Citation: McBrayer, R.H.; Pickens, J.M.; Witcher, A.L.; Wells, D.E.; Sibley, J.L. Effects of Nursery Container Color and Spacing on Root Zone Temperatures of ‘Soft Touch’ Holly. Agriculture 2022, 12, 2165. https://doi.org/10.3390/ agriculture12122165 Academic Editor: Jeb S. Fields Received: 11 November 2022 Accepted: 15 December 2022 Published: 16 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agriculture Communication Effects of Nursery Container Color and Spacing on Root Zone Temperatures of ‘Soft Touch’ Holly † R. Hunter McBrayer 1 , Jeremy M. Pickens 1, *, Anthony L. Witcher 2 , Daniel E. Wells 1 and Jeff L. Sibley 1 1 Department of Horticulture, Auburn University, Auburn, AL 36849, USA 2 Otis L. Floyd Nursery Research Center, Department of Agricultural and Environmental Sciences, Tennessee State University, McMinnville, TN 37110, USA * Correspondence: pickejm@auburn.edu † This manuscript is part of a Master thesis by the first author, available online at McBrayer Thesis 2021_Final.pdf (auburn.edu). Abstract: Newly up-potted ‘Soft Touch’ Japanese hollies (Ilex crenata ‘Soft Touch’) were grown in Mobile, AL in 1.5 L containers to evaluate the effects of growth from black or white container colors and container spacing (jammed or spaced) in relation to root zone temperature. Two treatments, container color and container spacing, were evaluated and root ratings were reported. At termination, an interaction was observed in growth from 43 to 141 days after potting between container color and spacing. Both white container treatments and the black-jammed treatment experienced 36% and 21% more growth than black-spaced plants. Root ratings for white containers (jammed and spaced) were 42% greater than for black-spaced. Black-jammed root ratings were 25% greater than black-spaced. Black-spaced containers experienced the greatest number of time intervals over the critical temperature of 39 ◦ C when compared to other treatments. Results suggest that ‘Soft Touch’ holly may be grown at final spacing when using white containers and have little impact from elevated root zone temperatures. Keywords: root zone temperature; container color; nursery production; abiotic stress 1. Introduction Since the late 1950s, the effects of supraoptimal root zone temperatures (RZT) have been known to impact production of containerized nursery stock [1]. Black plastic nursery containers absorb solar radiation, and heat energy is then reradiated to the container substrate. As the heat capacity of the substrate is higher than the surrounding air, the substrate gains heat faster than it can be lost, resulting in temperatures well over ambient. Root zone temperatures have been reported in excess of 50 ◦ C[2,3]. The effects on root growth can easily be observed on mature plants with little root growth occurring in the south-westerly portion of the container, as it has the greatest exposure to solar radiation. Many nursery growers are unaware of the impacts of RZT. With entire crop affected, it is difficult to know there is an issue without a comparison of plants with near-optimal RZT [4]. A considerable amount of research has been conducted towards alleviating elevated RZT [5]. Successful methods include pot-in-pot [6–8], container [9,10], shading [11,12], container material and design [13–15], and container color [15–17]. Irrigation has histor- ically been thought to alleviate elevated RZT; however, research has shown otherwise. Irrigation frequency has been demonstrated to have a small impact on RZT [18]. Martin and Ingram [19] demonstrated that it takes an unrealistic amount of water to cool RZT to optimum levels. A common practice among nursery growers is to “jam” or place containers “can-to- can” to reduce RZT and conserve space for newly potted material. By jamming plants, the container sidewalls are shielded from solar radiation. Plants are later spaced to allow for Agriculture 2022, 12, 2165. https://doi.org/10.3390/agriculture12122165 https://www.mdpi.com/journal/agriculture