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I have a pair of large (roughly 30' tall I guess) cherry trees. They're mossy. They don't produce well (at least not for the 2 seasons we've had them for). What they do produce is so high that I can't net or harvest the fruit safely. The branches are a mess.

I don't know the age (we've only owned the home for a year and a bit), and I get the impression from our neighbors and the overall state of the yard that the former owners weren't all that concerned with upkeep.

The birds get most of the fruit now, but the few cherries that are low enough to harvest are delicious.

  1. Can I aggressively prune them? When, how, and how much? Am I hoping for too much to want to get the trees down to about 15-20 feet tall?
  2. Is the moss a problem? Should I just pull it off, or do I need to treat it with something?
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The type of pruning you're looking for is called crown reduction. The main purpose of this is to reduce the height of the tree by cutting down its crown, as explained here.

Since you say your tree is 30+ feet high, it is advisable to get professional help instead of venturing out to do this on your own. Note that the pruning cuts that will be made are quite large and could lead to diseases in the tree. This can be mitigated by sealing the wound quickly with tar.

Crown reduction is inherently aggressive and could send the tree into a temporary shock. I would suggest that you reduce the crown and minimally prune the smallest of the messy branches the first year. The second year, you can remove more of the smaller branches (as you would in a normal pruning).

You will have to be patient with the tree and give it a year or two to recover and start producing a bountiful crop.

enter image description here

Image source: See link above

As for the moss (probably lichen) on the trunks, it shouldn't be a problem as the thick bark provides protection for the tree. There might be cause for concern if it spreads to the leaves, as it could be symptomatic of something else and prevent respiration/photosynthesis, but until then, it should be fine. I personally think trees with a bit of green moss growing on them look pretty.

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    +1 for linking to "How to Prune Trees" by the Forest Service. "This can be mitigated by sealing the wound quickly with tar", nowadays by the vast majority of arborist's it not recommended to seal tree prune cuts (if those cuts are done correctly). Tar sealing is "generally" considered old-school. – Mike Perry Jul 24 '11 at 21:56
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Mike Perry is right about NOT sealing wounds with tar or any sealant. This has been the unwavering advice for over 30 years, by tree care researchers such as the Forest Service and university horticulture departments. Look at it this way--you don't seal a human wound, you allow some bleeding to clean it, apply a modern germ killer (the old iodine and mercury solutions killed tissue as well as germs), and then encourage the body's natural regrowth of tissue. The same is true of trees.

When removing branches, cut very near the trunk, but beyond the first 1-2", called the "collar." This is where the healing cover called the "callus" starts to grow. Tree wounds' greatest enemy is fungus diseases. These are more likely to overwhelm the tree in big horizontal cuts, especially during damp weather, which fungi love. As you can see, height reduction, though sometimes necessary, is a long-term risk to the tree's survival. Still, you could drastically shorten and thin this tree, hoping it can cope with that--and plant dwarf or semi-dwarf fruit trees, allowing space to remove the older tree later.

Lichens are free-living, and get their nutrition from algae contained in their fungal walls, NOT from the tree! They grow thickest on older branches that are more open to sunlight—which is what makes it look like they are killing the branch. But they are only taking advantage of its increased sunlight. Some of these grayish lichens are very tasty to deer, which might be why they evolved to grow up in trees, not low down on rocks. They might block some sunlight from ripening fruit, though I haven’t seen or heard of this.

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