By Barrie Anderson
As I’m sitting here piecing together the latest tips for pruning deciduous trees, I can look out my window and see a beautiful Betula nigra (River Birch) in my neighbor’s landscape, and right next door on the other side, a massively, giant Acer saccharinum (Silver Maple). I would never have thought there is a common thread between Birches and Maples when it comes to pruning. Yet in spite of their many differences, especially size and look, both need to be pruned in the months of June and July when sap production is relatively lower. Both of these trees “bleed” heavily in late winter when the days are sunny, and warmer (relatively speaking for Minnesota winters), and the nighttime climate is just below freezing. These temperatures trigger a pressure build up in the roots of the Maples, and Birches as well, that pushes the sap up and into the limbs where it will freely flow out of any cut.
Pruning these trees when young, is critical for proper stature, and to reduce limb failure. This is what is known as developmental pruning. Being that a Silver Maple will reach at least 70-80 feet in height, it is fundamental to prune this specimen often while it’s young. Removing any crossover branches is necessary as it opens up the canopy of the tree allowing better air flow and light to enter. It is also essential to not leave a “stub” cut when pruning as this is an avenue for disease to enter straight into the tree. Indicated by the photo below is an example of a stub cut. This is a cut we do NOT want to have done to a tree. On the other end of the spectrum, a cut too close into the main leader of the tree is just as damaging as a stub cut. The reason is this type of cut eradicates the protective cells in the branch collar that are critical for the healing process.
Birch trees experience the same pressure build up in their roots when exposed to the same temperatures as Maples, thus, when cut in late winter, they too will “bleed” heavy amounts of sap. So, remember to prune Birch the same time as you prune Maples, in June and July. The same rules apply to Birches for developmental pruning as they do to Maples. The only difference is most Birch trees are sold in clumps, which means they will have a minimum of 2-3 main leaders. Treat each main leader as you would the main leader on a Maple. Make certain there are scaffold branches that create the canopy, prune out any crossover limbs and be certain to rid the tree of water sprout branches as well. In the photo below left, is a River Birch with 2 main leaders. It is perfectly normal for the other co-dominant leaders to list to one side. Here is a photo, below right, of several crossover branches that should’ve been removed when the tree was much younger. There is still time, being the crossover extensions in this particular photo are small, the cut will heal much quicker. However, take caution not to hesitate too long, as larger cut limbs yield larger wounds requiring more recovery time, and the taller the tree, the more difficult it is to maintain.
Juglans nigra. The Black Walnut. When it comes to growing nut trees in Minnesota I wonder if some gardeners don’t consider the land of 10,000 lakes as producing nuts. Black Walnut trees aren’t high on the list in any landscape in Minnesota because of one culprit: juglone. Juglone is a toxic chemical released by the Walnut to ensure its survival. It slowly poisons other trees and plants around it so it can enjoy all the nutrients and longevity it desires. Nearly every part of the tree has naturally occurring juglone, the roots, the buds, even the nut hulls, which makes it a daunting foe for any nearby plants. For developmental pruning on Black Walnut trees, if you wish to harvest nuts, it’s imperative to have the branches as low as 4 feet off the ground for ease during harvest. Also, make certain all of the side branches are lower than the highest point of the main leader. These subservient branches will assist in the easy harvesting of nuts. The critical juncture for pruning a Walnut is late winter/early spring prior to leaf buds opening. This provides a great visual for the gardener to accurately shape the Walnut.
If the desired look of a Black Walnut in the landscape is solely for providing shade, then the branches should start no lower than 6-8 feet off the ground. Maintain a yearly developmental pruning schedule for the first 5-7 years of planting a Black Walnut and it will mature into a spectacular specimen in any yard.
Photos courtesy of K. Messinger