In my unirrigated nursery, I put a layer of thin plastic at the base of planting holes to steer roots outward instead of downward in the pursuit of water. When I first tried this I feared that interfering with drainage might be very harmful- instead it turned out to be highly useful. There is heavy root colonization of soil contacting the plastic, apparently to sip the water that his trapped there. My trees grow more vigorously with the plastic than without and they are also a lot easier to dig.
I wonder if this method could be of use in growing plants like Musa.
As far as FN's comments about mulch, I believe that some mulches function as reservoirs if they hold a lot of water as do rotting wood chips. I believe the water is pulled by capillary action into the soil below as plants remove water from that soil. If I'm correct (and I've no research to back my belief) the more mulch the greater access to water. It wouldn't even have to be capillary pull, mychorizal fungi could also deliver the water to plants.