Weed management for nursery ornamental plants requires several applications of preemergence herbicides to control recurring weed emergence and to counter chemical dissipation and leaching losses. Herbicide losses from plant containers in runoff water may pose risks to surface and groundwater supplies. Sphagnum peat is a frequently used rooting medium. Limited information exists on the fate of herbicides applied to peat. The leaching of two commonly used chemicals in Europe by the containerized ornamental plant industry, isoxaben and BAS 479 14H, was measured. Herbicides were applied to 1-yr-old conifers (Juniperus communis, cv. Repanda [common juniper]) grown in pots containing sphagnum peat. The isoxaben-treated pots were placed on 1- by 1-m lysimeters that were buried in the center of replicate field plots. Pots were also placed in subirrigated sandbeds to determine herbicide dissipation and movement in irrigation water. With overhead irrigation, isoxaben had a half-life of 2 mo in the 0- to 5-cm depth of the peat container. More than 96% of applied isoxaben was recovered in the 0- to 5-cm depth. Less than 4% of the amount applied was found in the 5- to 10-cm depth, and none was detected in the 10- to 15-cm depth. No isoxaben residues were detected in water percolating through the field lysimeters. In subirrigated sandbeds, no isoxaben was detected in the water rising by capillary tension. Most of the applied isoxaben was found in the 0- to 5-cm layer of the peat container, where its half-life was also 2 mo. Similar results were observed in containers treated with BAS 479 14H, which had a half-life of 1.2 mo. The high organic matter content of the peat medium strongly sorbed both isoxaben and BAS 479 14H, and these herbicides remained primarily in the near-surface zone of the peat containers. No significant herbicide leaching or loss from the containers was found when subirrigated or irrigated by sprinkler.

We tested whether the efficacy of chemical weed control might change as atmospheric CO2 concentration [CO2] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO2]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot pigweed), a C4 species, and Chenopodium album L. (common lambsquarters), a C3 species, grown at near ambient (360 μmol mol−1) and twice ambient (720 μmol mol−1) [CO2] for 14 d following glyphosate application at rates of 0.00 (control), 0.112 kg ai ha−1 (0.1 X the commercial rate), and 1.12 kg ai ha−1 (1.0 X the commercial rate) in four separate trials. Irrespective of [CO2], growth of the C4 species, A. retroflexus, was significantly reduced and was eliminated altogether at glyphosate application rates of 0.112 and 1.12 kg ai ha−1, respectively However, in contrast to the ambient [CO2] treatment, an application rate of 0.112 kg ai ha−1 had no effect on growth, and a 1.12-kg ai ha−1 rate reduced but did not eliminate growth in elevated [CO2]-grown C. album. Although glyphosate tolerant does increase with plant size at the time of application, differences in glyphosate tolerance between CO2 treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO2] could increase glyphosate tolerance in a C3 weedy species. Changes in herbicide tolerance at elevated [CO2] could limit chemical weed control efficacy and increase weed–crop competition.