Answer: Unfortunately, in my experience the maple seedling/saplings tend to grow faster than the hedge, too! You will probably need to work with this for quite some time. First off, pull out any new seedlings as soon as they appear. Then maintain a mulch layer several inches thick to try to inhobit germination and make pulling any seedlings that do occur easier.
Cutting the trees will cause them to "sucker" and come back even thicker than before, so this is not really a solution. What you need to do is either dig them out by the roots (not very practical) or use an herbicide; in this case glyphosate applied to the "stems" of the maples according to the label instructions for woody plants should work.
Alternatively you can cut them off at the ground repeatedly and hope to eventually exhaust the root reserves. You can also try to smother them by cutting them off at the ground, then covering them with a thick layer of cardboard or newspaper and then topping that with several inches of organic mulch such as shredded bark. Should any make their way up throught all of that, simply repeat. Try to keep the mulch from touching the stems/trunks of the hedge, and be aware that the smothering process will also inhibit new shoots trying to sprout from the hedge.
Reducing the number of maples and their competing roots should help the hedge to grow better, as will maintaining a mulch layer. An occasional topdressing of compost as well as some balanced fertilizer such as a granular 5-10-5 according to the label instructions in early spring and late fall should also help. Finally, regular pruning beginning with cutting it back hard to rejuvenate it and then regularly maintaining it thereafter will also help. When you trim the hedge make sure to make it wider at the base and tapering toward the top so sunlight can reach the base of the hedge -- this helps to keep it thick from top to bottom.
You may also find, however, that the same tree is now shading much of the hedge, or that its roots and canopy are competing with the hedge for light and water and nutrients. If this is the case, it may be time to consider other options such as a fence or wall or a fence covered with vines instead of the hedge at least in some areas.
Good luck with your project!
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