Plant Early, Plant Thick and Plant Big to Get Ahead of Invasive Weeds in Riparian Areas, Early Research Shows

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Lespedeza encroaches on plantings in the wetland area of the Lonnie Poole

Photograph: Lespedeza encroaches on plantings in the wetland area of the Lonnie Poole.

Based on first year results from a three-year research project, a North Carolina State University researcher has some advice for land managers of riparian areas, especially if the areas have become overgrown with exotic invasive weeds: plant early, plant thick and plant big.

With $24,600 in grant funding from the Southern IPM Center in 2011, NCSU landscape extension specialist Dr. Barbara Fair planned to experiment with native seed mixes and some non-invasive adapted flowering shrubs to replace current overgrowths of invasive exotic weeds in riparian areas. She hypothesized that adapted species might be easier for landowners to grow and would fare better in areas disturbed by development.

Riparian areas typically include wetlands and surround bodies of water, so when soil is disturbed by tilling or other movement from development, invasive plants quickly outcompete native plants, which take longer to recover from disturbed environments.

Invasive exotics are also more flexible in terms of planting space; they amass thick clumps of plants that shade out smaller native wildflowers growing among them. Dr. Fair hypothesized that if populations of invasive weeds were tilled and treated with herbicide, landowners could plant a combination of native plants and adapted shrubs (such as Knockout Rose) that would be less expensive and easier to manage than a fully native flower population.

In several test plots near the 16th hole of the Lonnie Poole golf course, behind one of the ponds, Dr. Fair tilled the area and chemically treated the existing populations of exotic invasive plants. Then she planted a native seed mix including ornamental grasses, sedges, and coneflower, along with a mix of Carolina roses, native to North Carolina and Knockout Rose, a non-native, adapted species that is not invasive.

Unfortunately, delays in funding postponed the planting, and a wet summer and cold spring hampered seed growth. As the native seeds began to emerge in late spring and the roses slowly came out of dormancy, a rush of lespedeza and dog fennel quickly overpopulated the wetland plots, choking out most of the Carolina roses and many of the budding flower seedlings.

“I had envisioned a meadow with coneflower and roses that everyone could see from the course,” Dr. Fair said as we faced the overgrowth of lespedeza. “Unfortunately, as you can see, that’s not what happened.”

Although Dr. Fair’s wetland plantings were crowded out by the aggressive lespedeza, the plots in the higher, drier areas fared better.

The Knockout Rose planted in plots on higher ground from the pond survived; however, they had not grown in the year from their planting. Many of the Carolina roses planted in the wetland plots did not survive.

The native wildflowers from the seed mix fared better. Individual black-eyed Susans appear in isolated patches in the wetlands. In the other plots, they are as plentiful as the lespedeza. Other flowers and grasses, including panicum and native monarda, populate the higher plots as well.

The most interesting finding, Fair said, was in the control plot, which has some lespedeza but is overrun with Bermudagrass. The control plot was the only plot in which she did not plant anything. None of her plots were fertilized. Plots were irrigated only during establishment during the first summer, when it did not rain. Irrigation came this year with a wet spring and early summer, encouraging faster and thicker invasive weed growth.

Although the results were not what she had hoped, Fair’s findings will save other land managers time and money, especially in areas with intensive invasive weed pressure:

  • After initially eradicating and treating invasive weeds, let the site sit fallow for a number of months, and then treat emerging seedlings again with an aggressive herbicide.
  • Plant seeds and shrubs in early spring so they can be established before invasive seeds have begun to form seedlings.
  • Plant larger shrubs for planting.
  • Plant the shrubs about 4-5 feet spacing to overshadow possible invasive weeds underneath.

Next year, Fair plans to hand treat the lespedeza in some of the plots where plantings are faring well and follow her current recommendations to see how they change the landscape.

“I want something that people will enjoy looking at while they walk by,” she said. “If I could keep the lespedeza out, this would be an attractive feature.”

Contact: Dr. Barbara Fair, Assistant Professor, Horticultural Science, North Carolina State University, 919-513-2804 or Barbara_fair@ncsu.edu

Written by: Rosemary Hallberg, Communication Specialist, Southern Region IPM Center, 919-513-8182 or rhallberg@sripmc.org

Date: August 14, 2013

Updated on Feb 28, 2014
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