Is there a rough limit to how think a deciduous branch (e.g.
mulberry) can
be that can still be air layered? Is 3" too big?


Probably not for an air layer.


Also, does the cut still only need to be about 1-1 1/2 " even
for thicker
branches, or should it be proportional to the diameter?


Generally the cut should be at least as wide as the branch being
layered, though 3" may be excessive. Try 2".

Jim Lewis - - Tallahassee, FL - Our life is
frittered away by detail . . . . Simplify! Simplify. -- Henry
David Thoreau - Walden

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