I’d say you can prune any time of the year. While dormant, (deciduous) fruit trees store their energy in their root system; in spring, it emerges to feed the growth above.
With a young tree, you don’t want to lose any of that stored energy, so it’s best to remove unnecessary material before Spring. An older tree usually has an abundance of stored energy. If pruned in the winter, it has stored enough energy to reinvigorate the ‘entire tree.’ So, if you prune (remove) a substantial portion of that tree, its stored energy will activate hundreds of ‘latent buds’ within the bark, sending up a multitude of ‘water shoots.’ Had you ‘Summer pruned,’ not as much energy would have been stored, thus fewer worthless shoots.
It’s easiest to prune when the leaves are off the tree, less damaging to developing fruit and foliage, and easier to see what needs remove. But – to keep the stored energy balanced within mature trees, it’s likely ideal to remove large suckers when they’re fully leafed out and ‘growing.’ Removing them while growing will cut down on the volume of energy stored over the winter; instead of energy returning to the roots, you will have removed it.
However… there’s a tricky balance to maintain, and if the trees are only marginally healthy, it may not be a good idea to do the bulk of your pruning during the growing season. Also, older trees often develop (especially in Oregon) a thick layer of moss on their scaffold (main) limbs. Removing it and exposing the bark beneath can cause sun scold that can seriously damage those limbs.
Rejuvenating a severely neglected orchard is a delicate and labor intensive process… The structure, damage, and vigor of each tree is a factor. I love to prune … but will often shake my head sideways while passing an old orchard... And, having dealt with the same, I speak from experience and not just theory.
I’ve read it suggested to ‘go slow,’ removing the largest ‘upright’ massive ‘sucker (trees within a tree) shoot’s’ one year, and continue at a pace both you and the tree can sustain. And though the work is daunting, the results can be magnificent, especially if the trees retain a productive base structure. The tricky part is making the ‘big cuts’ first, then thinning what’s left. And, making large cuts 8 to 10 feet up …then safely removing it from the tree can be very dangerous – or exciting!
A big question, and a big job. But it can be done, and any time would likely work.
Welcome to the HOS Forum