Trees, Shrubs and Conifers: maple pruning, 1 by Weerobin

Weerobin wrote:
I have a couple maples which are in dire need of judicious pruning.
I know the prohibition against pruning maples in spring, since the sap is 'running'.
Can someone tell me when it's safe to proceed?
Here's a shrubby japanese maple which is remarkably congested.
During mid-summer and fall, the foliage is too thick to see the branch structure.
Now, with the branching structure visible, I'm eager to thin it out.
But I hold back because of the 'sap running' issue.
Should I go for it, or wait?
If I wait, how long?
Here's a shot of crown of the tree.