Determining Differential Tolerances of Newly Released vs. Traditional Cultivars of Common Ornamental Species to Preemergence Herbicides

Abstract

Abstract Hundreds of new woody ornamental plant cultivars are introduced into the nursery industry each year which have many desirable aesthetic traits. However, in recent years growers have reported a higher level of herbicide sensitivity with certain cultivars compared with older cultivars that have been in the trade for multiple years. The objective of this research was to determine the tolerance of 12 different cultivars of five ornamental species including four cultivars of Loropetalum chinense [‘Ruby’, ‘Shang-hi’ PP18331 (Purple Diamond®), ‘Irodori’ USPP 27713 (Jazz Hands®), and ‘PIILC-I’ (Crimson Fire™), and two cultivars of Gardenia jasminoides (‘Frostproof’ and ‘Buttons’), Lagerstroemia indica [‘JM7’ PP34092 (Thunderstruck™ Ruby) and ‘Tuscarora’], Rhododendron [‘Conlet’ PP12111 (Autumn Carnival Encore®) and ‘Fashion’], and Ligustrum sinense Sunshine (‘Sunshine’ PP20379 and ‘Variegatum’) to spray-applied applications of dimethenamid-P or isoxaben + prodiamine and granular applications of dimethenamid-P + pendimethalin and indaziflam. While little to no injury was observed in gardenia or crape myrtles, significant injury and differences among cultivars of the same species were observed in azalea, loropetalum, and ligustrum. Results indicate that all new cultivars should be evaluated for herbicide tolerance by growers prior to wide scale application as significant differences in both growth and injury ratings were observed between different cultivars of the same species. Species used in this study: Ruby Loropetalum (Loropetalum chinense (R.Br.) Oliv. ‘Ruby’); Purple Diamond® loropetalum (Loropetalum chinense ‘Shang-hi’ PP18331); Jazz Hands loropetalum (Loropetalum chinense ‘Irodori’ USPP 27713); Crimson Fire™ loropetalum (Loropetalum chinense var. rubrum ‘PIILC-I’); Frostproof gardenia (Gardenia jasminoides J.Ellis ‘Frostproof’); Buttons gardenia (Gardenia jasminoides ‘Buttons’); Thunderstruck™ Ruby crape myrtle (Lagerstroemia × ‘JM7’ PP34092); Tuscarora crape myrtle (Lagerstroemia indica L. ‘Tuscarora’); Autumn Carnival Encore® azalea (Rhododendron ‘Conlet’ PP12111); Fashion azalea (Rhododendron × ‘Fashion’); Sunshine ligustrum (Ligustrum sinense Lour. ‘Sunshine’ PP20379); Variegated ligustrum (Ligustrum sinense ‘Variegatum’). Chemicals used in this study: dimethenamid-P (Tower®), (S)-2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)-acetamide; dimethenamid-P+ pendimethalin (FreeHand®) (S)-2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide + N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenam; indaziflam (Marengo®G) N-[(1R,2S)-2,3-dihydro-2,6-dimethyl-1H-inden-1-yl]-6-[(1RS)-1 fluoroethyl]-1,3,5-triazine-2,4-diamine; prodiamine + isoxaben (Gemini® SC) 2,6-Dinitro-N1,N1-dipropyl-4-(trifluoromethyl)benzene-1,3-diamine + 2,6-Dimethoxy-N-[3-(3-methylpentan-3-yl)-1,2-oxazol-5-yl]benzamide.