I avoid spraying Roundup or 2-4-D around the daylilies during the times they are forming scapes or blooming, but after they are done, I have carefully blocked the daylilies with cardboard or tarps and sprayed along the bed edges. I would avoid using any sprays in the beds. It may not kill them, but my experience is that it can/will distort or ruin the bloom.
I am a fan of Snapshot. I find it works a little better than Preen. The key to either is getting it down before the weed seeds germinate, and then watering it in as per directions.
It depends where you live how often you need to apply pre emergent, and when. Your local master gardeners should be able to give you guidance on that, or a trusted employee at a local nursery. Where I live, I should apply twice per yer, but often I only manage once. This year I didn't mange to get any Snspshot down, and boy, can I tell a difference.
What a mess!
If you must use killer in the beds, try a sponge paint brush or disposable cotton glove to apply instead of spraying, and only apply to one weed at a time ( such as to poison ivy). Pull daylily foliage back, apply killer, and try to keep daylily foliage from touching where you applied until it is dry.
