Spiderwort. It produces a deep taproot, so isn't the easiest thing to pull out. Digging works better.

Thanks for the quick reply. After posting, I did try to pull some up, as you confirm, no luck there. If I dig it out of the daylilies, I would expect collateral damage. I'm thinking of cutting off the stems and dabbing the cut ends with full-strength weed killer, carefully so as not to harm the good guys. Any comment?

You're probably right about the collateral damage; are cultivars of daylilies more sensitive than the common orange type? The reason why I ask is nothing seems to kill or harm the common orange ones. Last year I dug up a bed of them and threw them bareroot across the street into an open field. . . they are flowering right now, don't ask me how. So who knows, you might be able to get away with digging that spiderwort out, carefully.

Thanks for the input. I think I'll try my cut-and-poison idea first and if the weed survives that, I'll try digging. I don't really know just how robust or sensitive the daylilies are.