Optimum N and P rates for greenhouse production of vegetative coleus using constant liquid feed

ABSTRACT

Many new coleus (C. hybridus cv.) cultivars are vegetatively propagated and require different fertilization practices from seed propagated cultivars. Two experiments were conducted to evaluate growth responses of Henna, Indian Summer, Mint Mocha, New Orleans Red, Red Head, and Trusty Rusty to 0, 70, 140, 280 and 420 mg·L⁻¹ nitrogen (N), and Henna, Mint Mocha, Red Head, and Trusty Rusty to 0, 6.2, 12.4, 24.8, or 49.6 mg·L⁻¹ phosphorus (P) to determine optimum constant liquid feed rates to produce marketable size plants from rooted cuttings. Positive growth responses in terms of biomass were found with increasing N rates but not P rates. For medium-sized cultivars such as Henna, Indian Summer, and New Orleans Red, quadratic responses were found in aboveground biomass, and N at 280 mg·L⁻¹ resulted in similar plant size and dry weight as those fertilized at 140 and 420 mg·L⁻¹ N. For large-sized cultivars such as Mint Mocha, Red Head and Trusty Rusty, plant dry weight responded linearly within the N range tested and were greatest at 420 mg·L⁻¹. However, plant visual quality was negatively affected by N rates at 280 and 420 mg·L⁻¹ in that, leaf color became less intense at these high N rates. Plants fertilized at 70 mg·L⁻¹ were smaller than those fertilized at 140 mg·L⁻¹, however, they received similar visual quality ratings because of more intense leaf color. Therefore, N at 70 to 140 mg·L⁻¹ can be used to grow most vegetative coleus for similar marketable quality. Mint Mocha and Henna were the only cultivars responded to P treatments that, 12.4 mg·L⁻¹ P rate resulted in greater biomass than the no-P control. All other cultivars had no response to supplemental P except a linear response in tissue P%. Therefore, supplemental P is not required during the 8 week production period when there is an initial P charge in the substrate. We found that substrate pH decreases with higher P rates, therefore supplemental P fertilizer can be used for adjusting pH. Both N and P rates found optimum in this study are lower than current industry practices (N at 150 to 250 mg·L⁻¹ and P at 24 mg·L⁻¹) and can significantly lower production cost and potential leaching of excessive nutrient into waterway. Nutrient treatments in further study on postharvest performance of vegetative coleus will be selected based on this study.

KEYWORDS:

• Bedding plant
• fertilization
• plant aesthetic quality