Wake Up, Trees- It's March
The weather is changing; the hard cold of winter replaced with intermittent warm and cold spells, sunny days and windy days. Our landscapes are waking up, leaf and flower buds swelling and opening, bees flying busily back and forth. It’s a great time to work in the landscape, too. Not too hot yet, but the promise of warmth feels good after the winter. March is the month we in New Mexico celebrate Arbor Day (the second Friday of March; this year that is March 13).
Arbor Day comes around in spring because early spring is a great time to plant trees. They go into the ground dormant and quiescent, they wake up in a new and hopefully final growing spot, ready to send their roots pushing through moist soil, their shoots up into fresh air and sunlight. I really like planting fruit trees in the spring, because if I mail order them in the fall, they arrive now as dormant, bare-root unbranched stems. Why do I like my trees this way, you may ask?
Trees are sedentary organisms. They don’t move about from spot to spot, unless we dig them up and move them. They don’t like having their root systems messed with. And, much experience and research has shown that smaller trees do much better post-planting than larger trees. They don’t need as much codling through the transition period from nursery tree to landscape citizen; that transition period is somewhat dictated by the size of the tree at planting. Larger trees may take a few years to get their roots out into the new landscape well enough to support strong shoot growth. Smaller trees can get established in one season, and will often outpace the growth of larger specimens, becoming larger within a decade or so.
Another reason I like small bare-root trees is that when I plant that root system, I immediately have root-to-native-soil contact. This is important because the growing media used in a nursery container has a very different texture than any native soil, and that texture difference creates a barrier to water movement. It is possible to end up with a root system drying out because the water isn’t moving from the native soil into the root ball to supply the existing roots, or to end up with a root system rotting away due water not moving from the root ball into the surrounding native soil. These problems are not absolute tree killers, but they can certainly be major tree stressors. With bare root stock, those issues evaporate.
Finally, in terms of fruit trees, there are many more varieties available from mail order houses than any local nursery.
Snip the Roots?
If you are planting a tree that was grown in a container, one of the most important things you can do for the long term health of that plant is to cut the roots. What??!! Are you crazy? What I’m talking about is selectively cutting any roots that have grown to the edge of the root ball, hit the container side, and started circling around. Cut them just before that elbow turn and the new roots that grow from the cut surface will continue out away from the trunk, as opposed to forming a wood noose that can eventually kill the tree.
Speaking of cutting, this is a good time to do some branch pruning as well. I don’t recommend pruning once the new season’s growth has popped out, until mid-summer or so. But if the buds are still swelling, you can still prune. Pruning may be one of the more misunderstood aspects of tree care – is it necessary? When should it be done? How should it be done?
I can’t answer all of that here, but a few key points need airing:
- One is that any pruning cut into living tissue wounds the tree, and the tree must spend some of its energy dealing with that wound.
- Trees don’t heal the way we do. Instead they attempt to lock the wound away behind barriers that prevent the growth of wood-decaying fungi from the wound surface into the tree.
How Trees Heal
There are two steps to this process:
- The tree brings anti-decay chemistry of its own making up to the wound site from below, attempting to create a barrier the fungi can’t get across.
- The tree also grows wound wood over the wound, preventing additional fungal spores from growing.
If both of these things are successful, we say the tree has compartmentalized the wound. The wound is still there, locked away inside the tree, but it’s stable and not a site of decay advancement.
When to Prune
In order to do those things, the tree needs to be metabolically active. Pruning cuts made now will sit a short while until the tree comes out of dormancy, at which time the processes will begin. If the cut was well placed and well executed, and if the stem cut is not too big, compartmentalization will occur successfully. Different species vary on how well they do the two things. Some, like cottonwood, are very poor at the chemical defense approach. Others, like pine trees, can do this well. On the species that are better compartmentalizers, a three-inch cut surface is about the limit that can be protected well. On the poor compartmentalizers, that may be a two-inch diameter limit. So, the best approach is to decide early in the life of a given branch whether it’s a keeper or not, and cut out the losers early.
Obviously, we often have to prune out larger branches. Sometimes that’s because of storm damage, or because we just realized they grow so low that we get a massive headache every time we walk under the branch. So, we make those pruning cuts knowing we are setting the tree up for a decay problem down the line. How fast that progresses is a matter of seemingly infinite variables, so no one can predict with certainty, but we do know the probability of problematic decay goes up with the size of the cut.
Before Pulling out the Pruning Saw...
This brings me to a last point about pruning. Trees feed themselves through the manufacture of sugars in their leaves. Pruning removes leaves. Fewer leaves means less food is being manufactured, which in turn means the tree has fewer resources to allocate to its various needs. A small decrease in that resource base isn’t critical to tree health, but as we approach 25% leaf loss in a single pruning season, it can be enough to cause problems. Again, no hard and fast rules – I had a young apple tree completely and utterly defoliated by a plague of grasshoppers in early July one year. By fall that tree had put out a whole new canopy of leaves and it was fine. But thinking of the trees welfare, go easy on the pruning. Have a good reason to cut a branch:
- It’s dead.
- It’s in the way.
- It’s poorly attached and likely to break off anyway.
There is no leaf on the tree that is not benefiting the tree from the perspective of making sugars. There is no branch that is stealing resources from the rest of the tree – if it ain’t paying its own way, the tree will cause it to die off and it will be shed. Thinning out a canopy to let more light into the interior will only cause the tree to grow more interior branches and leaves to take advantage of that light and space.
But mostly, don’t stress about this stuff. Read and learn, do and learn even more. Enjoy the sunshine and the clear blue sky!
PS – the Parks and Recreation Department will be giving away tree seedlings on March 14, from 9 to 11 in the morning, at three locations: Singing Arrow Community Center, Vista del Norte Park, and Desert Springs Park. Come early and get a free seedling!
Parks and Recreation Department
City of Albuquerque