Stop! Winter pruning Japanese maples

Discussion in 'Maples' started by JT1, Jul 18, 2019.

  1. JT1

    JT1 Contributor

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    I have spoken many times about how pruning Japanese maples over Winter is a bad idea despite many books encouraging such activity. I have found they heal or form wound wood faster when pruned in May. Below is a study finding Japanese maples do not heal from winter damage. When cuts are made when temperature is at 15C or 59F trees start to heal within a three week period.

    This information in combination of high risk of bacterial disease in late Winter thru early Spring (Feb-mid April in my area) should be evidence to restrict Winter pruning of live wood in Japanese maples.

    "During winter dormancy, temperate trees are capable of only a restricted response to wounding. In an experiment, we investigated the effect of wounding on Acer palmatum trees during winter-bud dormancy and found that in the cold (4 °C) temperature treatment, wound reactions were virtually absent. In the warm (15 °C) treatment, however, trees reacted actively to wounding within a three-week period by e.g. forming callus and local wound xylem. We conclude that temperature is an important factor in wound reactions during winter dormancy and may even induce the formation of callus and wound xylem within a three-week period."

    Why don't Japanese maples heal during cold dormant periods, it's though cold temperatures inhibit biological wound signals that triggers important hormones:

    "This suggests that in our experiment the local formation of wound xylem in A. palmatum was triggered by a wound signal that is active only at a higher temperature. The wound signal might comprise hormones such as jasmonates and ethylene, which are important in wound closure"

    Pruning during the dormant season increases the risk of bacterial diseases because the trees ability to compartmentalize and stop the spread of infection is inhibited by cold temperatures. The risk is highest in areas of cold and wet climates in Spring before the tree breaks dormancy. This may explain why in these areas Japanese maples are prone to black tips after Winter pruning. When I stopped Winter pruning I no longer get black tips in Coral bark maples and other prone cultivars.

    "compartmentalization by inhibitory compounds is a temperature-dependent physiological process similar to the formation of ligno-suberized layers, callus and wound xylem, which are all delayed at low temperatures during winter dormancy. Given that compartmentalization of tree wounds restricts moisture loss and damage from pathogens (Shigo 1984; Mireku and Wilkes 1989; Pearce 1996; Fink 1999), it thus seems likely that trees in areas with mild winters might cope better with the effects of wounding than trees in areas with cold winters."



    Early wound reactions of Japanese maple during winter dormancy: the effect of two contrasting temperature regimes - PubAg

    Full text:
    Early wound reactions of Japanese maple during winter dormancy: the effect of two contrasting temperature regimes

    Reconsider Fall pruning as it can be too cold to heal in some climates:

    "In a garden experiment, Copini et al. (2014) found that A. palmatum trees that had been wounded in October when most leaves had abscised had formed no local wound xylem or callus 14 or 28 days later: in the 14-day period the average temperature was 7.3 °C (±2.7) and in the 28-day period it was 8.2 °C (±3.3)."
     
    Last edited: Jul 19, 2019
    opusoculi, Margot, AlainK and 2 others like this.
  2. Acerholic

    Acerholic Member Maple Society

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    Hi JT1, Very interested in your posting, we have always pruned in Autumn after last leaves have dropped. This is due to nearly all books saying to avoid pruning in Spring due to rising sap. Have you had any problems with your maples with your method?
    We are about to move our whole collection (150 maples) in the Autumn and was going to prune then for house move, would you suggest leaving it to the Spring? Thanks for a really good post.
     
  3. JT1

    JT1 Contributor

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    The sap starts to flow usually in late January to early February, but it's weather dependent. Once the trees leaf out and put on their Spring push of growth you don't need to worry about the sap bleeding from pruning cuts. Pruning too early in Spring (after the flow of sap but still dormant) before the tree breaks dormancy carries the risk of bacterial infection. In my area the risk is highest for disease from February thru mid April when the ground is still very saturated from the Winter thaw.

    We remove grey dead twigs over Winter but we do not make the cut into live tissue. We cut the dead grey twigs back to live bark but do not cut the live bark, the pruning cut is made into grey dead tissue flush.

    If you are moving then I would go forward with your plans. It's better to prune and prevent accidentally breaking branches during the move. Since you have done Fall pruning in the past with no problems you shouldn't worry. Areas with a milder Fall may have adequate time for the tree to start the healing process and compartmentalize the wound from freeze damage and disease. Our area is very unpredictable as we can have snow for Halloween and prolonged periods below freezing in November. I am in a high risk area where your area carries less risk. The following season you may consider Spring pruning during the growing season, you may find the wounds heal quickly and your trees may be healthier as a result with less Winter damage.
     
  4. Acerholic

    Acerholic Member Maple Society

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    Hi JT1, many thanks for the great info, our Autumns are very mild indeed, so we will continue this season with pruning in October. It will be quite a hard prune this time to enable wrapping of branches to move safely. In 2020 we will try your method as it's always interesting to look at different ways to care for maples. My wife and I have been growing maples for 40+ years (hence the quantity), but one never stops learning. Thanks again.
     
  5. JT1

    JT1 Contributor

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    I agree, we are always learning too. I find our trees are great teachers when we devote our time but most importantly pay attention to what they are trying to tell us. Then the internet opens up a world of research / experience that I never would have access to otherwise. The effort we put into our collection repays us ten fold in ways I never thought possible. They would definitely be coming with us in a move to a new home.

    It's great to hear both you and your wife grow maples! Both my wife and I enjoy, but we are only about half the number and time growing. We always talk about what we might do if we ever move.

    Good luck on your move! I assume it's both nervousness and excitement. Nervous about moving all the trees and getting them to their new home, but at the same time exciting to get a new place to do things right. It's like starting over with having everything figured out. This should take things to the next level having experience as a solid foundation in finding the best spot for each tree / cultivar.

    Please keep us posted, take some pictures. I'm sure many people can benefit from the experience gained in moving such a large collection. Thanks for sharing and your feedback on the study.
     

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