maple tree helicopters
Thanks for using Ask a Master Gardener with your mulch question.
Generally, mulching should be done after the soil warms up, even though we have seen that the habit has become to apply mulch now while the soil is still cold. That habit causes the plants to slow down breaking dormancy and new growth. When I apply mulch, it is usually late May of early June.
That being said, your specific question is interesting. If you apply mulch before the seeds drop from the maple, then you have a more difficult time cleaning up any seeds to prevent the germination and resulting weed trees. If you apply the mulch after seed drop, then you do have the opportunity to clean up those seeds before applying the mulch, resulting in less weed problems. Failure to clean up the dropped seeds before mulching actually would encourage the seeds to germinate since they would have good contact with soil or organic matter (the mulch).
I was not able to find any research based information on this specific question. I believe the choice would be up to you as when to mulch to minimize your work load on weed maple trees that need to be managed. Soil temperature being taken into account, the direct answer could be after seed drop, since that occurs after the soil has warmed.
Here is a fact sheet from Penn State dealing with proper mulching of trees:
Please note that the mulch should be 2-4" and never be placed against the trunk of a tree. I hope this gives you direction. Thanks for your question.