I sincerely apologize for how long it's taken me to make this post!! I intended to complete my little "how to create a rain garden" series in a couple of weeks, and it took me well over a month! Ah well, the important part is that we're continuing now...
So you've selected a rain garden site and done the actual digging of the garden...now it's time for the fun part: the plants!
As in all gardens, rain or otherwise, the adage "right plant, right site" should be observed. All your plants will have to be able to tolerate repeated flooding as well as periods of dryness, unless your rain garden site is really swampy. For this reason it is generally recommended that rain gardens use native plants (meaning those native to your region).
Face it--at some point you'll probably be experiencing some degree of drought, and deep-rooted natives are usually better equipped to handle dry conditions as well as soak up intense amounts of water. On top of these qualities, native North American plants also host numerous pollinators and other forms of wildlife, including butterflies and birds.
So without further ado, here are recommendations for rain garden plants for sunny sites and shadier sites, all of which are hardy in approximately zones 4 to 8. I grow a number of these in my rain garden, and others I have researched as part of various article assignments. I've included flowers and sedges and one grass species. Sedges are really better suited to rain garden conditions and they sort of take the stylistic place of ornamental grasses in these settings.
Rain Garden Plants for Sun
Golden Alexander (Zizia aurea): Great plant! Host for swallowtail butterflies, will get very robust with lots of water and full sun.
Swamp Milkweed (Asclepias incarnata): Lovely pink color, really needs full sun. As a milkweed it's also a monarch host plant.
New England Aster (Symphyotrichum novae-angliae): Can be floppy, consider using on the berm.
Blue Flag Iris (I. virginica): Beautiful classic purple iris flowers. Needs full sun; mine are being dwarfed by larger plants and struggling. I suggest planting them when you first start the garden to avoid this situation.
Sneezeweed (Helenium autumnale): Good late-season yellow color after most plants are done flowering. Does not actually cause sneezing.
Obedient Plant (Physostegia virginiana): Beautiful, distinctively shaped pink flowers. Inaccurately named; not obedient at all. Will spread prodigiously in moist conditions; this is good if you need to fill space, otherwise be prepared to fight back.
Culver's Root (Veronicastrum virginicum): Reaches 5-7 tall with spikes of white flowers; grow it in the center of the rain garden so other surrounding plants will complement it.
Porcupine Sedge (Carex hystericina): Very robust in full sun.
Muskingum Sedge (C. muskingumensis): Can reach up to 3' tall; also takes partial shade.
Fox Sedge (C. stipata): Very interesting, spiky-looking inflorescences.
Prairie Dropseed (Sporobolus heterolepis): Beautiful soft texture, consider growing on the berm or a spot that is not the wettest in the rain garden.
Rain Garden Plants for Shade
Cardinal Flower (Lobelia cardinalis): Great red color and wonderful for hummingbirds. I have actually witnessed a hummingbird at mine here, in total suburban wasteland.
Great Blue Lobelia (L. siphilitica): Gorgeous blue, not as tall as cardinal flower.
Pink Turtlehead (Chelone obliqua): Unique purplish flower; rare plant.
Graceful Sedge (C. gracilima): Up to 2' tall, arching habit.
Bur Sedge (C. grayi): Distinctive inflorescences.
Pennsylvania Sedge (C. pensylvanica): Good for the berm, not the soggiest spot. Will spread slowly over time.
Astilbe (genus Astilbe): Please note: this is not a native plant, but a moisture-tolerant shade plant! I do not grow astilbes in my rain garden but they were once recommended to me as a potential choice by a reputable source in the midst of an interview. Since shade flowers are in short supply for these conditions, I figured it was worth mentioning, native or not.
I'm sorry I don't have pictures of all these plants, but for good photos and more information please see Illinois Wildflowers. They have a section on wetland wildflowers, as well as prairie ones (some of which are included in the list for sun here), and grasses and sedges. Although it is named "Illinois," the plants on this website are native to much of the Midwest and Eastern U.S.
So to Recap:
Remember, there are lots of other options out there, depending on where you garden! Always consult local sources for what is native to your area, and evaluate your particular site conditions for the appropriate plants. These are suggestions only!
Next up...rain garden maintenance!