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[Alan Haigh <alandhaigh@gmail.com>]

Have no fear of pruning your N. Spy right now.  A vigorous old tree can be pruned just about any time that the weather isn't going to drop below -25 degree F. within a couple of weeks and I don't know if that would hurt it- it's just the critical cut off according to the literature.  I have done major pruning on many varieties of old apple trees where the temps have fallen below -20 shortly after with no cambium damage around cuts.  As far as summer pruning, it is only a problem with young trees that you are trying to grow as fast as possible or trees with inadequate vigor.  Summer pruning is more de-invigorating than dormant pruning which can be a good thing.

 

I have found that even major pruning done at this time is actually safer than dorrnant pruning as far as preventing scorched bark on upper scaffolds.  I believe this is because there is so much leaf development at this time that adequate sap gets pulled through the cambium to keep temps down even when a lot of wood is removed.