maple pruning

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.

Another shot of the incredible thicket of branches.

By contrast, here's a shot of one of several other dwarf Jap maples,
which typically develop a few well-demarcated structural branches.

I'd love to go ahead and thin out the excessive twigginess of the above maple
before the leaves obscure the branching structure.
Is it safe to proceed now?'

Now isn't a good time (except for removing any dead twigs), it'll bleed sap profusely.

Personally, I'd not bother with pruning at all; as it grows, it'll self-thin to a large degree, with inner heavily shaded twigs dying out; then you can remove the dead twigs as/when they die off. The one in the bottom photo is more open at the base because it is much older.

Resin

weerobin i will have to disagree with resin - now when the tree is dormant is the best time to prune the maple - and that one looks like it will need a lot of thinning - i would not do it all at once and spread it out over a few years.

you may want to post this question over on the jm forum as well.

this link was just posted on another gardening site about prunning maples.

http://www.finegardening.com/how-to/articles/pruning-japanese-maples.aspx

Late winter now, maples are not dormant any more!

Resin

they are here

Have they started harvesting maple syrup around you yet? If yes, then the maples are no longer dormant. Dormancy in maples ceases long before leaf-out.

Resin

no maple syrup havesting here yet.

I'm surprised! I thought it would have started by now. This page http://ohioline.osu.edu/for-fact/0036.html says "on or before a certain date such as the second or third week of February". It has to be cold enough to stop the sap fermenting before it gets to the evaporation trays.

Resin

There's some maple syrup collecting going on in Cincinnati, OH this week. I'd suspect that Massachusetts won't be far off, but Vermont is going to have to thaw some (likely).

Then again, wha is practically a Green Mountain Boy...

Resin, the two trees I posted are roughly the same age.
I have often witnessed the self-pruning tendency of most JM's,
but the first one shows no sign whatsoever of self-pruning.
I didn't prune either one of these trees.

I'm probably ahead of wha, so I think I'll hold off on pruning.
I'm not sure when I'll be able to tackle it. It really is too dense in full foliage.
And either the weather isn't inviting or I just never think of it mid-winter!
Thanks for your help.