It depends entirely on what you planted and the media it was planted in. In my area, the focus on rain gardens is with mostly native plants and they require NO fertilization. And the planting medium is enriched as well, either by working compost into the existing soil or importing a rain garden soil mix. All the nutrients the plants need to survive are well provided for. In fact, because rain gardens here are intended primarily to limit storm and irrigation water run off, it is advised NOT to fertilize (or use any pesticides). An annual replacement or refreshing of an organic mulch will supply any nutrients that may be lacking.

Great, thank you.