Preemergent Control of Phyllanthus Species

Several preemergent herbicides were compared for control of the warm season annuals Phyllanthus urinaria and P. tenellus. Approximately 55 seeds per species were applied to each 3.8-liter container filled with a pine bark:Canadian sphagnum peat:sand medium (3:1:1, by vol.; initial pH = 5.6). The six treatments (including untreated pots), with four replications per treatment and three pots per replication, were applied on 6 May 1991. When total control was estimated to be below ca. 70% based on visual observations (consensus of two observers) of percent coverage of medium surface and number of Phyllanthus for more than half the pots within a treatment, weeds were harvested and treatments reapplied. Oxadiazon (4.5 kg ai/ha), dithiopyr (3.4 kg ai/ha), and isoxaben + oryzalin (5.6 kg ai/ha) provided 60 to 90 days control of P. urinaria} oxyfluorfen + pendimethalin (2.2 + 1.1 kg ai/ha) provided 60 days control. No treatment provided more than 30 days control of P. tenellus after the first application, but oxadiazon (4.5 kg ai/ha), dithiopyr (2.2 and 3.4 kg ai/ha), and oxyfluorfen + pendimethalin (2.2 + 1.1 kg ai/ha) pro vided 60 days control after the second application. Phyllanthus urinaria L. (chamberbitter, leafflower) and Phyllanthus tenellus Roxb. (long-stalked phyllanthus) are two summer annual weeds that are becoming major prob lems in turf and ornamentals in Florida and other states in the southeastern U.S. (Elmore, 1990). Only a limited amount of research has been published with regard to their control. Preemergent control of P. tenellus was ob tained with oxadiazon + metolachlor (Stamps, 1991) and oxadiazon 4simazine (Stamps and Poole, 1987). Wehtje et al. (1992) reported that oxadiazon, oxyfluorfen + oryzalin, and oxyfluorfen + pendimethalin reduced germination of P. urinaria while paraquat, glyphosate, oxyfluorfen, and acifluorfen provided excellent postemergent control. The objective of this experiment was to evaluate several preemergent herbicides for control of these two species under container nursery conditions. Florida Agricultural Experiment Station Series No. N-00663. The au thors gratefully acknowledge Monsanto, DowElanco, and Rhone-Poulenc for their support. 200 Materials and Methods Containers (3.8-liters) were filled with a medium of pine bark:Canadian sphagnum peat:sand (3:1:1, by vol.); initial pH was 5.6. One cubic meter of medium was amended with 6.1 kg dolomite, 3.1 kg triple superphos phate, 0.9 kg Micromax (12S-0.lB-0.5Cu-12Fe-2.5Mn0.05Mo-12Zn), and 6.1 kg Osmocote 18N-2.6P-9.9K (18-612). A top dressing of Osmocote 18-6-12 was applied 3 May and 6 August 1991. On 3 May, ca. 55 seeds per species were applied to each pot. The experiment was a completely randomized design with four replications per treatment and three pots per replication. On 6 May, the following treatments were applied: 2.2 and 3.4 kg ai/ha dithiopyr (1G), 4.5 kg ai/ha oxadiazon (2G), preformulated (3G) oxyfluorfen + pen dimethalin (2.3 + 1.1 kg ai/ha), and preformulated (80DF) oryzalin + isoxaben (4.5 + 1.1 kg ai/ha); untreated pots were controls. Individual aliquots of the granular her bicides were applied uniformly to each container. The dry flowable herbicide was applied as a broadcast spray using a compressed-air backpack sprayer which delivered 374 liters/ha through a single Teejet 8004 flat fan tip. Im mediately after application, ca. 0.75 cm water was applied via overhead irrigation. The experiment was conducted under full sun. Containers were irrigated daily with 0.75 cm the first 5 weeks, and 1.27 cm daily for the remainder of the experiment. Total rainfall during the entire exper iment was 75.9 cm. The number of seedlings of each Phyllanthus species was counted every 30 days; seedlings at the two-leaf stage were counted separately but were not classified by species. Weed control was assessed by the authors every 30 days. When it was determined that a treatment was providing less than 70% control (based on number, size, and percent of the medium surface covered by both Phyllanthus spp.), the weeds in all pots within a treatment were harvested and dried; the treatment was then reapplied. All treat ments were harvested 60 days after the second application primarily because cool weather was inhibiting growth. Two sets of controls were used to accommodate differences in lengths of efficacy one set was used for the preformulated (3G) oxyfluorfen + pendimethalin treatment and the other set was used for the remainder of the treatments. Results and Discussion All treatments provided good to excellent control of both Phyllanthus species for 30 days after the initial applica tion (Tables 1 and 2). Wehtje et al. (1990) also reported that oxadiazon and oxyfluorfen 4pendimethalin provided Proc. Fla. State Hort. Soc. 105: 1992. Table 1. Preemergent control of Phyllanthus urinaria (PU), Phyllanthus tenellus (PT), and total seedlings (SDLNGS; 2-leaf stage) of both species in a soilless medium after one application.