Where are you located? It's hard to imagine that watering for less than a minute for a tree planted in mid August could possibly lead to over-watering. In view of the limited info provided, I'd guess you haven't been watering enough!!

Has it rained in the time you planted it? Watering for less than a minute doesn't sound like enough water. I water my newly planted trees for an hour with my hose sprayer to remove the air pockets in the soil. You wanna flood the soil around the tree to get the air pockets out of the soil. You don't need to stand there for an hour you can attach the hose sprayer to a hand truck or something similar. Sunlight doesn't sound like the issue since Japanese Maple are naturally found in shaded location under forest tops.

I live in zone 9b in Elk Grove. No rain. The soil is very moist but not sopping wet down deep around the tree when I checked. It was fine when it was still in the pot and watered everyday. It started wilting about 2 days after planting in ground.

Something is not adding up......no rain and less than a minute of watering do not translate to very moist soil :-))

New plants need long and slow and deep watering. An hour for a small tree is overkill but they do need a good deep soaking, right at planting and then at least biweekly until the rainy season hits or the tree goes dormant. I'd guess a good deal of the wilting is due to transplant shock, which is almost impossible to avoid when planting at this time of year. Watering and some protection from afternoon heat (which it sounds like you have) are the only ways to overcome.

My experience is with a much younger Japanese maple, but similar growing conditions-mounded topsoil on top of clay soil, partial sun. I planted it this spring. I try to water mine lightly every single morning, and leave the heavy soakings to mother nature unless she is falling down on the job. By lightly I mean enough to fill the shallow cavity I created around the tree when I planted it.

Daily watering is not doing your tree any favors. It doesn't encourage any deeper root penetration and makes the tree more susceptible to drought or dry conditions. It is far better to water less frequently but slowly and deeply when you do. And it is risky to rely on Mother Nature to address the problem fully. Until the tree becomes fully established - not for at least 1-2 full growing seasons for a small tree - it is much better for the long term health of the tree if YOU assume the responsibility for proper watering and just let Ma Nature supplement as she sees fit :-))

My tree is very small and recently planted so I am trying to avoid leaf scorch because if I don't, I may end up with no leaves at all. And to do that, I have needed to water frequently in this very dry summer. Maybe it's because my soil mix is too loose or my mulch is too thin, but that is what the plant needs right now. And I do water it deeply once a week if rain does not come. I plan to ween it slowly to a normal watering schedule this fall. If a tree is used to daily watering in the pot, it seems logical to me that gradually retraining it to conditions in the ground would be less damaging than abruptly altering its whole world. I think I read somewhere that Japanese Maples require consistent moisture levels.