I wonder if TAMU has some testing available for ornamentals rather than forage? I think I may check into that.
The 2nd link suggested a cause might be 'over fertilization'. I have, in fact, wondered about that. Especially on the seedlings. But it's true, a container plant pretty much only gets what its given and its hard to know how fast what is given is used up. Especially when they get water as often as the daylilies do. It's not the same as being grown in the ground. There are both positive and negative aspects to container growing.
