With the mulch I'm already over 4" deep! I'll continue to experiment. If I get roots big enough to split this fall I think I will go deeper with some. Our issue is that it doesn't get that cold during the winter, we truly are no colder than zone 4B. Our problem is the inconsistent spring. I'm not sure I can plant deep enough to keep them from experiencing our crazy springs without sacrificing all of the blooms.
