ASHS 2024 Conference

Containerized coneflower (Echinacea sp.) production in greenhouses and nurseries often relies on commercial fertilizers, such as 20 nitrogen (N)–4.4 phosphorous (P)–16.6 potassium (K), applied at 100 to 200 mg·L−1 N; however, increasing N concentrations proportionately increase phosphorous pentoxide (P2O5) and thus, elemental P concentrations. As such, the recommended N fertilization supplies P concentrations of 21.8 to 43.6 mg·L−1 P, exceeding plant requirements and potentially leading to excessive stem elongation and P runoff, posing ecological risks. Therefore, the objective of this study was to evaluate the effects of P concentrations on the growth, ornamental value, and tissue mineral nutrient concentrations of coneflower (E. × hybrida Sombrero® Granada Gold ‘Balsomold’). A custom soilless substrate was formulated with (by vol.) 55% aged pine bark, 35% sphagnum peatmoss, and 10% perlite, and amended with 0.6 kg·m−3 wetting agent and 3.9 kg·m−3 dolomitic limestone to achieve a final pH of ≈5.6 to 5.8. Young plants of coneflower were individually transplanted into 16.5-cm-diameter (1.7 L) containers filled with the custom soilless substrate. Upon transplanting and throughout the experiment, plants were irrigated with nutrition solutions formulated from technical grade salts providing 0, 2.5, 5, 7.5, 10, 15, 20, or 30 mg·L–1 P. Plants were grown in a glass-glazed greenhouse at 20 °C under ambient daylight supplemented with a photosynthetic photo flux density of ≈120 µmol·m–2·s–1 delivered from light-emitting diode lamps from 0600 to 2200 hr (16-h photoperiod) to achieve a daily light integral of 14 mol·m–2·d–1. At anthesis (≈69 d after transplant), data were collected. In general, plant height, plant diameter, and shoot and root dry weights were significantly influenced by P concentrations, although to different magnitudes. Quadratic plateau models demonstrated plant height, plant diameter, and shoot dry weight were maximized at 52.7 cm, 36.5 cm, and 27.2 g, respectively, with P concentrations of 18.3, 15.9, and 23.4 mg·L−1 P, respectively. Root dry weight demonstrated a positive linear correlation with increasing P concentration where roots were 74% (3.3 g) larger at a P concentration of 30 than 0 mg·L−1 P. Concentrations of ≤ 7.5 mg·L−1 P resulted low (≤ 0.2% P) leaf tissue P concentrations and ≤ 2.5 mg·L−1 P resulted in low-quality plants with incipient symptoms of P deficiency. Collectively, these results indicate that a narrow range of P concentrations may be used to control coneflower growth and mitigate P pollution.