Hugelkultur is an ancient method of raised-bed gardening, one that utilizes fallen wood. It has been used in Europe for centuries. In German it translates to "mound culture." Building one of these mounds takes a bit of work, but it will last for a long time, and it will be a self-watering, self-feeding, and self-composting raised bed! I have built mine in the vegetable garden, but you could have one serving as a perennial flower bed.