When is the best time to trim a river birch?
Most trees should be pruned in mid-late winter. Insect and disease activity is not a threat at that time. River birch trees are pretty immune to most of these assaults and therefore can be pruned at other times of the year.
They "bleed" sap quite freely when cut in the spring. While this is usually not a problem for the tree, it can be disconcerting for the owner when sap drips on outdoor furniture, walkways and people.
The best time to trim River Birch is in the late summer - fall. This would be a good time. Never remove more than 1/3 of the crown. If your tree needs a serious haircut, try to stage the work over a year or two.
Here is a link to good information about trimming trees:
I hope this information is helpful. Please contact AaE again if you have further questions.