Best time to prune maple and birch trees depends on weather

2013-03-05T19:17:00Z Best time to prune maple and birch trees depends on weatherBy Molly Hackett - for the Ravalli Republic Ravalli Republic

Q: I am pruning my trees, but I remember hearing that I should not prune the maple tree until summer. Is that right?

A: Not exactly. Given the right kind of late winter weather – warm, sunny days and nights below freezing – pressure from the roots will push the sap up and out of any cut on the tree. The tree will “bleed” sap. This can happen with both maples and birches. Tapping the trees and collecting the leaking sap is the first step in making maple syrup.

As long as a maple or birch tree seals a cut reasonably soon, pruning will not harm it. On a young maple, loss of much sap would weaken the tree. On a mature tree, only sawing off a major limb would expose enough tissue to cause severe damage.

Sometimes gardeners avoid the problem by pruning maples after the buds have opened; by then the sap will no longer leak under pressure. Sometimes they prune in late winter but before the sap begins to run. You could prune your maple at either of those times.

If now is when you want to prune, experiment to see whether you have chosen a good day for the maple. Make one small cut and watch it for a few minutes. If sap runs at a fast drip and keeps on for several minutes, wait for the weather to change. If little sap appears, it is a good day to prune.

Birch trees should be pruned very little, and only when necessary. Pruning a birch will stimulate the growth of many water sprouts, no matter what time of year the pruning happens. Other trees to avoid pruning for the same reason include horse chestnuts and locusts.

Ring of bark on pruned apple tree

Q: An old apple tree on the property we bought last year shows where someone cut off a whole limb. Around the cut is a circle of what looks like bark, but it is swollen. Is this a bad sign? Should I do something for the tree, or does it have a disease which is likely to kill it?

A: You are looking at a tree trying to heal itself. On an uninjured tree the layers of bark protect the wood underneath, in the same way that our skin protects the muscle tissue beneath.

The big pruning cut which removed the limb exposed an area of unprotected wood. Your apple tree immediately began to cover the exposed wood by growing a ring of bark around the cut. The tree can cover the wound only slowly, working from the outside toward the center. The swollen ring of bark is like scar tissue. If you look next year, the ring should be wider and the area of wood in the center should be shrinking. Eventually the live tissue will grow to the center of the old wound, and no wood will show.

As long as the apple tree is healthy, you can practice watchful waiting. Arborists used to recommend painting over the cut with a special paint intended to serve as a tree Band-Aid. Occasionally one still sees a tree wound which has been painted over.

After a few decades of painting the open sores on trees, arborists learned that they were often doing more harm than good. If the wound is sterile, the tree paint separates it from any bacteria which may be floating around in the air. The contrary is also true. If there are bacteria on the wound surface, the paint seals them in, where they can grow undisturbed. When arborists realized that tree paint sometimes promoted bacterial rot, they stopped using it. Most tree wounds are healed by natural processes if the tissue is exposed to air.

If a pruning cut is horizontal, it can leave a wound where water collects. Standing water on the cut surface encourages bacteria to grow there. If your old pruning cut serves as a dish to collect water, you could help the tree by providing drainage.

Our dry climate benefits the healing process for trees. We do have fireblight bacteria in season, but several kinds of rot bacteria do not flourish here. The complications which attend a big pruning cut, though, do provide an illustration of why it is better to cut small and often. Smaller pruning cuts made earlier in the tree’s life heal far more quickly than big ones.

Proper apple tree pruning

Q: I think that I have just ruined my three-year-old apple tree. I pruned off a couple of very low branches, but I have never cut the central stem or leader. I have been looking at instructions on the internet which tell me that I should have cut it off last year. The tree is about 10 feet tall. Can I do anything to fix the mistake?

A: Yes. Disengage the panic button and quit worrying. You have not hurt the tree. There is no rigid system which is the only way to prune a tree, apple or otherwise. There are some general principles for pruning fruit trees. However, they tend to be modified or corrected every 20 years or so.

Remember that trees are individuals. They do not look exactly alike. They are shaped by wind, weather, and soil, as well as by pruning. They may be further shaped by unexpected attacks from deer, porcupines, voles, fireblight bacteria. The list goes on and on.

Why prune your apple tree? Because you want to harvest lots of apples for years to come. If you do not thin the branches, the tree will grow into a thick bush. The center will be too shady to ripen fruit, and your apple supply will be small. If you do not shorten the height of the tree, you will eventually need the local fire department and its ladders to reach your apples.

You noticed that some branches were too low and correctly cut them off. In 10 years or so you may think that more branches are too low. It would be fine to cut them at that point.

At 10 feet, you may well want to start limiting the height of your tree. Start the process by cutting off the top of the trunk. If you cut just above a branch, you will encourage that branch, and perhaps the next one down, to grow bigger. If you cut just above a bud, that bud will begin to develop into a branch.

Your apple tree is going to keep growing, and you will want to continue shaping it every year. Cut off branches which grow toward the center. That will help sunlight to penetrate to the middle. Cut off all the vertical water sprouts which start every year. They will make the tree too tall and will not bear apples. Look at your tree every winter and decide whether you like the shape of its framework. Prune off or leave branches accordingly. Let the tree grow at its own rate. With only three years in your landscape, it is still very young. It probably will not make its first apple for another two years or so.

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