There is no way a pre-emergent herbicide can distinguish between weeds and what we may consider non-weeds. They are simply all broadleafed plants and the effect should be the same. That's why it is not recommended to use pre-emergents where you will be sowing seeds or allowing self-sown seeds to germinate. I would attempt using Preen or some other pre-emergent before I'd resort to RoundUp, even in clear areas under the tree canopy. There is some evidence that repeated use of RoundUp across a tree's root system can have damaging effects.

I realize you may be dealing with a great many seedlings, but I'd recommend manual removal over and above anything else. I deal with freely seeding maples myself and understand what a PITA it is to get rid of them. Raking up the samaras when they fall would be my first step. The less of these that remain on the ground, the less germination that will take place and many fewer seedlings to deal with.

I second the idea of plucking them manually. Chemicals just aren't that specific. Think of it like spreading around rat poison and anti-freeze in the garage to rid yourself of a rodent problem. Them other plants your trying to grow there would be like letting your 1 year old play in the same garage. They're going to eat all the poison also.

Perhaps to give SOME silver lining to the trouble you're going through - I understand the pain, soon as my knee permits I have a number of ash seedlings to pull up - put a couple of them maple seedlings that come up intact in left over nursery pots and trade them to your neighbors for hostas or whatever you want more of.

Are you absolutely sure it's a red leafed Japanese maple and not a red leafed variety of another maple species? Each spring I find perhaps 5 or 6 seedlings from a 25 year old 20 foot Bloodgood, but that's it. A large number of seedlings would be much more common if it were another species.

If it really is a Japanese maple, the samaras hang on the branches for quite a long time. It should be fairly easy to take them off so they don't fall to the ground and germinate.

Mainegrower, it is a Bloodgood Japanese maple and is about 16 1/2 years old. From the other comments received, it sounds like there is no easy way to prevent the germination as Preen was not effective from last year, unless it was the timing of when the Preen was used.

With respect to using Roundup on the mulch in the open areas, there has been no ill effect on this tree or surrounding plants all of these years, so I will still continue to use that method. I too would prefer to not have to use it, but this bed is just one of two huge beds, each 90 feet long and 35 feet at their deepest points, not to mention other landscaping. Having to maintain them, work full time, and attend to other responsibilities, sometimes one cannot be as environmentally correct as desired.