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)."