Any organic fertilizer (or even dilute inorganic ones) can be beneficial. For a longer lasting punch how about mulching them with appropriate organic composts. This not only gives them added nutrients, but also fortifies the soil they are in for the longer haul. I do this each spring to my ferns and they love it.

Thanks...now if I could just get it to rain some more today so I don't have to run the sprinkler back there. The ferns do have a super organic mulch...I never ever rake ;). There's a four to five inch carpet of rotting leaves. So far they like it and did not even suffer much during the winter at all.