Those are awesome clumps of Stella! Eventually they will need to be divided and let me tell you, you're in for one heck of a big job! Simplist thing to do would be to use a very large, sharp knif and quarte the entire clump (once you manage to lift it out of the ground with a spading fork!) I'd also recommend doing it in the early spring right when they first start to grow, will make it a lot easier to find the beginning and the end of the clumps.

As I'm sure you know, Stella grows in a very tight clump and increases at the speed of sound! The longer you wait to divide Stella the more difficult the job becomes. Fortunately as long as the plant is blooming profusly and reblooming well, you can let it go. The simplest way to dead head Stella is to wait until all the blooms are spent then cut the scapes off. You could probably use hedge shears as it wouldn't really matter if you got a little foliage, ithe plant would quickly replace it.

To get better rebloom, you need to keep the plants well watered and fed, water being the more important of the two.

Stella should continue to produce blooms until a hard frost kills the buds/blooms. But I have seen them still trying to bloom up until a hard freeze puts the plant into dormancy!

Stella is EVERYWHERE and I do get a bit tired of seeing it used, almost exclusively in commercial applications, but it's still the best daylily on the market for that purpose, IMHO, with Happy Returns coming in second.

BTW, I have finally gotten down to only two clumps of Stella and perhaps a dozen little plants from tiny fans left over from my fall dig last year. Surprised me to see how many made it through the winter and actually produced bloom. I just scattered them here and there and barely covered them with a bit of earth scratched over them. Tough little Daylily!