Most of my daylilies are in pots, mostly 2-gallon, 3-gallon, and 5-gallon black nursery containers. I've tried larger containers, but the small difference in growth isn't worth the expense of filling them with potting soil, since the soil breaks down and requires replacement every couple of years. The 5-gallon nursery pot seems to have the ideal height-width proportion for good growth. Because it's dry here, I have saucers under most of them once they become root-bound.
