Overgrown, 50 year old taxus yews. If they are cut back to stumps, will they rejuvenate? Best season to do this?

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Answer 1

Don't count on it!. That is a very extreme measure and will create physiological stress which the trees may not be able to recover from. Not to mention - ugly.
It depends on the water availability and soil fertility. The poorer the growing conditions, soil and water management, the greater the chance of mortality.
Professional arborist best management standards for pruning are to not take out more than 25% of the foiage and wood in any one to two year time frame, especially for most conifers.

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