How do you all approach plant height? I'll use Astilbe chinensis 'Purpurkerze' PURPLE CANDLE as an example.

http://www.missouribotanicalga...
Chinese Astilbe (Astilbe rubra 'Purpurkerze')

Per MOBOT it "...forms a foliage mound (to 24” tall)..." with flowers on "...branching stems rising to 42” tall..."

As you can see the main data heading notes Height: 2 to 4 feet.

I would say the highest point possible of the plant is the height. So if it usually is 24" tall and then during bloom time goes to 48", then the height is 48" (and maybe with a comment posted at the bottom detailing this feature.)

A comment on one of the forums brought this to my attention. An individual noted their Astilbe grew to 4', in bloom, and she had only planned for a 2' plant. She seemed a bit peeved. Sooo 48 inches it is.

For daylilies, we use the height of the bloom stalks.