The 2012 ASHS Annual Conference

In containerized production of herbaceous ornamentals, weedy plant species compete for moisture and nutrients, often resulting in ornamentals with reduced growth and poor aesthetics.  Hand weeding is costly and labor intensive.  Many currently available herbicides labeled for use on agronomic crops cannot legally be used for weed control for many container-grown herbaceous ornamentals because the products have not been labeled for use on those crops.  The United States Department of Agriculture’s IR-4 Project coordinates research to determine the efficacy and safety of pest control products on horticultural crops.  The objectives of the 2011 study were to determine weed control efficacy and effects on crop plant growth of four preemergent  herbicides (Certainty, Echelon, Freehand, and Snapshot) on  thirteen species of container-grown herbaceous ornamentals (Artemisia frigida, Geranium macrorrhizum, Helianthemum nummularium, Hierochloe odorata, Lamiastrum galeobdolon, Liatris spicata, Matthiola incana, Phlox subulata, Santolina chamaecyparissus, Scabiosa columbaria, Tagetes erecta, Veronica spicata, and Veronica austriaca subsp. teucrium).  Although not all four products were evaluated on all thirteen species, two products were evaluated on Helianthemum nummularium and Liatris spicata.  Each product was evaluated at three rates: the suggested label rate (1X), two times the suggested label rate (2X), and four times the suggested label rate (4X).  Plants growing in the treated containers were compared to plants growing in the weedy control (WC) and weed-free control (WFC) containers.  Each treatment was replicated five times, and the experiments were conducted two times.  In all containers except the WFC containers, twenty seeds of each of five weedy plant species were sown into growing media prior to the first application of herbicide products.  For each experiment, the herbicide products were applied two times, with the second application 6 weeks after the first.  Counts of weedy plant species growing in each container were taken in August.  Height and width measurements were recorded for each plant at the beginning and end of each experiment. Phytotoxicity data were collected periodically after herbicide applications.  Each plant was harvested in mid August, placed in a paper bag, dried in a drying oven, and then weighed to determine dry mass.  Results indicate that weed control efficacy improved when herbicides were applied at rates greater than the 1X rate.  When applied at the three rates evaluated, Echelon, Freehand, and Snapshot had no adverse effects on the crop plants on which they evaluated.  Certainty at all three rates stunted the growth and damaged some of the Liatris spicata plants.