Do wildflowers help reduce runoff in roadside soils?

Road construction and roads themselves can cause more problems than just a rush hour traffic jam. My research focuses on using wildflowers to help reduce some of these problems – in addition to being pretty!

Paved surfaces, such as roads, cannot absorb rain during storms. Rain or snowmelt becomes runoff that flows to the soils along roads. During road construction these soils are disturbed and may have a limited ability to soak up runoff. Runoff from roads typically contains harmful pollutants and contaminants. Streams, rivers, and lakes can be harmed by these contaminants if the runoff is not infiltrated by soils on the roadside first. In regions that receive a lot of precipitation year-round, this can be a big problem.

Turf grass is typically planted in the soils along roadsides after construction. It provides an inexpensive, low-tech way to minimize the runoff leaving roads. Crews may work to improve the soil with tillage or compost to improve how this system absorbs runoff. However, managing the grass by mowing can compact soils which limits their ability to absorb water. This is especially true in southern areas with longer growing seasons.

field of multicolored wildflowers. wildflowers absorb roadside water runoff
Wildflowers are an alternative to traditional turf grass for vegetation cover along roadsides. Two coneflower variations of the Aster family are shown: the yellow flowers are Rudbeckia hirta (Black-eyed Susan)and the purple flowers are Echinacea purpurea (Purple Coneflower). Image credit: Jamie Luther

Compared to grass, wildflowers have the potential to remove more water through evapotranspiration. Wildflowers require less intensive maintenance and do not need frequent mowing. This may increase the benefits of soil improvement methods, such as tillage, and increase the ability of roadside soils to absorb runoff and filter pollutants long-term.

When working toward my Masters’ degree at North Carolina State University, I researched the use of alternative vegetation along roadsides. Our study compared the potential of tilled soils with planted with turf grass compared to tilled soil planted with wildflowers. We measured each system’s ability to infiltrate runoff from roads during rainstorms. We hoped to determine which system absorbed runoff better.

We established experimental plots next to highway roads to test the effects of different vegetation on the amount of runoff absorbed by the soils in each plot. We made plots with three types of soil treatments: existing grass (unaltered), tillage with grass, and tillage with wildflowers. We used wildflowers native to North Carolina, like coneflower and Black-Eyed Susan.

two people using equipment to create roadside soil plots in field
Experimental plots were established along highways to test soils with turf grass and with wildflower cover. Experimental plots were tilled before seeds were planted. Image credit: Jamie Luther

Water from rain, road runoff or snowmelt can move across the top of the soil – runoff – or into the soil – absorption. The hope is to find the best system to absorb water into the soil and not allow it to become runoff and keep it out of waterbodies.

After making our systems, we collected any runoff – the water that was not absorbed by the soil system. We collected the runoff water in a large tub. We also measured the bulk density (mass/volume) and the infiltration rates (amount of water absorbed per hour) in the soils of each plot at the end of the experiment. These are measures of how soil functions and how healthy it is.

It makes sense that the amount of runoff increased with the amount of rainfall in all plots; more rain means more runoff.

several research plots consisting of wildflowers and turf grass to test roadside runoff
The runoff from experimental plots was measured for one year after establishment. Wildflower plots looked much different than grassed plots. Image credit: Jamie Luther

After we tilled and planted with turf or wildflower seeds, runoff was lower in both turf grass and wildflower plots compared to the unaltered control plots. However, turf grass and wildflowers absorbed the same amount of runoff – neither was better than the other. There also wasn’t a difference in soil health or soil function.

So, just looking at the impact of a turf grass system or wildflower system on reducing runoff, they are the same. But there is a difference when one looks at the broader functions of turf grass and wildflower systems.

Wildflowers provide many co-benefits that are not provided by grasses. Wildflowers increase pollinator habitat and biodiversity. Wildflowers have deeper roots that can stabilize soils and reduce erosion. The need for mowing is decreased with wildflowers, reducing the compaction from the tires of mowers. Reducing the activities that cause compaction, like frequent mowing with heavy equipment, is essential to maintaining the soil improvements of tillage and vegetation growth through time.

six photos of different types of wildflowers. wildflowers benefit pollinators and the environment
Wildflowers have many co-benefits. In addition to providing vegetation cover for soils, they increase the habitat for pollinators, increase biodiversity, stabilize soils, and reduce erosion. Plus, they are a beautiful sight! The wildflower mix used in this study was comprised of various wildflowers native to North Carolina. Image credit: Jamie Luther

Wildflowers provide a great alternative to grasses for roadside vegetation. When paired with tillage, they can improve soil conditions, increase infiltration rates, decrease runoff from paved roads. Wildflowers not only are beautiful but provide food and homes for pollinators and other wildlife, improving the aesthetics and health of roadsides.

Answered by Erin Rivers, Utah State University

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