There is no herbicide that will target weeds without affecting other plants, but how they are exposed does matter for reducing that risk. For example, pre-emergent herbicides target germinating seeds, so if only weed seeds would be germinating in spring or fall (depending on the species), and no desirable wildflower seeds, than that might be an option if the product was labeled for use around wet habitats. Some post-emergent herbicides (those used on already-growing plants, not seeds) are more selective than others, meaning the range of plants that can be affected by the chemical can be broad; some only target grasses or only target broadleaf plants (non-grasses), but others do just about everything and caution needs to be used when applying any of them.

Since it would be ideal to avoid herbicide use wherever possible, in part because of the wildlife value that rain gardens can provide (and to avoid contaminating stormwater that the garden can't filter pesticides residues out of), other methods might be more practical to try. Is the ground mulched between plants to discourage weed germination and growth? Some mulch types might be less likely to erode down a slope than others, if part of the garden is on a bank. An alternative to mulch would be the use of more live plants, like native groundcovers to help serve the same function as much, preventing weed seeds from gaining a foothold.

If the surrounding yards are the continual source of weed seeds for the rain garden and need to be treated, the options will depend on what weeds are present (like annuals vs. perennials, spring vs. fall growers) and where they are growing (in flower beds, vegetable beds, or lawns). With more information like weed type (or photos for ID) and the setting in which they are proliferating, we can help to make recommendations as to a management approach.

Miri