Unlocking the potential of CEA lettuce, kale and arugula production

Kale and arugula, in addition to lettuce, are emerging as staple CEA crops as they grow relatively well under low to moderate light intensities, have short-production cycles, their biomass is harvestable and is sold on a weight basis. However, information on how temperature influences their rate of development is quite limited. Temperature plays a critical role in determining developmental rates, including leaf unfolding and crop timing. Additionally, temperature also affects bolting and leaf color, morphology, and texture. Each of these elements contributes to the overall marketability and appeal of the crop to consumers. Understanding how to properly control and manipulate temperature will provide crucial insights to enhance the quality and productivity of these leafy green crops.

Cardinal temperatures — the base, optimum and maximum temperatures for plant development — are vital metrics for growers aiming to make informed decisions about the growing conditions. Each of these temperatures represents critical thresholds in the plant’s developmental cycle. Development begins at temperatures above the base temperature; it then increases linearly as the temperature rises until it reaches the optimum temperature. Beyond the optimum, then development may plateau or decrease until it hits the maximum temperature, where it stops. Harnessing the knowledge of these cardinal temperatures can guide growers to create growing conditions to reach specific market dates and maximize the productivity and quality of their crops. In this study, we grew arugula, kale, and green and red lettuce at average daily temperatures ranging from 45 to 91 °F in both greenhouses and indoors to determine their base, optimum and maximum temperatures.

Study design

Seeds of arugula ‘Astro’, kale ‘Red Russian’, red oakleaf lettuce ‘Rouxaï RZ’ and green butterhead lettuce ‘Rex’ were sown and propagated for 11 days. These seedlings were transplanted into 6-inch containers and distributed into individual greenhouse or growth rooms. The indoor day/night (12 h/12 h) and average daily temperature (ADT) setpoints were 52/41 °F (ADT of 46 °F), 61/50 °F (55 °F), 70/59 °F (64 °F), 79/68 °F (73 °F), 88/77 °F (82 °F) or 97/86 °F (91 °F). The greenhouses were under constant temperatures of 45, 54, 63, 72, 81 and 90 °F. ‘Rex’, ‘Rouxaï RZ’, arugula and kale were harvested after 29, 32, 35 and 35 days, respectively. At harvest, crop height and width, leaf number, bolting or tip burn incidence, fresh mass, dry mass and foliage color were recorded.

Results

Rate of leaf unfolding and fresh mass gain

The minimum or base temperature for leaf unfolding was around 43, 40, 43 and 45 °F and increased to an optimum temperature of 70, 73 and 82 °F for arugula, kale, lettuce ‘Rex’ and ‘Rouxaï RZ’, respectively. Lastly, the maximum estimated temperature for leaf unfolding of arugula and kale and both lettuce cultivars was approximately 97 and 95 °F, respectively.

For fresh mass accumulation, the base temperature was 45 °F for arugula, 46 °F for kale and 47 °F for ‘Rex’ and ‘Rouxaï RZ’. Fresh mass gain was greatest at the optimum temperature of 73 °F for arugula and kale and 77 °F for ‘Rex’ and ‘Rouxaï RZ’. The maximum temperatures of arugula and kale fresh mass gain was estimated as 98 and 95 °F, while the maximum temperature of ‘Rex’ and ‘Rouxaï RZ’ was at 93 °F.

Fresh mass

Increasing the temperature from 46 to 64 °F resulted in the fresh mass of arugula increasing from 12 to 67 g (Fig. 1). Increasing the temperature from 82 to 91 °F resulted in a 21% reduction (14 g) in fresh mass (Fig. 1). Fresh mass of kale increased over 5-fold, from 12 to 72 g as the temperature increased from 46 to 64 °F (Fig. 2). Beyond 73 °F, the fresh mass of kale decreased by 41% (29 g), down to 40 g. For lettuce ‘Rex’ and ‘Rouxaï RZ’, as the ADT increased from 46 to 73 °F, fresh mass increased by 123 and 103 g; however, raising the temperature from 73 to 82 °F led to the fresh mass of ‘Rex’ and ‘Rouxaï RZ’ being 35% (33 g) and 14% (13 g) lower. The fresh mass of ‘Rex’ and ‘Rouxaï RZ’ was further reduced by 60% (77 g) and 55% (58 g) when the temperature increased from 82 to 91 °F.

Size, color, and tipburn

Arugula and kale were most compact at 46 °F (9.3 and 7.2 cm) and the tallest (19.9 and 21.5 cm) at 64 °F. Similarly, ‘Rex’ and ‘Rouxaï RZ’ were the shortest at 46 °F and largest at 64 to 82 °F. Leaves generally were longest and widest between temperatures of 64 and 73 °F, and leaves were small at the lowest and highest temperatures.

The foliage color of arugula was lightest at 47 and 91 °F. Kale grown at 64 °F was darker and grayer than those grown at 91 °F. ‘Rouxaï RZ’ was the darkest, grayest, and reddest at 47 °F, becoming lighter, greener, and less gray as temperature increases.

Tipburn was not present on either lettuce cultivar when they were grown with a difference between the day and night temperature. In the greenhouse, moderate tip burn occurred in ‘Rex’ when it was grown at constant temperatures from 64 to 91 °F. ‘Rouxaï RZ’ had minimal tip burn at all other ADTs and moderate severity at 64 °F.

Recommendations

We suggest growing leafy greens as close to the optimum temperature as possible for fresh mass gain and leaf unfolding, while balancing the expected yield gain to the cost of maintaining the setpoints. Therefore, 73 °F would be ideal for arugula and kale and 77 °F for the lettuce cultivars tested. Additionally, consider if your crop is being marketed for its color — for red lettuce and kale, foliage will be darker and redder when grown at cooler temperatures prior to harvest.

Sean Tarr is a research technician at Michigan State University and Roberto Lopez is an Associate Professor and Controlled Environment/ Floriculture Extension Specialist in the Department of Horticulture at MSU. The authors gratefully acknowledge Fluence Bioengineering for LEDs, JR Peters for fertilizer, East Jordan Plastics for containers, Rijk Zwaan and Ball Horticulture Co. for seeds, and the USDA National Institute of Food and Agriculture Hatch project nos. MICL02472 and USDA-NIFA Specialty Crop Research Initiative award no. 2019-51181-30017.