If possible, plant your rain garden as soon after possible after digging. If you cannot plant it within a week or so, mulch it lightly with fibrous wood mulch.
In Ohio, spring and late summer (before mid-September) are the preferred planting times. Planting during the peak of summer is fine if you're willing to water frequently.
It is important to mulch young rain gardens! If you don't mulch, the garden bed can seal up and prevent water from infiltrating. Mulching also prevents weeds and helps retain moisture so you won't have to water as often.
Another weed prevention tip – try laying down 6-8 sheets of newspaper prior to mulching. The newspaper will help prevent weeds from popping up and also with retaining moisture, plus, the paper will naturally break down over time.
Not all mulches work in rain gardens. Lightweight mulch and flat wood chips will float when it rains! Good results have been obtained using fibrous shredded mulch. Pine straw mulch can also work well, because the pine straw tends to lock together into one cohesive unit. Pine straw mulch also has natural properties that help with weed prevention.
Photo (above): Students at Goshen High School plant their rain garden.