Invasive and Noxious Weeds

Noxious weeds are weeds that, required by law, must be controlled. In Iowa, they consist primarily of invasive weeds, but do include two native species that can be troublesome in agricultural settings. Not all invasive weeds are declared noxious. 

Iowa Noxious Weed List & Laws

All landowners are required to control those weeds on their property that have been declared noxious by the State of Iowa or by Hardin County.



Noxious weeds growing within city limits, in abandoned cemeteries, along railroads, streets, and highways, as well as on farmland, or any private or public land, must be controlled. The Hardin County Weed Commissioner enforces the Iowa Noxious Weed Law and the County's Noxious Weed Control Program.

Many people are familiar with the concept of weeds in the context of their yard or garden. Weeds are simply undesirable plant species. The same principle holds true for Hardin County’s roadsides, natural areas, farm fields, etc., which are home to a diverse array of native plants. The number and variety of these native species is described by the term "biological diversity." Over the past 150 years, many non-native plants have been introduced to our region, both intentionally and accidentally. The vast majority of these plants coexist with native species and are ecologically harmless.

Canada Thistle

Canada Thistle

What Makes a Weed Noxious?

What makes a plant noxious in the context of Hardin County’s natural resources, is its negative impact on agriculture, or its ability to spread and crowd out native plants. Therefore, noxious weeds are distinguished from those weeds that occur in small numbers and are innocuous.

Noxious weeds are those able to reproduce in the wild, spread rapidly, which are difficult to control, or which cause the decline or loss of our native plants.

It is not completely understood why some weeds become noxious and some don’t. But because noxious weeds did not evolve locally, their populations are not held in check by natural predators or diseases, giving them a competitive edge over native plants. Noxious weed species can proliferate and spread over large areas. Some can completely displace other vegetation, forming a homogeneous (single species) cover. Contrast this situation with the rich variety and diversity of a native plant community such as is seen in prairies, wetlands, or forests.

Coming to Hardin County

Noxious weeds are exotic plants that have reached Hardin County and Iowa by escaping from gardens, being transported by hay or straw, air, dirt, tires, clothing, etc. They grow aggressively, lack natural enemies, and resist management methods. These species can move quickly into bare areas which have been disturbed by construction or erosion, have poor vegetative cover, or have other soil disturbances. Some of Iowa’s noxious weeds are spread by windblown seed, birds, or other organisms. Other weeds spread though poorly timed mowing or baling of hay to be fed to livestock or sold. And other noxious weeds spread by sending rhizomes (long underground roots) to uninfested areas.

Controlling Noxious Weeds

Most of Iowa’s noxious weeds can be easily controlled through proper management. Techniques to combat weeds include:

  • mowing
  • cutting
  • burning
  • competitive seeding
  • cultivation
  • herbicide usage

A primary key to weed control is early detection and management, before that weed has spread and built up a large seed bank and energy reserve.

Japanese Knotweed

Japanese Knotweed