Building Prairie Sod
If you want only a few prairie grasses or flowers for your beds, it's simple enough to order individual specimen plants and manage them as you would other perennials. Specimen plants work quite well in flowerbeds, border plantings, and wildflower islands surrounded by turfgrass. If your intent is to build a true prairie sod—an interdependent, intertwined community of plants—your best bet is to use a wildflower-and-prairie-grass seed mix that matches your site conditions and soil type. Once you have chosen your mix, here's how to prepare the site.
Site/seedbed: Prairie plants do best in an area open to sun and air—not surprising considering they evolved on the breezy, sun-drenched plains. Prairie plants require at least half a day of full sun and good air circulation, which discourages fungal diseases. Although it's difficult to achieve an entirely weed-free planting site, that is your goal. Prepare the site organically by choosing one of these three methods:
1. Place black plastic over the ground for an entire growing season. This deprives the weeds of moisture and sunlight and raises the temperature, literally cooking the soil.
2. Layer newspapers 10 sheets thick over the area you want to turn into a prairie garden, and then top the paper with a 2-foot layer of dead leaves and grass clippings in roughly equal amounts. Let the leaves-and-grass mixture compost for one growing season; then scrape off the top few inches until you get down to the easily crumbled, composted material. Seed directly into the compost. This way, you create a new seedbed and entomb any weeds in the soil below.
3. Remove the turf with a sod cutter or spade and sow seeds directly into the ground beneath. This only works if your lawn contains few weeds.
Seeding: When planting 1 acre or less, hand-broadcast the seeds. Prairie seeds can be tiny. To spread them evenly, bulk up the mix with sawdust or sand.
Regardless of where you live, you can plant in either spring or fall. When planting in spring, rake the seed lightly into the soil and roll the bed as you would when planting turfgrass. The seeds require firm contact with the soil. Keep the ground consistently moist until the seedlings are established.
If you plant in fall—usually from late September until the ground freezes—there's less need to rake or roll. In winter, as the ground heaves from the cold, the seeds will work their way into the ground. They germinate when warm weather returns in spring. This "dormant seeding" works especially well in hard-to-dig clay.
You will need to guard against erosion, particularly on slopes. Diboll recommends mulching the bed with a weed-free straw, such as winter wheat. Or you can add annual rye seed to the seed mix (at the rate of 15 pounds per acre) and raise a "nurse crop" that grows a few inches and stabilizes the soil through winter. In USDA Plant Hardiness Zones 1 to 5, the rye dies from the cold and helps fertilize the planting. In warmer zones, the annual rye will survive winter but will peter out the next season. "Even if it doesn't die off, it typically won't interfere with the planting," says Diboll.
Year one: Prairie seedlings grow slowly and will soon be outpaced by weeds. Yes, weeds. Despite all of your preparations, weeds still appear.
"The first year, it looks bad," warns Diboll. "All you see are weeds and these little prairie plants down near ground level."
Three or four times during the growing season, mow the bed with a weed and grass trimmer or hand scythe to a height of 6 inches—always before the weeds set seed. That way, you cut the weeds but spare the seedlings. Leave the cuttings in the bed as mulch.
Never try to pull any weeds by hand because you may disturb the emerging prairie plants.
Year two: To accommodate the increasing height of the prairie plants, raise your mowing height to 12 inches. Mow once or twice this season, beginning in June when biennial weeds, such as Queen Anne's lace, bull thistle, and sweet clover, are flowering but not yet setting seed. Mowing off the flower heads breaks the weeds' two-year cycle.
Annual weeds still come up, and you may notice some perennial nuisances, too, such as Canada thistle, dandelion, and bindweed. It's okay to pull weeds carefully by hand now that the prairie plants have developed root systems, but do so only after a good soaking rain, when the roots pull easily and you are less likely to disturb your prairie seedlings.
Year three: Now's the time to control the cool-season weeds, such as quackgrass and Kentucky bluegrass, that appear in the beds while the prairie plants are still dormant.
Early in the season—about the time maple trees begin to bud—mow the entire planting down to the ground and rake out the cut material. Or you can mimic nature and burn the prairie. (See "To Burn or Not to Burn" below.) Either treatment sets back the weeds, warms the soil, and encourages the reemergence of the warm-season prairie plants.
"It's precisely what happens in nature," says Diboll. "Prairies really take off after this treatment."
Subsequent years: By now you have a firmly established prairie sod of intertwined plants that fend fairly well for themselves. Occasionally, you may pull a weed or two, but burning or mowing in spring is usually all the annual maintenance required.