Gardeners' Holidays 2024: Pruning Day

Every year, one of the first jobs on our garden to-do list is pruning our plum trees. I've been dreading this task somewhat, as over the past couple of years our trees had become badly infected with what I assumed was canker, causing knobbly black growths like giant warts around their branches. All three trees had it to some extent, but the Mount Royal in particular hardly seemed to have a single branch that wasn't affected. 

However, when I tried researching tree canker to figure out the best way to tackle this, I began to realize that the growths on our trees didn't look much like the cankers in the pictures. I started searching for info on other problems affecting plum trees and eventually figured out that what ours actually had was a fungal disease aptly named "black knot." Sources on the Web disagree on how serious a problem this is. The province of Alberta warns, "The fungus continues to grow internally and externally, with the branch eventually becoming girdled and dying," while the University of Minnesota Extension says, "Many Prunus trees tolerate black knot. Tolerant trees have many galls throughout the tree with few negative effects on the health of the tree." Minnetonka Orchards takes a middle ground, saying "The tree may suffer from decreased fruit production, structural damage, and ultimately death if the infection is severe," but adding that "Mature trees are more resilient and may survive without any noticeable ill effects."

Based on the extraordinarily productive season we had for plums last year, my guess was that the disease wasn't affecting our trees too badly. I decided we wouldn't try to remove every single branch that was infected (which would probably be impossible anyway), but we'd take off the worst offenders. And, at the same time, we'd also prune out out any branches that were problematic in other ways: overlapping, inward-pointing, or in danger of impinging on the house, driveway, and sidewalk.

Unfortunately, this measured approach proved hard to stick to. We kept finding more and more branches that seemed to fall into the "worst offender" category, especially on the Mount Royal. We definitely violated the rule against cutting off more than one-quarter of the tree's crown, and eventually we just had to force ourselves to stop before we stripped it completely bare. Even now, in its largely denuded state, it still isn't entirely free of galls, but it looks a lot cleaner than it did before. And since our trees seem to operate on a two-year cycle—incredible productivity one year followed by nothing at all the next—it should have over a year to catch up and produce new, healthy growth before our next plum harvest.

Meanwhile, we had to dispose of the huge pile of pruned-off branches. All the sources I consulted emphasize that it's important to destroy branches infected with black knot as soon as possible, as they can otherwise continue to shed spores and spread the fungus. We planned to burn them in our little backyard fire pit, but the wood was too moist to light. Brian ended up having to supplement the fire with sticks from our stock of seasoned wood to get the plum branches dry enough to burn. Eventually he managed to destroy most of the diseased wood this way, leaving a small pile of healthy wood for later use.

There's still more to be done to prepare for the upcoming gardening season—pruning the rosebush, for one, and laying out the beds for the vegetable garden—but with all the cutting, hauling, and burning of branches, I think we've had enough of a yard-workout for one day.