I've been wondering the same thing and today, I just couldn't take looking at it anymore and I cut off all the dead branches. Its such a puny little plant, I hope it ends up being a lot bigger than it looks right now. This is my first ES, so don't neccessarily do what I did as I have no prior experience. :)

Maybe someone else will post a better response.

I live in Florida so I have a totally different climate, but my Endless Summer took awhile to leaf out. It started putting out some green and then went into a sort of suspended animation for about a month. It now looks great. It is a slow starter.

I'm also inexperienced, but last year (my first spring with ES) I posted the same question. I waited another month or so (into June) before chopping them off. Some of the old dead-looking canes did eventually develop leaves, and some didn't; I'm pretty sure that none of those older canes developed flowers last year, though. This year they leafed out earlier, but I still have some dead-looking wood. I think in colder zones the old wood gets damaged by cold easily. I'm guessing that you might have cut off some foliage, but that in zone 5 most of your flowers will come from new growth.

Like I said, though, I'm by no means an expert on this.

I cut back into a dead looking stem very conservatively and see if there is green around the perimeter of the cut. If not, its dead and I cut a bit deeper. I give each stem a chance to put out new growth from as high up the stem as possible. If there is the slightest chance that the terminal bud is alive, I don't touch that stem at all. Those would be the earliest flowers. Often this produces a lopsided plant, but I don't care. I rarely get a live terminal bud. Maybe 1 in 100.

After doing this for awhile, you will get a feel for how deep to cut right off the bat.

Then when all the leafing out has occurred, I go around and snip off anything I left behind that is truly dead.

I just don't cut the stems on ES. I'm sure I should, but it just gets so full that I never notice if there are dead stems inside.

I am in zone 5b/6a and I do not cut back the stems of my ES until late in June, because some of them are really alive and it's hard to tell at this point. Be patient :-)

I was just thinking of this thread as I happen to notice leaves starting about one inch from the tips of ES this am. You never know what's going on with those sticks. Don't cut too early.