If you smell citrus as you're driving down a road in southeastern Iowa next spring, it's probably because Chris Henze and his crew are battling Japanese knotweed with a concoction that smells like lemon.

They're using a highly specialized herbicide-application technique — invert emulsion — to attack infestations without jeopardizing desirable plants along the right of way or crops growing beyond nearby fences. Instead of the surfactant becoming liquid to match the properties of the herbicide, the herbicide thickens to the consistency of the lemon oil so the mix sticks only to its target.

“We don't have large infestations acre-wise, but we have 16 that cover several hundred feet and keep spreading each year,” says Henze, weed commissioner for the Johnson County Secondary Road Department. “If we don't flag them, our operators will mow the knotweed along with everything else in the right of way, which spreads the rhizomes and leads to new infestations.”

When Japanese knotweed moves in, wildflowers, low-growing grasses, and shrubs are completely overshadowed by a dense canopy, disrupting the local ecosystem by eliminating native plant species altogether. The knotweed thickets shroud driveway and intersection sightlines along some stretches of road, endangering drivers. Even though farmers have fought back with standard pesticides, Polygonum cuspidatum crept further into rights of way and across fences into farmland.

The department's goal was to stop the advancement without damaging neighboring property, row crops, aquatic areas, or ecosystems.

A general broadcast application would be inefficient because the overall acreage is a tiny percentage of the total roadway miles the department maintains. To identify the right herbicide and application technique, Henze turned to Chemo-Trol Inc. in Kansas City, Kan., a quality vegetation management certified applicator he's worked with in the past.

Because many of the infested ditch areas carry water during wet seasons, the company recommended that BASF's Habitat be applied via invert emulsion. Habitat is labeled for use in aquatic settings and eliminates spread of the weed by controlling it from the roots and rhizomes up; and mixing it with a surfactant — in this case a heavy lemon oil — would form a much denser substance as it leaves the nozzle.

“The liquid herbicide inverts from the droplet form you'd find in a regular spray to a thicker substance, a lot like mayonnaise in consistency,” says Steve Hansen, Chem-Trol's representative in Iowa City. The use of invert emulsion varies by region, weather patterns, and the type of land management goal in question; i.e., whether it's to maintain site clearance, maintain stormwater flows in a culvert, or prevent invasive species from encroaching on native species. The application can be done by hand and doesn't require a closer proximity than an applicator would need to apply a standard emulsion.

The lemon oil surfactant provides weight and density to the tank mix, allowing applicators to zero in on areas of infestation where crops or other desirable vegetation are nearby. It also sticks to and covers leaves and stems, driving the active ingredient into roots.

The treatments are conducted using a hand sprayer attached to a truck tank. “Our goal is to keep applications at zero-drift level,” Henze says.

Johnson County has had the most success when applied in early spring.

“In smaller, younger patches, we've been able to eliminate Japanese knotweed in just two years of treatment,” Henze says.

During the dormant season, crews often burn off the litter to make way for next year's treatment. “Doing a burn in the fall gives us better access to sprouts that come up in the spring. If none appear, we can reseed or let native plants get better access to sunlight and soil nutrients,” Henze adds.

After the burn, crews reseed native warm-season grasses such as Canada wild rye and big and little bluestem as well as wildflowers such as black-eyed Susan and partridge pea.

Residents like the restored landscape almost as much as the wildlife.

“When we added the spray program, we were a little worried about how landowners would feel about it,” Henze says. “But they've responded very well, and we make every effort to keep them informed about the treatments.”

— Blair is a sales specialist for BASF Professional Vegetation Management. He recently served on the board of the National Roadside Vegetation Management Association.