The one question that decides the timing
When to prune shrubs comes down to where the flower buds are. Some shrubs bloom on old wood: they form next spring’s buds on this year’s growth, over the summer and into early fall, and carry them through the winter. Cut those shrubs in October or March and you cut off the flowers before they open.
Other shrubs bloom on new wood: they make the buds on stems that grow in the same season they flower. Those can be cut hard in late winter and will still bloom that summer.
Most people ask in September and October, when the yard starts to look shaggy. That is the season when the answer for most shrubs is to leave them alone. If a shrub flowered last spring, its buds for next year are already on it. The exception is dead, broken or diseased wood, which can come out at any time of year.
| Shrub | Blooms on | Prune when |
|---|---|---|
| Forsythia | Old wood | Within a few weeks after flowering |
| Lilac | Old wood | Right after flowering; the window is short |
| Azalea | Old wood | Right after flowering |
| Rhododendron | Old wood | Right after flowering; snap off spent trusses by hand |
| Viburnum (most) | Old wood | Right after flowering |
| Weigela | Old wood | Right after flowering |
| Mock orange | Old wood | Right after flowering |
| Bigleaf hydrangea | Mostly old wood | Spring, once buds swell: dead stems and tips only |
| Oakleaf hydrangea | Old wood | Just after it blooms, if at all |
| Panicle hydrangea | New wood | Late winter, back to a framework of thicker stems |
| Smooth hydrangea | New wood | Late winter; can go nearly to the ground |
| Butterfly bush | New wood | Late winter, to live wood |
| Rose of Sharon | New wood | Late winter |
| Summer-flowering spirea | New wood | Late winter |
| Repeat-blooming roses (most) | New wood | Late winter, when the forsythia blooms |
| Privet hedge | – | Late spring to early summer, plus a lighter midsummer trim |
| Arborvitae | – | Late spring to early summer; lightly, only into green |
Spring bloomers: right after the flowers fade
Forsythia, lilac, azalea, rhododendron, most viburnums, weigela and mock orange all bloom on old wood. Prune them within a few weeks of the flowers dropping, while the new shoots are still soft. Wait until late summer and you are already cutting into next year’s buds. Lilac and rhododendron set those buds early, so the window is short. On rhododendrons, snap the spent flower trusses off by hand and take care not to damage the new buds just below them.
Two hydrangeas belong in this group and need care. Bigleaf hydrangea, the mophead and lacecap type, flowers mostly on old wood. Cutting it back in fall or early spring is the usual reason one grows leaves and no flowers.
Leave the stems standing through winter. In spring, once the buds swell and you can see which stems are alive, cut out the dead ones and trim tips back to the first strong pair of buds. Reblooming varieties also flower on new wood, but they still bloom best if the old stems are left alone.
Oakleaf hydrangea also flowers on old wood. It rarely needs more than the removal of a crossing or worn-out stem just after it blooms.
Summer bloomers: late winter, before growth starts
Panicle hydrangea, smooth hydrangea with its big round white heads, butterfly bush, rose of Sharon, summer-flowering spirea and most modern repeat-blooming roses flower on new wood.
Late winter into early spring is the time, roughly late February through March in southeastern Pennsylvania. The plant is dormant, the branch structure is easy to see without leaves, and the cuts close quickly once growth starts. A common rule of thumb for roses is to prune when the forsythia blooms.
These shrubs take hard cutting. Smooth hydrangea can go nearly to the ground every year. Panicle hydrangea is better cut back to a permanent framework of thicker stems, so the flower heads have something to hold them up. Butterfly bush can die back over a cold winter here, so cut it to live wood once you can see where the green starts. DCNR lists it as invasive in Pennsylvania, so cut off spent flowers before they set seed.
Hedges and evergreens: early summer, and nothing late
Shear formal hedges and evergreens after the spring flush of growth has expanded and begun to firm up, which is usually late spring into early summer. Fast growers such as privet may need a second, lighter trim in midsummer. Finish by late summer. A shear in September or October pushes out soft new growth that has no time to harden before the first hard frost, and that growth browns and dies over the winter.
Most needled evergreens, arborvitae and juniper among them, do not regrow from bare interior wood. The green is a shell on the outside, and behind it the branches are brown and leafless. Cut back past the green and that spot stays bare for good. Trim arborvitae lightly and often, and only into green growth. Yew, holly, privet and boxwood are the exceptions: they will push new shoots from older wood and can be cut back hard.
A hedge should be cut a little wider at the bottom than at the top, so sunlight reaches the lower branches. Cut it straight up, or wider at the top, and the top shades the base. The lower branches thin out and die, and you get a hedge with bare legs that no amount of trimming brings back.
| Window | Prune | Leave alone |
|---|---|---|
| Late winter, February to March | Summer bloomers on new wood: panicle and smooth hydrangea, butterfly bush, rose of Sharon, roses. Renewal cuts on overgrown deciduous shrubs. | Spring bloomers |
| Early spring, at bud break | Bigleaf hydrangea: dead stems out, tips back to the first strong buds once you can see them | Anything more on bigleaf hydrangea |
| Mid to late spring, April to June | Spring bloomers within a few weeks of flowering: forsythia first, then lilac and azalea, then rhododendron | – |
| Late spring to early summer | The main shear on hedges and evergreens once the new flush has expanded; arborvitae lightly | Bare interior wood on arborvitae |
| Midsummer, July to August | Oakleaf hydrangea after it blooms, if it needs anything; a second light trim on fast hedges such as privet | Shearing after late summer |
| Fall, September to November | Dead, damaged and diseased wood only | Shearing, shaping, cutting back spring bloomers |
Overgrown shrubs and the one-third rule
As a rule, take no more than about a third of a shrub’s live growth in a single season. Past that, the plant loses too much of its leaf area to feed its roots, and many species answer with a mass of weak, upright water sprouts.
A forsythia or lilac that has eaten a window is usually fixed by renewal pruning over three years:
- Each year, cut a third of the oldest, thickest stems right down to the ground.
- By the third year every stem is younger and the shrub is smaller.
- It flowers along its full height again, not just at the top.
Some tough multi-stemmed shrubs, privet and forsythia among them, will also survive being cut nearly to the ground all at once in late winter. It works, but you lose that year’s spring flowers and the shrub looks rough for a season. Save it for a shrub that is otherwise headed for removal.
Clean cuts, and why sheared shrubs stop flowering
Match the tool to the stem:
- Bypass pruners on live stems. Anvil pruners crush the stem.
- Loppers on anything thicker than a finger.
- A pruning saw beyond that.
Keep blades sharp, and wipe them with rubbing alcohol between plants if you are cutting out disease. Cut just above an outward-facing bud, or back to the branch or stem it came from. Do not leave a stub, which dies back and lets in decay.
Removing a whole stem back to its origin is a thinning cut, and it opens the shrub to light. Cutting a stem partway down is a heading cut, and it makes the shrub bushier at that point.
Hedge shears make nothing but heading cuts. That is fine on a privet hedge, which is meant to be a dense green wall. On a flowering shrub it does two kinds of damage.
Shear an azalea or forsythia into a ball every year and you get a thick outer shell of short twigs that shades out the inside. If the shearing happens at the wrong time, you also remove most of the flower buds. The shrub ends up green all year with a few flowers on the outside, or none at all.