I deal with higher temperatures than you would, but I have been growing Rose of Sharon's for almost 30 years. Although they can tolerate different conditions. I find they really like water, especially the double varieties. I have planted them fairly close to roof drip lines and near the water spigot so they would get hose run off. I think the sprinkling can may not be providing enough water to the deeper part of roots. Try doubling or tripling the water. Wait 15 to 20 minutes then water a second time, and another 15 minutes to water a third time. Then I would take regular miracle grow, mix at a light strength and pour that around the base of the plant. After they are 4 or 5 years old they are better about not dropping buds and leaves. You should be noticing transpiration on the Rose of Sharon leaves, if it needs water. If you are going through daily transpiration cycles. (Meaning the leaves droop alot during the day but perk up at night) The plant leaves will turn yellow and fall off. I would assume at 3 years your Rose of Sharon should be at least 4 ft?
