form there is a practical reason to wait for spring as well: a head full of dried flowers holds wet snow. General guidance: if a heavy, wet snowfall lands on it, brush it off gently with a broom rather than letting it bend the head.
How to Prune the Head
Use the same heading cut as on the shrub: just above a fat bud, about 6 mm (a quarter inch) above it, as the University of Minnesota describes. General guidance: cut each of last year’s stems back to two or three pairs of buds from where it started, which keeps the head compact and round instead of growing taller and floppier each year. Remove any stem that crosses into the middle of the head or rubs another.
Do not cut into the main trunk or the short, thick branches at the top of it. Those are the framework the nursery spent years building, and a hydrangea tree is slow to rebuild one.
A tree-form head is all small heading cuts, which is shears work; the lopper is for a suckering stem at the base.
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Trunk Shoots and Suckers
General guidance: a panicle hydrangea wants to be a multi-stemmed shrub, and a tree form keeps trying to go back. Rub off any shoots that appear along the trunk while they are still soft, and cut any suckers that come up from the ground at the base, flush with the soil. Left alone, they take over and the plant turns back into a shrub with a stick in the middle of it.
Staking and Wind
N.C. State lists wind damage among the problems panicle hydrangea can have, and the University of Massachusetts notes the heavy flower clusters make the branches arch. A tree form carries all that weight on one stem. General guidance: keep a stake on a young tree for its first few years, with a soft, loose tie that you check each spring so it does not cut into the bark, and plant it out of the strongest wind.
Winter Care in Canada
Panicle hydrangea is the hardiest of the species, so the head needs no wrapping in most of the country. The trunk is the exposed part. The University of Minnesota recommends fencing hydrangeas with wire about 60 cm (24 inches) tall against rabbits in winter; on a tree form, a guard around the trunk does the same job.
A hydrangea tree in a pot is a different matter. This site uses a rule of thumb that a plant in a pot behaves about two hardiness zones colder than the same spot in the ground, because the roots sit in cold air on every side; see our balcony garden guide for the table. Move a potted tree into an unheated garage or against a sheltered wall for winter, or plant it in the ground. More on the whole subject is in Hydrangea Winter Care in Canada.
Water and Mulch
N.C. State says panicle hydrangea is shallow-rooted and benefits from mulch, and the University of Massachusetts suggests about 2.5 cm (one inch) of water a week, more in hot, dry weather. Keep the mulch a hand’s width back from the trunk. For the shrub version of the same plant, see How to Prune Limelight Hydrangea, and the pruning chart covers every other hydrangea.
Frequently Asked Questions
When should I prune a hydrangea tree in Canada?
In late winter or early spring, before the buds open. Tree-form hydrangeas are usually panicle hydrangeas, which flower on new wood, so a spring cut costs no flowers.
How do you prune a tree-form hydrangea?
Cut each of last year's stems in the head back to a fat bud, keeping a compact, rounded shape, and remove crossing stems. Do not cut into the trunk or the main branches at the top of it.
What do I do with shoots growing from the trunk of my hydrangea tree?
Rub them off while they are soft, and cut suckers at the base flush with the soil. Otherwise the plant gradually turns back into a shrub.
Does a hydrangea tree need winter protection in Canada?
The head usually does not, because panicle hydrangea is very hardy. Protect the trunk from rabbits, and treat a tree in a pot as about two zones less hardy than one in the ground.
Should I stake my hydrangea tree?
For the first few years, yes. The heavy flower heads sit on a single stem, and N.C. State lists wind damage among panicle hydrangea's problems. Use a soft, loose tie and check it each spring.
Why is my hydrangea tree flopping?
Usually the stems in the head have grown long and thin. Cutting them back to two or three pairs of buds each spring keeps them short and stiff enough to carry the flowers.
Sources
- University of Minnesota Extension — Pruning hydrangeas for best bloom
- University of Massachusetts Amherst Extension — Growing Hydrangeas
- N.C. State Extension Gardener Plant Toolbox — Hydrangea paniculata
- Canadian zones and frost dates: this site’s city data, from Natural Resources Canada and Environment and Climate Change Canada.