NC State horticulture experts work to restore the garden beauty to rain gardens

Scientists at North Carolina State University are working to keep rain gardens beautiful and functional.

Originally designed as natural water filtration systems for urban runoff, capturing and treating water coming from a roof or parking lot rather than allowing the water to flow into a storm drain, rain gardens have become scenic areas of their own. Or at least they’re supposed to be. Like other gardens, they are plagued with insects and weeds, perhaps more so because they have to endure extreme conditions and are composed of sandy soils, perfect for any weed to sprout. The weeds make the garden unsightly, and untamed weeds can even force out the preferred plants. Dr. Helen Kraus, a horticulturist at NC State University, used a $30,000 Southern Region IPM Enhancement Grant to survey common weeds in rain gardens and find control measures for them.

Because rain garden plants and soils are used to filter nutrients, chemicals and sediment out of the stormwater and return purified water to ground water, rain garden managers can’t use herbicides. One of the only options for weed control is hand pulling, done by maintenance staff or volunteers. However, because weeds are usually not removed consistently enough to keep the populations in check, they grow to levels that require hours of labor to reduce to manageable levels. Weed control in most rain gardens is abandoned once labor becomes too costly to keep up with weeds. The result is often the removal of the rain garden itself.

Kudzu covers a rain garden at a high school.

Kudzu covers a rain garden at a high school.

Also called bioretention cells, rain gardens were first designed by agricultural and environmental engineers to remove pollution from water runoff. The water runs into the garden from either pavement or a roof and is filtered by the sand and silt layer so it can be absorbed by the plants. Water that the plants don’t use continues down through the soil to the groundwater level. As rain gardens became more publicly popular, and managers at schools and other businesses wanted to install rain gardens to address flooding, Kraus was asked to recommend plants and soil types.

After reviewing existing rain gardens, Kraus and landscape designer, Anne Spafford authored a book with recommendations, Rain Gardening in the South.

“We talked about how to design soils, how to pick plants to capture rainwater better,” said Kraus. “As we did the research over the four to five year period, we became aware that weeds were also a problem.”

To get a better idea of how much of a problem weeds were, she and her colleagues surveyed 74 gardens across North Carolina and Virginia this past year. Some gardens were so overrun with weeds that the garden was no longer attractive. One garden at a Charlotte high school was completely covered with kudzu.

After the survey, Kraus and her colleagues developed a list of the top 10 most common weed species plaguing rain gardens (numbers in parentheses are the frequency of occurrence in the rain gardens sampled):

  1. Tree seedlings (64%)
  2. Lespedeza (39%)
  3. Oxalis (34%)
  4. Dog fennel (33%)
  5. American burn weed (31%)
  6. Yellow nut sedge (31%)
  7. Poke weed (20%
  8. Sowthistle (20%)
  9. Spotted spurge (20%)
  10. Blackberry (19%)

In addition to cataloguing weed species, Kraus’s team also measured ornamental plant health, presence of mulch and substrate pH.

“We needed to know if the plants were thriving,” Kraus said. “Some of the gardens were several years old and if the gardener knew how tall the plants were when they were first planted, we could tell how well they had grown. We also took soil samples to measure the pH and found that the soil was very acid, running about 4 on the pH scale.”

A variety of plants are used for rain gardens, ranging from trees and shrubs to grasses and perennials. Plants must be hearty, able to withstand periods of heavy dousing often followed by long periods of drought. The soil used is low in organic matter and clay, so conventional herbicides are not an option for weed control because they may leach through the soil substrate to the ground water or damage the desired plants. Hand pulling, which is the typical second option for weed control, is not considered because of the time and labor costs. So Kraus is looking at other options for weed control.

“Now that we have a list of weeds, we can look at natural and chemical controls,” she said. “Can we use some herbicide labeled for wetlands? Some of that will depend on the difference in organic matter.”

She also said that the pH needed to be raised to assist with plant health.

“Sand changes pH easily because it doesn’t have much buffer capacity,” she said. “But how much lime to add will be different from typical recommendations.”

To keep weeds out and rain gardens functioning, she said, they need to test a variety of plants and recommend the ones that will grow and remove nutrients the best. Although a garden of vigorous plants would not completely crowd out weeds, they might reduce weed populations to the point where they could be controlled by reduced-risk herbicides.

To test different herbicides, she and her colleagues have built 20 rain gardens in which they can apply herbicides and then evaluate the movement of these chemicals in water. Any herbicide that does not damage the ornamental plants, or move with water, would then be listed as an option. If no herbicide options are found, she said, they will have to recommend returning to mechanical weed control methods such as hand-pulling.

She and her colleagues are hoping that they can find a solution that is effective and efficient.

“People will give up on the gardens if we can’t figure out a way to make them look better,” she said.

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