Bonsai Heresy’s Chapter 25 and Last Summer Pruning Date
Chapter 25 of Bonsai Heresy is about the mistaken notions of avoiding nitrogen in our fall fertilizing by using 0-10-10. The problem it is trying to correct, frost damage from still soft shoots, is more often a mis-timed pruning in the summer.
This is easiest to see in deciduous trees, which may be pruned in late spring / early summer to good effect. That is, good regrowth. The comments in Bonsai Heresy are directed to primarily temperate plants, and in temperate regions June and July are good months to prune back many deciduous trees, which will reflush with new growth.
If, however, strong pruning is done in August, there may be a reflush that is uncomfortably close to the first fall frost. Then you may well get frost damage from that non-hardened growth. And that is totally unrelated to nitrogen.
As Heresy explains in depth, citing several research papers in support, fall fertilizing with nitrogen is not a problem for temperate plants, in fact, it is a good idea. The inclusion of nitrogen supports several critical things we want in our bonsai, such as greater resistance to cold, and better growth in the springtime.
Here are a few photos of the type of strong pruning that we’re talking about.
Stewartia just before ‘last chance’ pruning of the summer…the window of opportunity closing at the end of July in many temperate areas.
Following strong pruning. The large extension was pruned back, having been left for several years (a Trident Maple might have reached this size in one year, but Stewartia are slooooow.) The smaller shoots were also trimmed, and some leaves were cut in half. The combined reduction of shoot and leaf will create enough of an imbalance in this strongly growing tree to force a resurgence, and, likely, inner budding as well. The smaller trunk will be pruned back further once the side shoots strengthen, likely in a year or two. BUT, the point of this article is that the early summer timing of this regrowth will harden off well before first frost (in my area), and has nothing to do with limiting nitrogen in the fall. This tree will continue to have fertilizer with nitrogen through the fall for the winter hardiness it offers.
Intermittently this blog will expand on the ideas presented in Bonsai Heresy: 56 Myths Exposed Using Science and Tradition. If you wish to follow in more depth, please order a copy at Stone Lantern.