I've scoured the internet and am still unsure what these plants are! They're starting to die off and I'm trying to do some research on what can be done to save them.
They have somewhat feathery silver/green leaves and stems that crawl along the ground and then curve upward into a foot-long stalk with seed pods (but no flowers that I've seen).
Growing conditions: SE Michigan, full sun. They were planted 1-2 Summers ago and were going strong up until about a month ago. My current best guess is that these are wormwood which, I'm told, do better without a lot of water and are susceptible to root rot if they get too much. That seems contrary to what I'm seeing, though; the plants appear to be withering and those closest to a downspout are the doing the best. Any suggestions other than watering, which I am doing (carefully and in small amounts)?