Effects of Frequent Shearing on Root Growth and Mycorrhizal Colonization of Two Landscape Shrubs

Lencophyllum frutescens I. M. Johnst. (Texas sage) and Nerium oleander L. (oleander) shrubs grown for 2 years in the southwest United States under well-watered conditions in outdoor field plots were either sheared every 6 weeks or not pruned (control) to determine if frequent shearing had an effect on root growth and mycorrhizal colonization. During February and June of the second year after transplanting, leaf gas exchange, shoot and root growth, and arbuscular mycorrhizal fungi (AMF) colonization of shrubs were studied. Shearing reduced shrub volume of Texas sage and oleander by 84% and 82%, respectively. Leaf carbon assimilation (A) and conductance of both shrub taxa were stimulated by frequent shearing, especially during June. Shearing decreased root mass density (RAID) and root length density (RLD) of Texas sage but had no impact on RMD or RLD of oleander. Shearing decreased the length of Texas sage roots colonized by AM F but increased AM F colonization of oleander roots. Soil respiration and temperatures were less under all shrubs that were frequently sheared than those that were not pruned and were higher under all shrubs in June than in February. From these data we conclude that under well-watered conditions, the rejuvenative capacity and resilience of oleander to the practice of frequent shearing is greater than Texas sage and recommend that Texas sage shrubs not be frequently sheared in southwest landscapes.