Abstract
Mulching landscape beds is a common task for landscapers seeking to affect soil conditions and reduce weed pressure. This study investigated the effects of three pine (Pinus sp.) straw mulch depths (5, 10, and 15 cm) on soil moisture/temperature modulation during late winter/spring. No differences in soil volumetric water content were observed; however, increasing mulch depth to ≥10 cm decreased fluctuations in temperature. This research provides a better understanding of the effect of mulch depth and potential environmental benefits so that landscape contractors can determine cost-benefits of mulching applications.
Mulches are used to moderate soil moisture and temperature, suppress weed growth, and maintain aesthetic appeal in the landscape (Chalker-Scott 2007; Cregg and Schutzki 2009; Duryea et al. 1999). Benefits of mulch vary based on material type, application depth, and site conditions (Chen et al. 2013). One of the more common mulches used in the southeastern United States is pine (Pinus sp.) straw, a readily available resource throughout the region (Singh et al. 2021). Debate over the proper application depth of pine straw exists, with recommendations ranging from 2 to 8 inches (Bachman 2020; Westerfield 2022; Williamson and McLeod-Scott 2016). It is a common practice to re-apply/topdress pine straw at least semiannually (Owings 2009); however, optimizing the initial application depth, particularly during transition seasons, bears the most importance in mulch management plans. Although widely available throughout the southeastern United States and comparable in price to other mulch types, maximizing the value of applied pine straw is both economically and environmentally important to landscape professionals and their clientele, particularly during the installation phase (Pope 1991). Therefore, this study was conducted to elucidate the effects that initial pine straw application depth has on soil moisture retention and soil temperature modulation over the course of a spring season in southeastern Louisiana, USA. This short communication aims to provide a concise, direct assessment on the effects of initial pine straw depth application in landscape soils.
Materials and methods
Site preparation and establishment
An 80-m2 plot (4 m × 20 m) was prepared at the Louisiana State University Agricultural Center Hammond Research Station located in Hammond, LA, USA (lat. 30.5044°N, long. 90.4612°W). Wherein, the soil, a fine-loamy, siliceous, semiactive, thermic typic Hapludult, was cultivated to a depth of 10 cm and amended with a locally sourced landscape mix (composed of pine bark, sand, dolomitic lime, micronutrients, and magnesium sulfate) to provide uniform soil conditions representative of landscape beds (Phillips Bark Processing Co., Brookhaven, MS, USA). Twenty individual 1-m2 plots were partitioned, with 50 cm between plots north to south and 1 m separating the two rows of plots east to west. Locally sourced pine straw from loblolly pine (Pinus taeda) was applied at depths of 5, 10, or 15 cm on 14 Feb 2023 with no pine straw as the control. Plots had no plant material installed to more clearly assess the impact of mulch depth alone. Straw depth was randomly assigned to plots, with n = 5 for each treatment. The experimental area was irrigated weekly (every Wednesday) using 360° sprayer (model XS360TS Adj True Spray; Rain Bird Corp., Azusa, CA, USA) applicators located 30 cm above the soil surface within the center of each experimental unit. A total of 9.3 L·m2 of water was applied per irrigation event. The study was conducted from 14 Feb to 8 May 2023.
Data collection
A soil moisture sensor (Teros 12; METER Group Inc., Pullman, WA, USA) was installed in the center of each experimental unit at 15 cm beneath the soil surface to monitor soil volumetric water content (VWC) and temperature. All soil sensors along with an on-site tipping bucket rain gauge (TR-525I; Texas Electronics, Dallas, TX, USA) were connected to a data logger (CR1000x; Campbell Scientific, Logan, UT, USA) that collected data every 10 min to generate hourly averages for both soil VWC and temperature from 14 Feb to 8 May 2023. Daily averages were derived from all measurements over a 24-h period with flux calculated as maximum-minimum for soil moisture and temperature. Ambient air temperature throughout the study was compiled from the Hammond Northshore Regional Airport weather station (Hammond, LA, USA).
Data were analyzed using statistical analysis software (JMP Pro version 17.0.0; SAS Institute Inc., Cary, NC, USA) to conduct a one-way analysis of variance on mulch depth effects on soil VWCs and temperature modulation. Means were separated following Tukey’s test at P = 0.05 significance.
Results and discussion
Soil moisture content
There were no differences in average soil VWC between mulched plots on any individual date throughout the study (Fig. 1). This was likely influenced by the low evaporative demand throughout the season, as well as the frequency with which plots received irrigation and precipitation. The flux between the maximum and minimum VWC of each day exhibited differences, in which a greater difference was observed in the unmulched plots vs. the plots mulched with 15 cm of pine straw on 16 of the 84 d in this study. These dates were typically at least 24 to 48 h after any irrigation or rainfall and reflected a more rapid drying of the soil in uncovered, unmulched plots.
