A 1/4-1/2" is just fine. These seeds germinate very easily and quickly. I've grown them for the last few years. This year I had an abundance of seed so I sowed several seeds into each hole in cellpacks so that there was a nice clump when they came up. When they were about 4 inches tall I tranplanted them into 6" pots-two cellpack clumps each. This will give me a nice large clump to put into my large pots and half barrels. I also sell some at my yearly plant sale.
They will grow green at first, begin to get a purple rib up the middle of the blade when they are about 6-8" tall and the leaf won't turn purple until they get larger and are set out or planted out in full sun. Then they colour up very quickly.

wyndyacre,
Thanks for the info. As popular as these are I was surprized I could not find the depth anywhere!!

Thanks,
Lee

Lee,
Interested in a seed trade? You've got mail!