Ornamental bedding plant operations in the United States are continually transitioning to the production of edible crops (Chidiac, 2017), particularly high-value leafy greens and herbs. However, culture systems used to produce edible and ornamental crops tend to differ in system design and nutrient and water delivery. Fresh market basil (Ocimum basilicum), for example, is typically produced in hydroponic systems in which plant roots are submerged in nutrient solution to promote maximum growth and yield (Gómez et al., 2019; Resh, 2012). Bedding plants are often produced in soilless substrate and containers and periodically irrigated with a fertilizer solution using drip emitters or ebb-and-flood irrigation (Lieth and Oki, 2008). An important decision for bedding plant growers transitioning to edible crop production is whether to invest in new hydroponic equipment or modify existing culture systems (Chidiac, 2017).
Nutrient film technique (NFT) and deep water culture (DWC) are common hydroponic systems used commercially for the production of leafy greens and herbs (Gómez et al., 2019; Resh, 2012; Walters and Currey, 2015a; Wolf et al., 2005). The NFT systems contain plants within narrow gutters or troughs, where root systems are exposed to a continually flowing thin film of nutrient solution. Alternatively, DWC systems expose plant roots to a relatively large volume of nutrient solution, typically ranging from 6 to 8 inches in depth, where roots are completely submerged in nutrient solution. With both systems, commercial producers maintain strict control over solution pH, nutrient concentrations, oxygen levels, and pathogens to ensure optimal plant growth and yield (Currey, 2017; Resh, 2012). The NFT systems are popular among bedding plant growers transitioning to leafy greens and herb production, partially because NFT systems are relatively low-cost and troughs are at a comfortable working level for employees (Walters and Currey, 2015b).
Chidiac (2017) designed a novel soilless substrate system—a shallow aggregate ebb-and-flood (SAEF) system—for bedding plant growers transitioning to edible crop production. The SAEF system consists of modifying traditional ebb-and-flood irrigation systems by adding a layer of clay aggregate substrate to the bottom of the flood table. Seedlings are transplanted and grown directly in the substrate, which is periodically subirrigated with nutrient solution, similar to ebb-and-flood irrigation with container crop production. The SAEF system is intended to allow bedding plant growers to easily modify ebb-and-flood systems for food crop production without having to purchase NFT and DWC systems (Chidiac, 2017). However, the viability of modifying an ebb-and-flow system in this manner for edible crops needs to be evaluated by comparing plant growth and yield of SAEF systems to those of traditional NFT and DWC hydroponics.
Bedding plant growers sometimes purchase and transplant large seedling plugs to finishing containers as a strategy to improve production and space-use efficiencies (Fisher et al., 2006). A similar strategy may also apply to edible crop seedlings transplanted to hydroponic systems. For bedding plants, large seedlings finish rapidly after being transplanted to containers because plants are developmentally mature, thus reducing the time between transplant and a finished crop and allowing for additional crop cycles (Fisher et al., 2006). However, large seedlings require additional time for adequate root and shoot growth during propagation, as well as trays with larger root volumes to avoid issues with root restriction and excessive drying of the root zone (Fisher et al., 2006; Latimer, 1991; van Iersel, 1997). There is a need to investigate the potential of transplanting large seedling plugs to hydroponic systems to reduce the time between transplant and harvest without compromising yield.
The objective of the first experiment was to evaluate edible crop growth in a novel SAEF culture system using basil as a model crop, with an emphasis on comparing basil growth and morphology to plants produced in traditional NFT and DWC hydroponic systems. We hypothesized that basil yield would be lower in the SAEF system compared with DWC, based on previous research comparing plant growth of hydroponic and substrate cultures (Blok et al., 2017). The objective of the second experiment was to evaluate the effect of seedling size and the time of transplant to NFT hydroponic systems on final yield. Large seedlings of bedding plants are typically grown in trays with relatively low cell counts and large root volumes to avoid root restriction (Fisher et al., 2006); therefore, seedling size in this study is discussed in terms of tray cell count and root volume. We hypothesized that basil seedlings grown in the largest root volume would have the greatest yield as a result of decreased restriction of root growth.
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