In a text message this morning (Jan. 16), a concerned landscape maintenance contractor said “I’m pruning Japanese maples and I see a lot of bleeding on cuts. Any concerns?”
Since I’ve known him for many years and figured he could take a joke I responded: “Not unless it is your blood. The sap flow will stop within a couple days. There is nothing to do for it. I checked Vertrees’ ‘Japanese Maples.’ He recommends dormant pruning to be done from late November to early January.”
That’s the short answer. Since time is wasting for dormant season pruning, let’s look a little deeper into the topic.
First, “Vertrees” refers to J.D. Vertrees’ wonderful book “Japanese Maples,” a compendium of hundreds of Japanese maple varieties with guidance on their propagation and care.
On pruning, he wrote: “Where plants and great expanses allow it, it is magnificent to permit these trees to grow unhindered. However unless one wants to have a very tall Japanese maple of an upright cultivar (cultivated variety), top shaping and pruning should be started rather early in the life of the tree.”
In other words: Think ahead and prune the young tree to develop the desired structure.
He wrote from the perspective of his arboretum in Roseburg, Oregon, which is a bit more wintry than Napa Valley in winter. With that in mind, it is already getting late for Japanese maple pruning. The sap is flowing.
The reference to “bleeding” is common and convenient, but the great tree anatomy and biology researcher Alex Shigo called it “a very poor term…another tree-human term that should be avoided.” He preferred “sap flow.”
Probably referring to the sugar maples of New England, Dr. Shigo recommended pruning maples “during the end of the dormant period.”
His work described “compartmentalization,” a “dynamic defense process that starts after injury.” In his theoretical model, consisting of four virtual “walls” inside the tree, “Wall 1” resists vertical (lengthwise) spread of pathogens by blocking wounded vessels.
When a branch is severed the tree walls off the wound with various materials: gums, granular materials, air pockets and varnish-like material.
“Arboriculture” (Harris, Clark, Matheny) discusses the subject of timing, stating that spring is not favorable for pruning because tissues soft and loose during active growth are more vulnerable to being torn.
Their review of the literature indicates that wounds on mature trees, particularly deciduous trees such as maples, can “bleed” heavily, sometimes resulting in bark injury below the cut, slowing wound closure.
Larger wounds are more difficult to close, so making small diameter pruning cuts, 3 inches and smaller, can minimize the sap flow.
Over-pruning can be more harmful than pruning at the wrong time of year. Time and time again I have seen Japanese maples that have had large limbs removed resulting in severe sun injury.
Trees shade themselves. Severe pruning removes shade and exposes branch surfaces to direct afternoon sunlight. Especially in drought-stressed trees, over-heated tissues die and branches become infected with opportunistic disease and decay fungi.
Japanese maples are particularly susceptible to this.
Sometimes large branches need to be removed because they have died of disease, but too often they are removed to “improve the view of the branch structure.”
It brings to mind a collection of prized lace-leaf Japanese maples in a Napa Valley garden. The trees were pruned for thinning to show off the gracefully cascading, undulating, and twisting branches, but every one of them had severe sun injuries on the upper surface of the upper branches.
With artificial shading and good irrigation practices such trees might eventually recover.
It is far better to start with good cultural practices and just take it easy with the pruning tools.