Common reed (Phragmites australis) is an invasive perennial grass in aquatic and riparian environments across the United States, forming monotypic stands that displace native vegetation that provides food and cover for wildlife. Genetic variation in global populations of common reed has given rise to two invasive haplotypes, I and M, in the United States. Our objectives were to (1) determine if any differences in herbicide efficacy exist with respect to common reed haplotypes I and M and (2) screen for other labeled aquatic herbicides that may have activity on common reed haplotypes I and M, most notably imazamox and diquat. A replicated outdoor mesocosm study was conducted in 1,136-L (300-gal) tanks using haplotypes I and M of common reed. Restriction fragment length polymorphism methodologies were used to verify the identification of I and M haplotypes used in this study. Diquat at 2.2 (1.9) and 4.5 (4.0) kg ai ha⁻¹ (lb ai ac⁻¹), glyphosate at 2.1 (1.8) and 4.2 (3.7) kg ae ha⁻¹ (lb ae ac⁻¹), imazamox at 0.6 (0.5) and 1.1 (0.9) kg ai ha⁻¹ (lb ai ac⁻¹), imazapyr at 0.8 (0.7) and 1.7 (1.5) kg ai ha⁻¹ (lb ai ac⁻¹), and triclopyr at 3.4 (3.0) and 6.7 (5.9) kg ae ha⁻¹ (lb ae ac⁻¹) were applied to the foliage of common reed. After 12 wk, no difference (P  =  0.28) in herbicide tolerance was seen between the two haplotypes with respect to biomass. The 4.2-kg ae ha⁻¹ rate of glyphosate and the 0.8- and 1.7 kg ai ha⁻¹ rates of imazapyr reduced common reed by > 90% at 12 wk after treatment (WAT). Imazamox at 0.6 and 1.1 kg ai ha⁻¹, and triclopyr at 3.4 and 6.7 kg ae ha⁻¹ reduced common reed biomass (62–86%) at 12 WAT, though regrowth occurred. Diquat did not significantly reduce biomass by 12 wk. Glyphosate and imazapyr were the only herbicides that resulted in > 90% biomass reduction and corroborate control from previous studies.

Nomenclature: Diquat; glyphosate; imazamox; imazapyr; triclopyr; common reed, Phragmites australis (Cav.) Trin. ex Steud PHRCO.

Management Implications: Common reed (Phragmites australis) is an invasive perennial grass in aquatic and riparian environments across the United States. Common reed has high genetic variability, with two unique haplotypes, I and M, having the greatest distribution. Our common garden mesocosm study indicated that there was no difference in the susceptibility of the two most common haplotypes (I and M) to a selection of aquatic herbicides. Glyphosate and imazapyr at all rates evaluated provided excellent control of common reed and would be recommended for large-scale applications. Triclopyr provided good control only at the maximum application rate, and regrowth tended be quicker (as observed new shoot growth) than in plants treated with either glyphosate or imazapyr, but may be a more selective option depending upon associated species. Identifying effective herbicides and use patterns for controlling common reed will provide land managers with necessary information on effective selective and nonselective control of this invasive plant in aquatic systems.