Assessing tolerance of longleaf pine understory herbaceous plants to herbicide applications in a container nursery
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Renewed efforts in longleaf pine (Pinus palustris Mill.) ecosystem restoration has increased interest in the commercial production of understory herbaceous species. Successful establishment of understory herbaceous species is enhanced when using quality nursery-grown plants that have a better chance of survival after outplanting. Nursery growing practices have not been clearly identified for propagating longleaf pine understory herbaceous species, especially for controlling weeds that can reduce plant growth in containers. To identify potential herbicides that could safely be applied postemergence to herbaceous plants in containers, trials were installed at the North Carolina Forest Service Nursery in Goldsboro, NC. Different container sets of wiregrass (Aristida stricta Michx.) and muhly grass [Muhlenbergia expansa (Poir.) Trin.] seedlings were treated at two growth stages with pendimethalin, oxyfluorfen, lactofen, or oxadiazon. One-year-old wiregrass was also treated 6 months after being outplanted at the nursery with the same treatments except that clopyralid was tested instead of oxadiazon. Oxyfluorfen killed all seedlings tested within 2 weeks of treatment with the exception of 7-week-old container-grown muhly grass. Lactofen and oxadiazon applications at 2 and 7 weeks post-sowing killed both grass species and caused moderate injury, respectively. Lactofen caused foliar damage, while pendimethalin and clopyralid did not affect outplanted wiregrass. Pendimethalin applied 7 weeks post-sowing did not injure muhly grass seedlings but did cause more severe injury when applied 2 weeks post-sowing. Results of the trials indicate that pendimethalin may be safe to use over-the-top of 7-week-old container-grown muhly grass and 1-year-old outplanted wiregrass as a preemergence application to targeted weeds.