Most of the leaves on my potted Japanese red maple would turn brown and dropped after first breakout from winter dormancy. This has happened the last two years on the 5 gallon and the l gallon pots. The plants are on the south-facing balcony, so there are at least 3 hours of sun daily. I kept the plants well watered. The Japanese green maples are thriving under the same conditions.
It is a *general* rule that green leaf acer palmatums are more sun tolerant than red leaf acer palmatums. Even though your red leaf maples are getting only 3 hours of afternoon sun it is obviously enough to cause damage. You don't say where you live, but if you live in a southern region this would be more of a concern than if you lived in a northern region. Afternoon sun is more intense than morning sun. Most maple people suggest morning sun, afternoon shade. Since moving your maples is not an option you may think about putting up some kind of shade cloth or screen to protect your red leaf maples from the harsh afternoon sun. Perhaps those roll up bamboo or white cotton window shades. Layne
