It my understanding that daylilies increase during periods of active growth and I have found that to be true in both my Michigan garden (zone 5) and my North Carolina garden (zone 7). One period of active growth begins two weeks after the daylily has stopped blooming as daylilies "rest for a bit" after their bloom. During that rest period, it is a good time to divide them and line them out unless you are living in the deep south where rot becomes a problem if you divide in hot weather. Early bloomers will increase at a different time than late bloomers....but they both will be increasing 2 weeks after their bloom has ended so I'm lining out all summer long for optimum increase as I'm fertilizing line outs during that period of active growth and watering them well! The other period of active growth is in the spring when plants are coming up. Increase may not happen if plants are not well cared for....watered, fertilized, etc. or if you have selected a daylily that is not suitable for your climate.
