Answer: You are probably right on both counts, shade and tree roots both will slow growth. There might also be some salt washing onto the soil from the road? Maple trees are notoriously difficult to garden beneath due to their greedy roots and dense canopy. And any time a long row of plants is planted in different growing conditions from one end to the other you will see a noticeable difference in the line of plants. Unfortunately, there is not a lot you can do about it. Even if you fertilize with extra nitrogen to compensate, the maple roots will just grow that much faster, too. You might consider using a decorative fence at that end, starting it about where the hedge plants seem to deteriorate. That way it would look symmetrical. I'm sorry I can't be more encouraging.
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