Late summer pruning: what happens, what won’t, and why

In the fall a gardener’s fancy lightly turns to thoughts of pruning (with apologies to Alfred, Lord Tennyson).  In particular, people worry that pruning too late in the summer or early fall will stimulate plants to send out new growth, which is then damaged by freezing temperatures. Let’s dissect what actually happens when woody plants are pruned during this time.

Sumac leaves in full autumn glory.

First, we need to separate temperate trees and shrubs from tropical and subtropical species. For the most part, the latter don’t become winter dormant: pruning them at any time means you will have regrowth as long as there are sufficient resources. If planted in more temperate zones, they will continue to grow until they are killed by freeze damage. Instead, we’ll look at temperate species and how they are adapted to surviving winter conditions.

Tropical woody plants like this jade are not winter dormant species. Don’t leave them outside even if you think they are protected (lesson learned).

I wrote a couple of posts last year on cold hardiness (here and here), so I won’t repeat those discussions on how plants survive freezing. Instead, we’ll focus on the process of HOW plants enter winter dormancy and become cold hardy. It’s a two-step process that depends on two different environmental factors: one that never changes from year to year, and one that certainly can.

The first step to dormancy is initiated right after the summer solstice. Plants are exquisitely adapted to changes in the light-to-dark ratio, and days begin shortening after the summer solstice. The changes that occur are largely biochemical, but you can also see some changes in plants themselves. Many trees and shrubs slow their growth during this time so that fewer young leaves and shoots are produced. Instead, resources are put into the existing foliage, or flowers for summer bloomers. Excess resources are routed to woody parts of the plant for storage.

Light and dark ratios vary with latitude, but the seasonal changes are always the same time of year.

From a practical standpoint, this means that when you prune trees and shrubs where growth has stopped, you will NOT get regrowth. The vegetative buds below the pruning cut are dormant. The tricky thing is that the exact time when the switch is thrown varies by species and is affected by environmental conditions. Careful observation will allow you to estimate when the plants will no longer produce new growth.

Some temperate species naturally put on a spurt of late summer growth. The leaves on these new Japanese maple shoots generally die from cold damage, but the branches themselves survive.

The second step begins when night temperatures cool to near freezing, which is not a predictable date. Because many of the biochemical and physiological processes have already begun or are finished, the response to cold night temperatures is rapid and visible. Leaf colors change as the plant begins breaking down leaf materials for mobilization and storage elsewhere in preparation for winter dormancy.

This katsura has started the process of autumn leaf senescence.

This process, honed over millions of years, is unfortunately not infallible especially under abnormal environmental conditions. Two examples spring to mind:

  • High intensity street lights. If the normal light-to-dark ratio change is interrupted by significant levels of night light, the first step of dormancy is hijacked. You can see what happens in these previous blog posts here and here.
That street light in the middle has kept the nearby leaves green while those farther away are senescing.
  • Unseasonably cold weather. With climate change, we are seeing wild shifts in all sorts of weather patterns, including the date of the first hard freeze. Hard, early freezes are not the same as a light evening frost. You can see what happens here:
A hard freeze in early November fried the leaves on this hydrangea.

Given normal conditions, however, temperate trees and shrubs are well on their way to full winter dormancy by late summer and early fall. Pruning them is not going to induce new growth.

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

9 thoughts on “Late summer pruning: what happens, what won’t, and why”

  1. Your discussion of late summer pruning was exquisitely clear, Linda.
    The photos and their captions added valuable depth to the blog.
    Thank you!

  2. Will pruning at this time remove any resources the plant may need before spring pruning? “Excess resources are routed to woody parts of the plant for storage.” Thanks, Brandon

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