If you have formal shrubs or hedges—the kind you shear with a power hedge trimmer rather than pruning branches out by hand—it’s time for their last haircut of 2017.
“Early August is the very latest you should shear shrubs,” says Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum.
If you shear them too late, you’ll make them vulnerable to damage from the cold in late autumn or winter.
Shearing shrubs prompts them to grow many new twigs that have thin, tender green skin. If twigs start growing after mid-August, that thin skin won’t have time to mature into tough, protective bark. Winter’s cold blasts will freeze the twigs and dry them out.
Selective pruning—the kind you do with hand pruners—also stimulates new growth, though not to the same degree. It’s a technique you can use to give a more natural shape to either evergreen or deciduous shrubs (the ones that lose their leaves).
To avoid pushing deciduous shrubs to grow vulnerable new twigs, wait to prune them until they have lost all their leaves and gone dormant for the winter.
"Once the plants are dormant, pruning won’t stimulate them,” Yiesla says.
Generally, you can safely prune deciduous shrubs between Thanksgiving and St. Patrick’s Day.
When you prune flowering shrubs, it’s also important to consider when they bloom so you don’t prune off flower buds that already have formed. Shrubs that bloom in summer and fall can be pruned when they’re dormant in the winter, but spring-flowering shrubs should not be pruned until after they finish blooming in spring.
Although winter is a good time for selective pruning, it’s the wrong time for shearing. That’s because most formal hedges and shrubs are evergreens, such as yew or boxwood, that never go entirely dormant. Wait to shear hedges and formal shrubs until just before new growth starts in spring.
“New growth in spring won’t be in danger,” Yiesla says.
If you prune or shear at the wrong time, it won’t be a disaster. It just makes it likely that you will need to snip out some dried-out brown twigs from your shrubs or hedges in spring.