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Remedial Pruning
by Des Kennedy
by Des Kennedy

Des Kennedy is a celebrated public speaker, having performed at numerous conferences, schools, festivals, botanical gardens, art galleries, garden shows and wilderness gatherings in Canada and the U.S. His humour, irreverence and passion for gardening and the natural world have made him a 'must see' speaker in demand across the country.

January 1, 2006

Even in less catastrophic circumstances than the epic winter ice storms in Quebec and Ontario several years ago, winter storms can wreak tremendous damage on trees of all sizes. Preventive pruning beforehand may diminish the opportunities for disaster, but every once in a while, unusually fierce storms of wind or ice or snow are going to take a toll even among the most meticulously cared-for trees.

Several years ago one of our Douglas firs - a stout tree at least 60 cm in diameter - was literally torn in half by a freak wind storm. A huge cleft opened down the centre of its trunk so that you could actually look straight through the bole and out the other side. In another storm, two winters ago, a heavy dump of wet snow tore just about every limb from a big ornamental plum tree.

After events like these, post-traumatic pruning becomes the order of the day. When big trees near to hydro lines or buildings are involved, we wisely leave the lopping and pruning to professionals equipped with hydraulic lifts, power tools and pulley systems for lowering big limbs. But in less hazardous circumstances, it may fall to the home gardener to get out and do the necessary remedial pruning Possibly the most painful decision is the first one: trying to determine if a tree is damaged beyond repair. We have a young redbud that has repeatedly suffered cracked branches in its crown, an example of a tree that may force one to conclude that starting from scratch with a new specimen is preferable to ongoing damage control.

On the other hand, the ornamental plum that was stripped almost down to bare trunk has rebounded astonishingly, replacing its crown within two growing seasons. Some species will bounce back from a bad mauling better than others, the standard rule being the less desirable the species, the better its chances of recovery.

Large limbs that have cracked or sheared off the trunk are cleaned up with the principles that apply to any dormant season pruning. Proper pruning tools properly sharpened are, of course, a sine qua non. Lateral branches that have been damaged or badly bent can be cut back to about one-third of their original length, the cut being made to a side shoot growing in the desired direction.

To completely remove thick branches that have been damaged, we take the same cautionary approach we would were we simply pruning to shape. We want to avoid having the weight of the branch tear it away from the tree, causing even more damage to the trunk. The branch is first reduced to a manageable length by cutting it at about 40 cm from the trunk. First you make an undercut about one-third of the way through the branch, then complete the cut from the top. Sometimes its advisable to tie the branch being cut to another sturdy branch higher up, to prevent the cut limb from crashing down and doing damage. Once down to a 40 cm stub, the undercut/overcut routine is repeated, this time close to the trunk.

It used to be we were advised to make these stump cuts flush against the trunk, but now some arborists advise that we leave a residual "turtleneck" on the trunk to aid in the tree's healing. Any ragged edges around the cut are cleaned off with a sharp knife.

What about painting over pruning cuts and other wounds? The standard practise used to be that any cut greater than 5 cm in diameter would be painted with antiseptic tree paint. The idea was to fill the pores of exposed sapwood, staunch bleeding or evaporation of sap, and prevent fungal spores, bacteria and wood-tunneling insects from infesting the living tissue.

Some "good enough" traditional sealants probably did more harm than good - coal tar, roofing tar and oil-based paints all contain ingredients toxic to exposed living wood cells. Latex paints are not toxic but need to be applied repeatedly to have any effect. A wound dressing that cracks, blisters or peels off has to be replaced.

Again, some arborists now argue that even large cuts should be left undressed for the tree to heal itself by walling off the damaged area, forming a reaction zone. Callus tissue, growing inwards from the cut edges will in time completely seal the cut surface, denying oxygen to any invading pathogens.

One thing there's no argument about: many trees have a remarkable capacity to withstand a savage beating and bounce back. The big Douglas fir I mentioned is still standing, several years after splitting up the centre, its wounds now sealed with new bark. Small consolation, I know, for when a grand tree is destroyed in a winter storm, but still grounds for optimism that nature's resilience is in its own way as powerful as her destructiveness.

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