Evaluating Recovery of Cupressaceae Taxa After Flooding at Contrasting Temperatures1

Chamaecyparis are ornamental plants that are used extensively in temperate-zone landscapes worldwide. However, due to their low tolerance to environmental stresses Chamaecyparis often perform poorly in urban landscapes. The objective of this research was to evaluate rootstocks of selected Cupressaceae taxa to determine their adaptability to poor drainage and high temperatures found commonly in urban landscapes. To accomplish this objective, 10 taxa (Chamaecyparis, Platycladus, Thuga spp., and x Cupressocyparis leylandii) were grown in 2.8 liter (#1) black plastic containers with a pinebark:sand (8:1 by vol) medium for 19 weeks in two greenhouses with 9/ 15 hr day/night temperatures of either 22/18C (72/64F) or 30/26C (86/79F). Half the plants in each greenhouse were flooded for 4 weeks. Root relative growth rate (RGR) was a better indicator of plant performance under flooded conditions compared to shoot RGR. At 22/18C (72/64F), root RGR of Chamaecyparis thyoides and Thuja ‘Green Giant’ only declined 10% and 11% between nonflooded and flooded plants, respectively. Chamaecyparis obtusa and x Cupressocyparis leylandii had the largest percentage decline in root RGR from nonflooded to flooded plants with 71% and 213%, respectively. Chamaecyparis thyoides had the highest root RGR at 30/ 26C (86/79F) in both nonflooded and flooded conditions with a 19% decrease in root RGR between nonflooded and flooded. Chamaecyparis lawsoniana and T. ‘Green Giant’ were ranked 2 and 3 in flooded conditions at 30/26C (86/79F); however, percentage decline increased to 43% and 46%, respectively. At this temperature, the remaining seven taxa had greater than 50% decline in root RGR in flooded conditions compared to nonflooded plants. In nonflooded conditions, shoot and root RGR of all species decreased from 22/18C (72/64F) to 30/26C (86/79F) except for Chamaecyparis thyoides. The shoot and root RGR of Chamaecyparis thyoides grown in 22/18C (72/64F) and 30/26C (86/79F) were similar in nonflooded conditions. Chamaecyparis thyoides demonstrated excellent tolerance to flooding and temperature and could be a desirable understock for other Chamaecyparis when grown in poorly drained locations. Index words: environmental stress, falsecypress, whitecedar, arborvitae, landscape, hypoxia, root. Taxa used in this study: Chamaecyparis lawsoniana (A. Murr.) Parl, Lawson falsecypress; Chamaecyparis nootkatensis (D. Don) Spach, Alaska cedar; Chamaecyparis obtusa (Siebold & Zucc.) Endl., Hinoki falsecypress; Chamaecyparis pisifera (Siebold & Zucc.) Endl., Sawara falsecypress; Chamaecyparis thyoides (L.) BSP, Atlantic whitecedar; x Cupressocyparis leylandii Dallim & A. B. Jacks, Leyland cypress; Platycladus orientalis (L.) Franco, Oriental arborvitae; Thuja occidentalis L., eastern arborvitae; Thuja ‘Green Giant’ arborvitae; Thuja plicata J. Donn ex D. Don, giant arborvitae or western redcedar. Significance to the Nursery Industry Chamaecyparis are landscape plants used extensively in temperate-zone landscapes worldwide. However, poor adaptability to extremes of heat or flooding often limits use of these plants in stressful sites. This research identified a wide range of tolerance to root-zone flooding and temperature among different members of the Cupressaceae that are potential understocks for Chamaecyparis. Grafting may provide an effective means for enhancing adaptability and tolerance to environmental and biological stresses by exploiting superior taxa for use as rootstocks. Additional research is currently focusing on interspecific and intergeneric graft compatability among selected species and clones. Introduction The genus Chamaecyparis includes many desirable and commercially important taxa. Although there are only six to seven species in the genus, the taxa vary considerably with over 240 cultivars of Chamaecyparis lawsoniana alone (14). Unfortunately, many Chamaecyparis taxa are native to cool, temperate climates and may perform poorly in stressful landscape situations, particularly under conditions of poor drainage (hypoxia), high temperatures, and presence of Phytophthora pathogens (5, 12). Chamaecyparis lawsoniana and Chamaecyparis nootkatensis, for example, are both native to the cool climate of the Pacific Northwest United States and often have poor survival in less than ideal landscape settings. In addition, Weathers (21) offered anecdotal evidence that Chamaecyparis obtusa will not survive in clay soil in the mid-south (Durham, NC) region of the United States. For a variety of reasons, urban soils often have poor drainage resulting in saturated soils for extented periods (4). In well-structured soils, a day or two of heavy rainfall and high temperatures can induce anaerobic conditions in spring and summer, when plant growth rates and microbial activity are high (9). These short-term anaerobic periods may cause damage to root systems. However in urban soils, waterlogging and anaerobiosis may occur for extended periods (11), causing severe restrictions to root growth and even death of the plant. Raulston (18) stated that the key to plant adaptability in many landscape situations was root survival under hot, wet conditions.