Effects of Timing and Rate of Glyphosate Application on Toxicity to Selected Woody Ornamentals

Abstract

Abstract Thirteen species of woody ornamentals were treated over-the-top with glyphosate in a 6 × 6, rate by time factorial experiment. The influence of application timing on glyphosate phytotoxicity was significant for all species. The times of maximum tolerance and injury were species dependent. Species were organized into 4 response groups based on the effects of application time. Group 1 species, including ajuga (Ajuga reptans L.), azalea (Rhododendron obtusum Planch. ‘Coral Bells’), and a variegated liriope (Liriope muscari L.H. Bailey), were injured on all application dates. Species in groups 2, 3, and 4 exhibited tolerance to fall applications of glyphosate. Group 2, including wax leaf privet (Ligustrum japonicum Thunb.), sustained maximal injury from spring applications. Group 3 species, including Compacta holly (Ilex crenata Thunb. ‘Compacta’), were injured most by summer applications of glyphosate. However, Blue Rug juniper (Juniperus horizontalis Moench ‘Wiltonii’), a representative of group 4, was tolerant of glyphosate applications, sustaining only temporary tip chlorosis from spring and early summer treatments. First season evaluations were not sufficient to describe the ultimate effects of glyphosate on plant quality. Visual and objective evaluations in the 2nd growth season also were necessary. Chemical name used: N-(phosphonomethyl) glycine (glyphosate).