It can be a tricky proposition to rely on deciduous trees to remove excess water. Just because a plant looks dormant, it's roots are not necessarily not growing. A worst case scenario would be planting a willow anywhere in the vicinity of a septic system or a leaking pipe/drip system. In Jim's part of the country as well as many others Acer palmatum or other small statured Acers should do well. I have a raised bed made of corrugated steel sheets that measures 4' wide, 3 1/2' deep and 20' in length in which I grow vegetables. It took a year to fill as it started as a compost pile, and additional material was added as soon as available.It is constructed with a solid sheet of the same material as a bottom, unattached. Drainage is excellent, but I noticed that growth had all but stopped no matter what kind of vegetables I planted. Digging around the outside perimeter I found that the ideal conditions in the raised bed had encouraged roots to encroach from a tree 30' away. They had entered at the junction of the walls and the bottom. Having been cut off a couple of feet away from the bed and a trench 1 1/2' wide and a 1' deep filled with sand for easy checking of encroachment, I think I can control them in the future. Practicing the French intensive method of vegetable gardening, I will have to wait for the severed roots to rot in place. The fine feeder roots have become a solid mat in the planter, to hard to dig out. I punched as many holes as possible ( I got tired), and added 5 pounds of earthworms. Let them do the hard work. Gary in Hilo, HI