By Barbara Leach, Horticulture Techniciation, VCE-Roanoke
December 13, 2017
Pruning is surgical amputation, and like surgery on people, the key to the patient’s survival is in understanding how healing occurs. Severed limbs can only heal at nodes, buds or crotches. This is contrary to everything we know about healing in our own bodies. Imagine if you could only heal at your shoulder, elbow, wrist, or knuckles. If your limb was severed at the forearm, all the tissue back to the elbow would die. In trees and shrubs, if a cut is made at an internode (that blank wood between branches, crotches, or buds) instead of just beyond a node, the stub will block healing. That leads to rot starting in the stub and working its way down the limb. Although not visible, a lot of damage is being done inside. We have all seen old, rotten trees that are hollowed out in the middle. Usually that means rot started somewhere above (perhaps in an ice or wind break) and worked its way down the limb. In time, the rot column increases in girth, until only a thin shell of wood remains around a hollow center. That is an accident waiting to happen. Imagine a tree in a parking lot that has been topped. From below, we think it is sound. You may even see a proliferation of new sprouts at the limb ends, but those come from latent buds that are weakly attached to begin with and then once rot starts behind them there is nothing sound to attach to. So, the clock is ticking. I wonder when the limb will fall?
Species vary, but this generally means you should never cut a limb in the internode unless it is a tiny twig and you are willing to shorten rather than extend the plant’s life. Cuts should be made just above a crotch, node, or bud. If everything beyond that is removed, an undamaged bud at that node will sprout, or a side limb will continue making a strong and healthy limb that is attached correctly. If you get that much right, even if you mistakenly cut back to a bud that faces the interior of the plant and it later sprouts a limb that goes the wrong direction, you will have an opportunity to correct that in future years by taking it back to an outward facing limb or bud. The remaining tissue at your cut will begin generating wound wood that will eventually seal off the whole cut, making it sound and strong. As you are driving, observe roadside trees that have had limbs removed. You will see many that have smooth circular bark patterns where the wound-wood has sealed off an old cut.
Learning to prune is a visual thing so here are some links to help you understand: https://www.na.fs.fed.us/spfo/pubs/howtos/ht_prune/htprune-rev-2012-screen.pdf, http://pubs.ext.vt.edu/430/430-458/430-458.html, and https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-210/430-210_pdf.pdf Additionally, there are many good videos on the internet. Always check for a reliable educational source, however.
Early spring blooming plants should be pruned right after bloom, but before the heat of mid-summer. Things that bloom from mid-summer on, or aren’t grown for bloom are usually best pruned when dormant. That is from Nov to Feb in our area. Trees that are free bleeders, such as elms, birches, maples, walnuts, mulberries and poplars should be pruned in late fall or early winter, avoiding any spring pruning when bleeding would be unsightly and attract insects.
Be safe. Leave the big stuff to experts. Know whether you need a surgeon or an undertaker. Check credentials to make sure candidates actually know how to prune living trees properly and understand insects and disease or if their expertise is cutting down dead trees. Both will call themselves arborists.