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I have been reading about how to prune grape vines and olive trees, and in multiple occasions it is recommended, when performing a cut, to leave a short collar from its base, (possibly in proportion to the diameter of the branch being cut, usually around 1-2cm), for these two reasons:

  • to avoid inner branches from drying up, since as the sap-flow retraces, branches at the edge might be left outside the (modified) flow.
  • to protect the base branch from fungi and parasites in general, as the small leftover protuberances drying up, are unattractive for parasites, so it performs a function similar to the bark.

Is this correct? Can this be considered a rule (of thump) that works on many plants of ligneous structure?

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If we examine the structure of grape vine shoots closely we find that the internodes have an apparently hard, mature, woody exterior, but the interior pith is very soft and vulnerable to drying. So while the exterior wood can resist drying the bud may still desiccate from the inside if it is cut too close. Leaving a small extension allows a little interior drying back, but as it does so the pith hardens and so protects the proximate bud tissue from drying.

Timing of pruning is an important consideration; grape vines tend to bleed quite profusely once the sap begins to rise, so desiccation is less of an issue if the pruning is late. Winter pruning is more convenient since there is more time, so having the cut ends standing vulnerable in low humidity conditions makes careful pruning more of an issue.

If we compare the grape vine to, for example, apple wood we see that the apple has a dense pith, so is not as liable to drying therefore close cutting is permissible and preferred since new wood can completely enclose the cut end, reducing risk of infection. So one of the factors is soft pith which in vines is more evident in young wood than in old.

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