Answer: Droopy leaves on your maple may indicate heat stress or water stress and it's sometimes hard to distinquish between the two. If it started out healthy in the spring and begins to look a little stressed in the heat of summer, I'd suspect heat stress, especially if the tree looks fine in the morning and then begins to droop in the afternoon.
You can help your tree by making sure it has an adequate supply of moisture - but don't overwater; that can exclude oxygen and defeat the purpose. Try watering your tree deeply, then waiting 3-4 days and digging a hole near the roots. If the soil is moist 3 inches below the surface, you won't need to water for a few days; if it's dry, it's time to water again. I build watering basins beneath my trees. Once each week I flood the basin, allow it to drain, then flood it a second time. This concentrates the water directly over the root mass and allows it to trickle down and wet the entire root mass. Give this watering technique a try.
Best wishes with your landscape!
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