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Being spring in my part of the world, I have recently been pruning my young trees and roses.

Having read and watched a bunch of educational content to remind myself of the do's and don'ts of pruning. I often hear about how you should either correct or prune branches/stems which are either too vertical (because they will hog all of the sap flow) or too horizontal (because they will not get enough sap flow) obviously taking into consideration whether or not your tree/bush has a central leader. Ideally, all remaining branches should be 45 degrees off vertical, or as near as possible.

I'd like to know if that is factual, and also how sap flow relates to angle in that way. I'd have thought that because of gravity it would be the opposite!

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It turns out that IS factual and the science to back it up is as follows:

We know that sap flow happens because of capillary action inside the plant's capillaries. Capillary action works optimally in a vertical capillary (one thats pointing straight up) and increasingly poorly as the vertical angle of the capillary decreases (as the capillary angles toward the horizontal)

This is because the upward force experienced at the meniscus of the liquid in a capillary is directly related to the contact angle of the meniscus on the walls of the capillary. If the contact angle varies around the meniscus (as it would when the capillary is tilted away from vertical) then the resulting upward force is sub-optimal

The math is a little hairy, but this video explains the principal quite well

https://youtu.be/PKJ6Ai21JuE

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