I have a similar problem with hydrangeas. I've been debating whether it is better to put down more water at once during the day (get those roots to keep reaching down?) or add water again later in the day (if I do this, aren't I just refilling the top part of the soil where the ground is drying up? Are the roots down deeper just sucking the water up and if the second watering isn't the full amount, is it going deep enough?). And is there a good way to automate this so I don't have to monitor on a daily basis?
I'm sure there is some research out on the web if I can just find it, and then understand it.
What have you been thinking?