Answer: Vine maples can develop crispy leaves it hot summer sunshine. Since your tree is in a planter, the roots can get quite hot, too, which will affect the foliage on your maple. If you love the site you've placed your tree, you might be able to shield the planter by grouping some smaller pots on the sunny side of the planter to help shade the roots and keep them cooler. Or, if you don't mind moving your maple, try to find a spot with morning sunshine and afternoon shade.
Another thing you might try doing is flooding the soil to leach out any accumulated salts (which can also turn the leaves a crispy brown). This is how I do it: water your tree as usual, then flood the soil until water runs out of the drainage holes and no more air bubbles appear at the surface; wait 15 minutes for it to drain, then flood it a second time; repeat a third time. This will leach salts out of the soil and also eliminate any air pockets that may have developed around the roots.
I'm sure you'll see an improvement after moving your vine maple to a shadier site, or protecting the planter from hot summer sunshine and flooding the soil periodically.
Best wishes with your garden!
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