My seasonal stream has enough sand deposited that I can just walk near it or in it without sinking in at all. I know this sounds crazy: but so far, I have just been pulling weeds out by hand. It means, of course, that they have to get about 18 inches high before they count. As for grasses I got desperate this spring when four Sunsprite and one Mardi Gras were being threatened with about the same fate as your Sea Foams. They had spent all of last year buried deeply in native grass and I lost two or three roses in the thick of it. So I used Ornamec - available from Wildseed Farms. There are one or two spots where I've left native grasses in place - clump forming ones that make head-high seed stalks simply because mulch would wash away.
There's a stretch that is unplanted. It's mostly grass beneath junipers. This I mow with a string trimmer maybe twice per season. I think my garden is on a much smaller scale than yours. And I have not been aggressive about managing the edges. I'll plant a bed, get it under some moderate level of control, then plant another. Outside those bounds things are totally au natural, which means they blend perfectly with all the rest of the properties around us.
Assuming they had gotten established, I'd bet those Sea Foams will be back. And they'll probably bloom better than ever.