Soil temperature
Application of mulch had substantial effects on the soil temperature. Differences in average soil temperature were observed on 59 of the 84 d (Fig. 2), 56 of which had warmer temperatures in the unmulched plot compared with the plots mulched with 15 cm of pine straw. The 3 d where this phenomenon was reversed were during times of cooler weather, reflecting the capacity of the mulched plot to buffer sudden shifts in ambient air temperature in either direction. The flux between the daily high and daily low temperature of the soil offers insight toward the temperature-buffering capacity of different pine straw depths. On 83 of the 84 d in this study, there were differences in temperature flux, where in all cases the unmulched areas exhibited greater flux than the plots mulched with 15 cm of pine straw. Mulching plots with 5 cm of pine straw rarely (3 of the 83 d) reduced temperature flux compared with the control; however, mulching plots with 10 cm of pine straw reduced flux compared with the control on 82 of the 83 d.
Discussion
The results of our study were similar to Singh et al. (2021), in which application of pine straw maintained greater soil moisture and buffered soil temperature fluctuations more effectively than unmulched areas. The moderation of soil temperature provided by mulch was particularly evident in this study, as the equivalent soil VWC levels between plots would limit the impact of soil-water acting as the temperature buffer. Unmulched areas were more sensitive to temperature shifts, most notably during the relatively low temperatures experienced from 12 to 20 Mar. During this period, unmulched plots that had been consistently warmer throughout the study cooled faster than their mulched counterparts. Similarly, temperature in these unmulched plots increased more rapidly than in the mulched plots as ambient temperatures increased.
Expanding upon the temperature effects observed in this study and the fact that the soil moisture was equivalent between plots, it is likely that the physical barrier to light transmission was a primary factor in temperature moderation. Mulches limit light transmission (Chalker-Scott 2007; Petrikovszki et al. 2020; Saha et al. 2018), making it indeed likely that the shading and light exclusion provided by the mulch, especially at thicker application depths, limited the amount of solar radiation on the soil surface, thus mitigating temperature increases.
It is necessary to balance the value of thicker application depths with the greater material costs that requires. Proponents of thicker pine straw application depths often recognize the compression that occurs over time (Westerfield 2022), particularly after rainfall/irrigation. Therefore, a balance between the environmental benefits of thicker mulch and the economic and labor implications of applying thicker mulch/replenishing compressed mulch must be considered. Although relatively excessive mulch application depths may be environmentally unwarranted at installation, they may be economically advisable compared with replenishing compressed mulch in the future.
Conclusions
Pine straw mulch provides greater soil moisture and temperature insulation compared with unmulched areas. Although both moisture and temperature benefits were provided when mulching with 5 cm of pine straw, ≥10 cm of pine straw provided the most consistent benefits in both facets. Landscapers must weigh the mild additional benefits of applying >10 cm of pine straw with the increased up-front material and labor costs required. Although greater application depths are unlikely to provide further insulative benefits, they may ensure more longevity in the landscape as the mulch compresses and reduce or remove the need to re-apply pine straw throughout the season, in effect offering more efficiency from an operational perspective. Further research investigating the effects of pine straw depths on soil conditions, limitation on light transmittance, and the longevity of pine straw over a longer duration can better inform landscape contractors on mulching best management practices.
Units
References cited
Bachman G. Mulches for the landscape. 2020. Mississippi State Univ Ext Pub 2301.
Chalker-Scott L. 2007. Impact of mulches on landscape plants and the environment – A review. J Environ Hortic. 25:239–249. https://doi.org/10.24266/0738-2898-25.4.239.
Chen Y, Strahan RE, Bracy RP. 2013. Effects of mulching and preemergence herbicide placement on yellow nutsedge control and ornamental plant quality in landscape beds. HortTechnology. 23:651–658. https://doi.org/10.21273/HORTTECH.23.5.651.
Cregg BM, Schutzki R. 2009. Weed control and organic mulches affect physiology and growth of landscape shrubs. HortScience. 44:1419–1424. https://doi.org/10.21273/HORTSCI.44.5.1419.
Duryea ML, English RJ, Hermansen LA. 1999. A comparison of landscape mulches: Chemical, allelopathic, and decomposition properties. J Arboric. 25:88–97. https://doi.org/10.48044/jauf.1999.014.
Owings AD. 2009. Pine straw makes excellent mulch. https://www.lsuagcenter.com/portals/communications/news/news_archive/2009/march/news_you_can_use/pine-straw-makes-excellent-mulch. [accessed 7 Jul 2023].
Petrikovszki R, Zalai M, Tóthné Bogdányi F, Tóth F. 2020. The effect of organic mulching and irrigation on the weed species composition and the soil weed seed bank of tomato. Plants. 9:1–15. https://doi.org/10.3390/plants9010066.
Pope TE. 1991. Pine straw: Mother nature’s mulch. Louisiana Dept Agric For Pub 2387.
Saha D, Marble SC, Pearson BJ. 2018. Allelopathic effects of common landscape and nursery mulch materials on weed control. Front Plant Sci. 9:1–5. https://doi.org/10.3389/fpls.2018.00733.
Singh Z, Maggard A, Barlow R, Kush J. 2021. A comparison of the attributes of pine straw from southern pine species. J Environ Hortic. 39:115–122. https://doi.org/10.24266/0738-2898-39.3.115.
Westerfield RR. 2022. Mulching vegetables. Univ Georgia Ext Pub 984.
Williamson J, McLeod-Scott J. 2016. Mulch. Clemson Coop Ext HGIC Ext Pub 1604.