Damon E. Abdi and Jeb S. Fields

The Louisiana landscape is routinely challenged by unpredictable and unusual weather conditions. In 2022 and 2023, temperature extremes exerted pressure upon ornamental plants. Notably, in 2023, the mid-March frost felt throughout Louisiana diminished the displays of spring flowers and yielded tip die-back throughout the region. As summer advanced, weather conditions were far more consistent but perhaps even more cumbersome for plants. The unusual, extended drought felt throughout the South failed to offer respite from the sustained heat generously heaped upon the soil. Providing resilience to environmental extremes such as the ones experienced this past year is a deep, complex issue. To combat this challenge, researchers at the LSU AgCenter Hammond Research Station took a surface-level approach.

Soil surfaces are the canvas upon which our communities are built, with landscapes providing the natural escape from the concrete jungle. Ensuring the health of our soil is essential to sustaining landscapes and providing protection from temperature and precipitation extremes can be accomplished by several methods already employed by landscapers. Mulching, one of the more common aspects of a landscape contractor’s portfolio, utilizes a range of materials, such as pine straw or pine bark, as a physical soil covering that insulates soils from temperature shifts and rapid evaporation while limiting the emergence of weeds. Ground covers serve a similar purpose in supporting soils, albeit through living plant material with a full spectrum of features to enhance the landscape. Ground cover species, which are low-growing, spreading plants, not only provide soil insulation but manage soil moisture by limiting evaporation during periods of low precipitation and offer transpiration, the process of expelling water vapor through the foliage, as a method to evacuate excessive water during wet periods.

Despite the known benefits of these two features, there is a relative lack of data available to support more informed application of mulch and implementation of ground covers. We set out to quantify the effects mulch and ground covers have on soil conditions. Pine straw mulch, one of the more common materials used in the southern U.S., is typically applied at depths between 2 to 8 inches in landscape beds. Throughout spring 2023, three pine straw depths (2, 4 and 6 inches) were compared to bare soil plots for the capacity to provide resistance to temperature fluctuations. Plots were approximately 11 square feet with a soil temperature/moisture sensor buried 6 inches below the soil surface in the center of each plot measuring conditions every 10 minutes. The results indicated that without any pine straw, soil temperatures fluctuated rapidly in response to shifts in the ambient air temperature. With increasing amounts of pine straw, these fluctuations became more muted; however, there were diminishing returns when pine straw was applied at depths greater than 4 inches in this study.

Further studies at the Hammond Research Station investigating ground covers yielded considerable insights toward the spread of these low-growing species as well as the impacts they have on soil conditions. A spectrum of ground cover species was selected comprising annuals, tender perennials and some of the more common hardy perennials. These species were compared for their rate of filling in the open gaps between plantings with foliage, wherein typical planting densities were compared with plantings at a reduced density (more space between plants) in similar 11-square-foot plots. Many annual ground covers, such as Evolvulus and Ipomea, exhibited rapid growth — effectively covering the soil plots in the span of several weeks. Many perennial species, such as Liriope and Ophiopogon, benefitted from initially dense plantings to achieve the desired full coverage, as they did not spread to the same extent.

While providing coverage to the soil offers shielding from the sun and enhanced water infiltration, different ground covers exert different effects upon soil conditions. A fast-growing ground cover, Wedelia, was used as a model plant for rapid soil coverage despite its rapidly spreading habit lending credence to its consideration as a weedy invasive. This species was compared with Liriope, a ground cover with a clumping form that is popular in landscapes throughout the South, and bare soil plots in a study assessing ground cover effects on soil moisture and temperature. The results of that study indicated that regardless of whether overhead or microirrigation was employed, both species exhibited similar spread and growth patterns; however, soil moisture and temperature beneath spreading ground covers was lower than both the bunch-form and bare plots. While the efficiency in extracting water from the soil may be of benefit in some locations, particularly where ground cover spread can be managed, the comparatively limited water removal of Liriope suggests its value as a ground cover in sites lacking routine irrigation.

To the naked eye, bare soil in the landscape is unsightly; however, the impact that weather fluctuations have on these uncovered areas extends beyond aesthetics. Failure to moderate soil conditions in the landscape reduces resilience to extreme weather events and may lead to plant injury in the landscape. Using a combination of common landscape practices, such as mulching and ground covers, can help provide the support to sustain healthy landscapes throughout Louisiana — regardless of what Mother Nature throws at us.

Damon E. Abdi is an assistant professor of landscape horticulture, and Jeb S. Fields is an assistant professor and ornamental horticulture specialist, both at the LSU AgCenter Hammond Research Station.

This article appears in the winter 2024 edition of Louisiana Agriculture.

An attractively designed landscape utilizing a ground cover (Asiatic Jasmine). Photo by Damon Abdi

Sedum is used as a ground cover and is arranged to spell out LSU at the AgCenter Hammond Research Station. Photo by Damon Abdi