Andrew:
Very easy. Drill, or melt about 6-8, 1/2" - 3/4" holes into the bottom SIDES of the bucket. Don't drill holes into the bottom, only the bottom-sides. Why? If you sink the buckets into the surrounding soil, the roots will wander out of these drainage holes, and eventually grow into the soil. When the time comes to lift the buckets out of the ground, all you have to do is run a spade down the sides of the bucket and sever the roots. If you drill holes into the bottom of the bucket, the roots will be growing out into the soil and you will not be able to sever the roots without first digging out the bucket. You will curse a lot...and, wish you put holes in the correct spots. I made this mistake. I used to dig out many buckets. I made up some curses that could be in the Book-of-Records.
Bottom-sides only, and you're done.
Frank