2018 ASHS Annual Conference

Chemical plant growth regulators (PGRs) are commonly used to control growth of containerized plants. However, commercial PGRs are not labeled for containerized culinary herbs and alternatively growth control strategies must be used. The objectives of our research were to quantify the impact of restricting P on the growth and tissue nutrient concentration of basil (Ocimum basilicum L.), dill (Anethum graveolens L.), and parsley [Petroselinum crispum (Mill.) Fuss.], and common sage (Salvia officinalis L.). Seedlings were individually transplanted into 4.5-inch diameter round containers filled with a substrate comprised of (by vol.) 80% ground sphagnum peat moss and 20% coarse perlite and adjusted to a pH of ~6.0 using calcitic limestone. Beginning at transplanting, plants were irrigated with nutrient solutions providing 0, 5, 10, 20, or 40 mg∙L^–1 P formulated from technical-grade compounds. the N concentration was 150 mg∙L^–1 and, along with all other macro- and micronutrients except for P, was consistent across all solutions. Four weeks after seedlings after seedlings were transplanted and P treatments began, data were collected on plant height and width, node number, leaf area and internode and branch length. Shoots were harvested and dried for 3 d, after which mass was recorded and shoots were submitted to a commercial laboratory for tissue analyses. For all four species, plant height and width increased as P increased. However, the relationship between P concentrations and plant height was non-linear. As P concentration increased above 0 plant height increased until a maximum height was achieved height increases plateaued. Height increased for basil, dill parsley, and sage as phosphorous increased from 0 to 20 mg∙L^–1, but did not increase as concentration further increased to 40 mg∙L^–1. Restricting P applications to 5 mg∙L^–1 resulted in plants that were 12.0 cm (29%), 9.0 cm (32%), 4.6 cm (20%), and 6.7 cm (27%) for basil, dill, parsley, and sage, respectively. The effect of P on width followed a similar trend. In addition to height and width, restricting phosphorous also restricted branching and leaf expansion. While P concentrations that produced compact plants also resulted in tissue P concentrations that were below recommendation tissue concentration minimum values, visible P deficiency symptoms were not manifested. The results of this study illustrate that restricting P can suppress plant growth of containerized herbs, resulting in plants that are proportional to their containers and lack visible deficiency symptoms that may diminish retail marketability.