For Japanese plum (and other prunus) I read we want 3 or 4 main scaffold branches off the main trunk at between 18 and 28 inches above ground. My newly planted 28 inch "whips" (just one small trunk, no branches yet) have 20 - 30 buds growing fast (each about an inch long). Should I keep 3 or 4 of those buds and remove all the rest right now? Or is there a reason to wait? Seems a little dangerous to bet on just 3 or 4 of all those buds right now.
Most of the shoots will probably only grow a few inches long and then stop growing. A few of the shoots will probably become dominant shoots and put on the most growth and become the main future branches of the tree.
Pruning at the time of new growth can take a lot of energy from the tree, which is especially needed in a younger tree, since it has more limited resources than a larger tree. I would wait until the dormant season and see how the tree takes shape its first year before doing any pruning.
As far as pruning goes, it seems like everyone has their own personal pruning preferences. I personally like minimally pruning my fruittrees, and have had really good results with it so far. Most of the trees have grown really quickly and are extremely healthy.
It sounds like you have some nice healthy trees, hope they continue to thrive!
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It might be good to add your area or climate to your profile as it can change the advice sometimes. For example in my cold maritime climate we try to avoid pruning prunus species in winter so they don't potentially become prone to silverleaf disease. If it's a first year whip I would just cut the only leader you have got to a height you desire and leave it to branch out. This would be done for me on a decent day in spring to help avoid any disease issues and still get it to adapt to its new shape as early as possible.
Also if any fruit sets for the first few years remove it as it can affect the future development of the branch.
Thanks, guys, that all makes sense. Henry, our exchange in terms of what the trees are actually doing (budding) can then apply to any/all trees doing same at any/all locations/seasons, it seems. The low-chill varieties selected for zones like mine (9a/9b) are a little crazy - flowering multiple, unpredictable times through winter/spring. I had a few plums ripen in December! Now more flowers! I think we can master the complexity of biodiversity and climate diversity with some persistence!