There’s a lot to do in the garden in early fall: planting bulbs, dividing perennials, preparing vegetable beds for next season and watering. But there’s one thing you shouldn’t do for several more weeks until the leaves fall: prune trees and shrubs.

“Pruning in early fall, before woody plants are fully dormant, is more likely to stimulate new growth,” said Sharon Yesla, plant knowledge specialist in the Plant Clinic at Morton Arboretum in Lisle. “This new growth will not be protected from the cold, dry winds that come in winter.”

If you wait until deciduous trees and shrubs enter winter dormancy, you will avoid the problem. Pruning does not stimulate a dormant plant.

The new branches are green, soft, moist and tender. It takes several weeks for their soft skin to harden into a tough bark that can protect their internal tissues from the cold and trap moisture so they don’t dry out and die.

“Any new branches that grow in early fall won’t have enough time to harden off before we start getting really cold weather,” Jesla said. “They’ll be sitting ducks.”

Pruning early in the fall also risks the spread of diseases and insect pests. During warm weather, the tissues of the shrub or tree are active and growing, as are bacteria, viruses, and fungi that cause plant diseases. Pathogens or insects can easily get into 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 pruning be safe? Once the bushes are completely dormant. “There is no specific date,” she said. Dormancy can come early or late from year to year. Warm weather in the fall will slow the process.

Bare branches are a good indicator since shedding leaves is part of the process of entering a dormant state for deciduous shrubs and trees. “It’s always better to wait a little longer until it gets nice and cool,” Yiesla said.

“As our weather becomes increasingly variable, you really need to follow what you see in your plants, rather than relying on the calendar,” she said. If you are planning to prune a large tree, the general advice from an arborist 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 year-round never go completely dormant, even in very cold February, so pruning them even in late fall or winter can result in weak new growth. “Don’t prune evergreens in the fall at all,” said Yesla. “Wait to prune until spring, when the danger of a hard freeze has passed.”

What you can do for your evergreen trees in the fall is water them. “Evergreen plants need a lot of water in their roots, stems and leaves to avoid drying out during the winter,” she said. Continue to water evergreens deeply until the ground freezes and make sure there is a layer of mulch over their roots to retain moisture in the soil.

“Do the same for any trees or shrubs you have planted within the past two years,” Yiesla said. New plants have underdeveloped root systems, so they need extra help storing the water they’ll need to avoid winter drought and begin growth in the spring.

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

For advice on trees and plants, contact the Plant Clinic at The Morton Arboretum (630-719-2424, Beth Potts is a staff writer at The Arboretum.

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