Intermittent Airflow to Prevent Tip Burn and Maximize Cost Savings in Vertical Farms

Abstract

Controlled environment agriculture (CEA) and the technology integral to creating highly customizable and optimal environments for the plants grown within have given growers the opportunity to greatly increase production yields while simultaneously decreasing resource input. Vertical farming, in particular, allows for significant improvements in yield, quality, and consistency of harvests, while also greatly reducing the spacial requirements necessary for plant cultivation. Though this agricultural method is exemplary in its goals to maximize resource use efficiency, leafy greens such as lettuce grown in this environment are susceptible to the physiological disorder of tip burn. Tip burn in leafy greens manifests itself as visible necrosis of plant leaf tips and is caused by calcium deficiency within the plant. This calcium deficiency can be the result of accelerated growth conditions within the vertical farm, nutrient deficiencies within the fertilizers provided, or lowered transpiration due to a lack of airflow within the system. Suboptimal airflow at the leaf canopy leads to the formation of a thick boundary layer across the leaf surface, stifling the exchange of gases between the leaf and the surrounding environment. To encourage airflow and reduce tip burn occurrences, prior research has shown that vertical airflow blowing down, perpendicular to the plant canopy and leaf surfaces at a speed of 0.3-1.0 m s-1 can mitigate tip burn within a hydroponic system. The lack of research into this topic can be partly explained by the high variability within the vertical farming community, as there is very little standardization in the commercial vertical farming production system designs and practices. This study, conducted at the University of Arizona Controlled Environment Agriculture Center (UA CEAC), incorporated vertical airflow techniques and control strategies to determine the possible electrical energy savings associated with intermittent airflow as a method to reduce electrical energy costs. Within the UA CEAC’s vertical farm, oakleaf lettuce (cv. Rouxai) was grown for 28 days under a daily light integral (DLI) of 13 mol m-2 d-1, with three treatment levels of airflow; constant vertical airflow for all 28 days (Control), constant vertical airflow for the final 14 days of growth (Treatment 1), and vertical airflow during the photoperiod (16 hours per day) for the final 14 days of growth (Treatment 2). The primary focus of this study was to explore the efficacy of intermittent airflow on mitigating tip burn in hydroponically grown lettuce in a vertical farm setting, ultimately allowing growers to maximize electrical use efficiency with respect to fan electrical consumption. Using a tip burn rating system ranging from 0 (no tip burn occurrence) to 5 (severe visual necrosis of leaves), both the number of tip burn occurrences, as well as the severity of the occurrence were recorded to determine the most efficient method of airflow distribution. Both treatments (Treatment 1 and Treatment 2) performed comparably with the goal of mitigating the widespread occurrence of tip burn within the sampled crop, with Treatment 1 performing the best. Based on the rating system employed, tip burn ratings of 0-2 are still considered marketable heads of lettuce, since their tip burn is very minimal and would be unnoticeable by consumers. Ratings of 3 and higher are considered to be wastes, as they would be unmarketable for growers, and thus represent a loss in marketable yield. Tip burn was shown to occur in 11.1% of control plants, 19.4% in Treatment 1, and 37% in Treatment 2 plants. Of these occurrences, the control had 0% waste, Treatment 1 showed 3.7% of the harvest wasted, and Treatment 2 had 6.5% of the harvest over the established rating threshold. Broken down further, 9.3% of the plants within the Control had a tip burn rating of 1, while 1.9% had a tip burn rating of 2. In Treatment 1, 10.2% had a tip burn rating of 1, 5.6% tip burn rating of 2, 1.9% tip burn rating of 3, and 1.9% tip burn rating of 4. Treatment 2 was observed to have the following breakdown; 21.3% had a tip burn rating of 1, 9.3% had a tip burn rating of 2, 3.7% had a tip burn rating of 3, and 2.8% had tip burn 4. A tip burn rating of 5 was not present throughout any of the treatment levels.