Answer: Unfortunately I am not certain why your tree changed this summer. Generally speaking when a red-leafed cultivar turns greenish in the summer it is usually related to too much shade. This would typically happen gradually over time as a neighboring tree grew and cast a larger shade pattern each year, or you would certainly notice if a new building went up and was suddenly shading it. Some of the red leafed forms will also turn greenish by mid summer -- especially in the warmer climates in more southern areas -- in response to the warmer summer temperatures, so it may have been related to temperature patterns this year. These trees do prefer a slightly acid soil. If for example you limed your lawn (surrounding or adjacent to the tree) heavily in the past year, it might possibly have an effect although it is not something I personally have seen happen. Lime works slowly through the soil so its effect would not be immediate, it would take some months for it to work down into the soil at the depth where the roots are. This could conceivably shock the plant somewhat. Lime moves through the soil slowly but surely so if you think this is what happened, all you need to do is wait for it to naturally decrease and allow the pH to drift down again to its previous pH. If the tree seems healthy otherwise, I would wait and see how it does next summer. In the meantime you could also ask your local county extension if they are aware of any other similar trees changing color this past summer.
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