So we have a bed on the south side of the house that's currently planted with a whole bunch of flowers, including rose, day lilly, and a few that I can't identify. We've made a hugulkultur bed and are familiar with sheet mulching, but I don't want to kill all of the things already living there since they're nice and bloom at different points in the summer. What I'd like to do is build up the bed much higher (several feet if possible) and extend it outwards from the house 2 feet or so (currently that area is grass), and put keyhole bed spaces in there. Should I just pursue sheet mulching with a cardboard layer over the grass part, and perhaps sheet mulch without the cardboard for the rest of the bed? Will the flowers poke through? As I said, they all grow and bloom at different points of the summer, so it's hard at any given point to see how big each plant extends.
I am not totally sure of all the dynamics here, but I suggest that you pot-up the perennial flowers, then build the bed that you want, sheet mulch/compost, and then plant into holes made in the sheet mulch.
That seems like the best way to preserve the flowers you want.
I agree with Andrew. I just recently did this when making a small raised bed into a larger hugulkultur bed; I first dug up the existing plants and then transplanted them back into the new bed. I've been watering them regularly--I'll probably water them for about 2-3 weeks, until they establish.
I have sheet mulched a bed with existing plants, and some of them survived and some didn't. The brambles had no trouble poking through, but the spring bulbs never showed again.