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I have some fruit trees on dwarfing rootstocks. By the nature of the rootstocks they have a lesser overall root foundation than other trees. If I apply bone meal or other things that encourage stronger root development, will this in fact increase the root strength of my dwarf trees? Would it also potentially cause the dwarfing nature of the rootstock to be negated because the tree has a better root system than the dwarf stock was intended for?

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The answer to the first question is two questions: Have you had a soil test done? If not, do so before adding any soil amendments. If you have, is your soil deficient in calcium and phosphorus? If yes, will you be adding only the amounts recommended in the test? If yes, then the roots will benefit.

A short answer to your second question is "No", but the long answer is more interesting. Surprisingly, scientists have a couple of good theories as to how dwarfing rootstocks actually regulate the growth of the scion, but no actual 100% ironclad scientifically proven mechanism. Personally, I suspect that there are multiple ways in which this occurs. Many of the theories involve hormones that regulate scion growth. See here for an interesting paper on the subject.

A couple of quotes from the paper:

By reducing tree size, dwarfing root-stocks produce less vegetative growth and yields, on a per-tree basis, are lower than those for a tree on a vigorous rootstock

...rootstocks differ in their inherent capacity to produce hormones and the ways in which these biologically active substances are transported within the root-stock and/or scion.

Dwarfing rootstocks dwarf scions due to genetic reasons and not due to any environmental pressures. So, adding bonemeal to your trees' soil will have no effect on the growth rate or overall size of the scion.

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  • As a follow up question, given all the criteria, if it's needed and it improves root growth on dwarf rootstocks can it improve it enough to no longer need permanent staking? Typically dwarf trees need permanent staking because their root systems are pretty underdeveloped. Dec 1, 2022 at 20:50
  • No, you'll probably still need to stake the trees, even if they are phosphorus and calcium deficient at the moment. The weight of the fruit, IIRC, in combination with a smaller root run than on a standard tree is the reason dwarf trees need to be staked.
    – Jurp
    Dec 1, 2022 at 22:47

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