The verdict from our local garden expert: voles. Moles make the tunnels to get the grubs, and voles use the tunnels to eat the plant roots. Voles look like mice but are vegetarians.
Fortunately, being native species, many of these plants had gone to seed over the past few years, and we refrained from pulling out many of the seedlings which were scattered around the garden. Although a big section of the garden was completely empty this spring, seedlings were found and relocated. They just won't bloom much this year. Some other plants were simply allowed to expand into the empty areas. Bee Balm and Giant Hyssops were given free rein. Brown Eyed Susans (Rudbeckia hirta), a plant that can be an annual, biennial, or even perennial, reseeded aggressively and had popped up all over. They bloom quickly, and so were given a lot of up front space to do their thing.
Although there was some money in the pool for purchasing new plants, the spring was so dry that wells were reported to be at 10%, and there is no water onsite. So that option was postponed. But what about next winter? We may need to tidy up the garden and clear out the leaves, which encourage voles by providing cover.