· Brent Wilson's Answer
· Brian is right. Here's a few more tips:
The browning tips of leaves could be caused by consistently wet soil or from leaf scorch caused from too much direct sunlight, heat, or from water on the foliage during the heat of the day.
When growing Japanese Maples in containers you want to keep the soil moist or damp, but not too wet. Too, make sure the container drains well from the bottom. When growing Japanese Maples in contaniers, I usually use a soil mix that is about 1/3 native soil, 1/3 premium potting mix, and 1/3 regular grade potting soil. This, combined with adequate drainage holes in the bottom of the container, should provide good drainage.
All day direct sunlight can cause scorching of the leaves during exceptionally hot summers. If plants are watered during the afternoon hours, and water gets on the foliage, this can cause scorching as well. As Brian already said, Japanese Maples prefer morning sun with afternoon shade or dappled/filtered light all day.
I have several Japanese Maples growing in large containers on my back deck. These are all over 5 years old. I remember back during the first couple of years they had problems with leaf scorch and the tips of leaves turning brown. Younger trees seem to be more susceptible and won't tolerate soil that is too dry or too wet or sunlight that is too hot. Seems that once the trees are more established and mature, leaf scorch or browning isn't as a much a problem.
What to do with your trees now? I would say to monitor watering closely for the rest of this season. Apply a layer of mulch atop the soil for winter and don't water near as much, only if soil becomes dry. Next spring the foliage should come out fine. If your trees are in full sun, I would suggest moving them to a location where they will get some shade during the mid afternoon hours.
Hope this info helped.)