pretty much what JB said; I've put out my over-wintered plants early before and they didn't like the temp change, so now I tend to wait until almost June. I did put out some water lettuce, more as sacrifical plants for the fish. The frost we had last week did in some but there are some still alive. Hyacinth definitely would rather have warmer water. I'd say by the third week of May should work. Again, watch for any temps below 40 as the annual types don't do well then. I put out my taro and papyrus late May. I have some parrot's feather struggling but alive in the pond. The pennywort is doing okay.
So, very soon but I'd wait at least a week or so, if I were buying anything or bringing up overwintered plants. And, if you are overwintering, they really need to be hardened off by being protected from the sun for a week or so, then gradually allowing full sun. I've lost more plants (not lost but leaves were definitely sun burned) this way. I generally put them in buckets on the east side for morning sun then gradually work them into a southern exposure.
