In regards to the height I'd think there might be at least two things to consider. First would be if the plant cooperates and actually reaches its registered height. My growing conditions often inhibit plants (not just daylilies) from managing that. To say they are inconsistent would be an understatement.
The other thing would be the foliage height. Isn't registered height only considering the scapes? Even the descriptions of daylilies often refer to 'small fans' as part of their description implying that the foliage among daylilies varies. I'm growing UFs and spiders mostly. There are differences in the foliage among the plants even when you separate them by ploidy, which makes for another difference on what I'm seeing so far. To me there is a lot more difference between tetraploid vs diploid fan growth and not much at all in how the blooms look and fare here. I think those growth habits might mess with the best laid plans
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