When will it be safe to prune this fall?

There’s plenty to do in the garden in early autumn: planting bulbs, dividing perennials, preparing vegetable beds for next season and watering. But there’s one thing you should not do for several more weeks until the leaves have fallen: prune trees and shrubs.

“Pruning in early fall, before woody plants are fully dormant, would likely stimulate new growth,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “That new growth would not be protected against the cold and drying winds that are coming in winter.”

If you wait until deciduous trees and shrubs have entered their winter resting state of dormancy, you’ll avoid the problem. Pruning does not stimulate a dormant plant.

New twigs are green, supple, moist and tender. It takes several weeks for their soft skin to harden into stiff bark that can protect their inner tissue from cold and seal in moisture so they don’t dry out and die.

“Any new twigs that grow in early fall would not have time to harden off before we start getting really cold weather,” Yiesla said. “They’d be sitting ducks.”

Early fall pruning also risks spreading disease and insect pests. During warm weather, the tissues of a shrub or tree are active and growing, and so are the bacteria, viruses and fungi that cause plant diseases. Pathogens or insects can easily enter pruning wounds.

For all these reasons, pruning in the dormant season is best not only for shrubs but also for trees such as oaks and maples.

When will it be safe to prune? Once shrubs are fully dormant. “There’s no firm date,” she said. Dormancy can come earlier or later from one year to the next. Warm weather in autumn will slow down the process.

Bare branches are a good indicator since discarding leaves is part of the process of entering dormancy for deciduous shrubs and trees. “It’s always safer to wait a little longer until it’s good and cold,” Yiesla said.

“Since our weather is becoming increasingly variable, you really need to go by what you see in your plants, rather than depending on the calendar,” she said. If you are planning the pruning of a large tree, the Arboretum’s general advice is that it is safest to prune shade trees between November and February. 

Evergreen trees and shrubs are a special case. Plants that keep their green leaves all year never go completely dormant, even in the most bitterly cold February, so pruning them even in late fall or winter can prompt vulnerable new growth. “Don’t prune evergreens in autumn at all,” Yiesla said. “Wait to prune them until spring, when the risk of a hard freeze is over.”

What you can do for your evergreens in the fall is to water them. “Evergreens need plenty of water in their roots, stems and leaves to avoid drying out over the winter,” she said. Continue watering evergreens deeply until the ground freezes, and make sure they have a layer of mulch over their roots to hold that moisture in the soil.

“Do the same for any trees or shrubs you’ve planted in the last two years,” Yiesla said. New plants have underdeveloped root systems, so they need extra help to store up the water they will need to avoid drying out in winter and start growth in spring.

“In early autumn, you can best help all your woody plants if you delay pruning and focus on watering,” she said.

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.