COLUMBUS, Ohio - Plan to build a wetland in a field that used to grow crops? Then plan to help Mother Nature start the nutrient-filtering, water-cleaning, swamp-loving plants you need in it.

A new Ohio case study finds that if you rely on passive establishment alone - if you let wind, wildlife and the seed bank do all the work - more than half the plants that sprout on their own won't be wanted wetland species. They'll be weeds such as thistles, for example, or invasive types such as common reed.

Scientists with Ohio State University and the USDA recommend instead that you lend a hand: plant or seed desired species, from redtop to cattail to blue flag iris and others.

Doing that, the researchers say, should lead to the site having more than 50 percent wetland species within six years of construction.

"Non-wetland species don't provide the same kind of beneficial improvement of water quality" as wetland species do, said Norm Fausey, one of the study's authors and a research scientist with the USDA-Agricultural Research Service Soil Drainage Research Unit on Ohio State's Columbus campus. "Secondly, they don't provide good habitat for wildlife, a goal of many conservation programs.

"There's a real push to get more wetlands into agricultural areas to reduce the off-site delivery of agricultural chemicals," Fausey said. "But you need plants that survive in the water to give you that benefit."

Through a range of natural processes - decomposition, denitrification, more - wetlands settle sediment and filter excess nutrients. Hydrophytic plants - ones adapted to saturated soil conditions - drive the work. They also give food, shelter and breeding sites to gamebirds, songbirds, waterfowl and game mammals.

While other, larger studies have shown that passive establishment alone can work fine - that in only a few years a passive site can have wetland vegetation equal to or even better than a planted or seeded site - this study finds it not the case if the site is former farmland.

Fausey and colleagues published their findings in the September 2006 issue of the Ohio Journal of Science.

"If a field has been farmed for 80 or 100 years with the goal of keeping weeds out, the seed sources will be very much diminished," Fausey explained.

>From 1998 to 2001 the scientists evaluated three constructed wetlands, all of them built in 1995 next to farm fields in northwest Ohio - in Van Wert, Fulton and Defiance counties - to see how the sites grew plants on their own and exactly what types popped up.

Based on the results, the team concluded "that for constructed wetlands built in agricultural settings, planting or seeding of desired species will be required to supplement the existing sources of wetland vegetation species."

Fausey suggests transplanting plants or harvesting seeds from nearby ditches and wetlands (cheap, effective; get permission of course if on others' land); buying wetland plants from nurseries (effective but expensive); or seeding the area with purchased seed (cheaper than plants but less effective; including wetland-plant seeds in erosion-control seedings could someday be an option).

Regardless, he said, be sure to use locally adapted plants and seeds, not ones from other regions.

"One of the reasons for doing the study is that planting is expensive, so having a source of materials that make a good wetland is important," Fausey said. "We wanted to evaluate natural revegetation as an option: How does it stack up? We learned a lot in terms of what's the available material for generating the kinds of plants you want."

Sedges, pondweeds, bulrushes, cattails, spikerush, rice cut grass and narrowleaf willow were among the 35 wetland species found at one or more of the sites.

Non-wetland species, 42 in all, included bromes, thistles, fleabanes, plantains, clovers and goldenrods.

Wetlands on farms "are long-term investments," Fausey noted. "They're typically a permanent addition to the landscape, and they'll mature over time. The question is, how can we encourage it to happen as quickly as possible?"

Co-researchers on the study were Lee Luckeydoo, formerly of the USDA-ARS Soil Drainage Research Unit, now of the Botanical Research Institute of Texas, and Ohio State's Craig B. Davis, School of Environment and Natural Resources; Emilie Regnier, Department of Horticulture and Crop Science; and Larry Brown, Department of Food, Agricultural and Biological Engineering.

Brown holds joint appointments with the Ohio Agricultural Research and Development Center (OARDC) and with Ohio State University Extension, the research and outreach arms, respectively, of Ohio State's College of Food, Agricultural, and Environmental Sciences.

Davis and Regnier hold appointments with OARDC.

SOURCE: Ohio State University news release.