Dialing in microgreens production

Microgreens are defined as a wide range of vegetable and herb seedlings that are harvested shortly after the emergence of the first true leaf and prior to leaf expansion/senescence of cotyledons. Microgreens represent a quick turn, potentially high value crop for greenhouses and indoor production. Because there is a wide diversity of species used, recommendations for cultural practices are lacking for many. Relatively little information is available on the responses of microgreens to light and carbon dioxide.

Because many microgreens start with a relatively large seed that can provide initial nutrition to the developing seedling, it is thought they are relatively less responsive to light and CO₂ than their full-size counterparts (like baby greens or head lettuce). However, experimental data is needed to evaluate these claims.

The objective of this study was to determine how three species of microgreens would respond to daily light integral (DLI) varying from 3 to 12 mol·m^-2·d^-1 and carbon dioxide enrichment from 400 ppm (ambient) to 1,000 ppm.

Experimental methods

Three microgreen species — arugula (Eruca sativa), mizuna (Brassica rapa var. japonica) and mustard (Brassica juncea ‘Garnet Giant’) — were chosen for this study. The seeds were purchased from Johnny’s Selected Seeds and were selected to represent a diversity in sensory attributes for both vision and taste. We also choose these three because they share similarities in seed size and number of days from seeding to harvest.

Previous experiments were conducted to determine cultural parameters such as seed density, fertilizer and substrate. We used flats with 2401 (24-cell) inserts with a peat-based potting mix (Lambert LM-111) prewatered to a moisture content ratio of 1:1 (peat-lite mix:fertilizer solution) by weight with Jack’s 21-5-20 Liquid Feed at 150 ppm nitrogen. Seeds were sown on the top of the substrate at a rate of 125 seeds per cell (equivalent to 3,000 seeds per 20-inch by 10-inch flat). Seeds were then misted with the same fertilizer solution.

For germination, the trays were covered with a propagation dome covered with a standard black flat to restrict light. Seeds were then germinated in darkness at 73° F until 95% of the seedlings were 1 centimeter in height. Germination times were 42 hours for arugula, 46 hours for mizuna and 48 hours for mustard. After germination, microgreens were placed into the treatments described below with 12 cells per species per treatment.

The experiment used two adjacent controlled environment chambers at 73° F, each with its own mini acrylic chambers, which allowed for control of CO₂, temperature and relative humidity. For each experimental run, one DLI (from 3, 6, 9 and 12 mol·m^-2·d^-1) was selected. The growth chambers had T5 cool white fluorescent lights. Light was supplied over a 14-hour photoperiod. Shade cloth (50%) was used for the lowest DLI.

Each of the four acrylic mini chambers was randomly assigned one of the CO₂ concentrations (400, 600, 800 and 1000 ppm). Flats were watered as needed (roughly every three days) via subirrigation with Jack’s 21-5-20 at 150 ppm N. The experiment was repeated for each DLI and replicated a total of three times.

Fresh weight

For mizuna and mustard, there was a linear increase in fresh weight as DLI increased from 3 to 12 mol·m-2·d-1 (Figure 1). Fresh weight increased by 22 to 25% from the lowest to the highest DLI treatment. Arugula was more responsive to DLI; fresh weight increased by 51% from 3 to 9 mol·m^-2·d^-1 and then exhibited little further gains from 9 to 12 mol·m^-2·d^-1.

All species responded to carbon dioxide enrichment in a linear fashion, showing about an 11% yield increase from 400 to 1,000 ppm CO₂. Put another way, CO₂ enrichment could be used to reduce the amount of lighting needed. For example, fresh weight of mizuna at a DLI of 9 mol·m^-2·d^-1 with 1,000 ppm CO₂ was greater than a DLI of 12 mol·m^-2·d^-1 with ambient CO₂ (400 ppm).

Height

Average plant height for mizuna and mustard decreased linearly as DLI increased from 3 to 12 mol·m^-2·d^-1 (Figure 2). Decreases amounted to reductions in plant height of 0.9 of a centimeter for mizuna and 1.3 centimeters for mustard as DLI increased from 3 to 12 mol·m^-2·d^-1 at 400 ppm CO2.

For arugula, plant height was essentially unaffected by DLI but did decrease at the highest treatment (9 mol·m^-2·d^-1). CO2 also had a subtle effect where plant height increased slightly (0.4 of a centimeter averaged across species and light levels) as CO₂ increased from 400 to 1000 ppm.

Days to harvest

The days to harvest varied by species and was about 12 days for arugula and mustard and 10.5 days for mizuna at the lowest DLI (Figure 3). Days to harvest decreased for all species by about two days as DLI increased to 12 mol·m^-2·d^-1. The CO₂ concentration did not influence plant developmental stage (i.e., days from seeding to harvest).

Bottom line

While microgreens do not appear to require DLIs as high as mature leafy greens crops (ex. 17 mol·m^-2·d^-1 for head lettuce) or fruiting crops (ex. 25+ mol· m^-2·d^-1 for tomatoes), microgreens may benefit from increasing DLI up to 9 mol· m^-2·d^-1 for arugula and 12 mol·m^-2·d^-1 (and potentially beyond) for mizuna and mustard.

Growers should understand their ambient DLI (such as by purchasing a quantum sensor connected to a datalogger) to then assess if supplemental light makes sense (or during which months it makes sense). Besides increasing fresh weight, increased DLI reduced days to harvest by about two days, resulting in quicker crop turns.

CO₂ enrichment from ambient to 1,000 ppm increased harvested fresh weight by 11%. Thus, growers should assess the economics of CO₂ enrichment, and in closed production systems (i.e., no open ventilation), there may be an economic returning of CO₂ enrichment.

Plant height decreased by up to 1.3 centimeters with increasing DLI. For some growers, if microgreens are too compact in height, they can be difficult to harvest. If these growers use higher light intensities, they may want to consider measures to increase stem height, such as a slightly longer germination period in the darkness.

More research is needed to quantify how other microgreens species/cultivars respond to light and carbon dioxide. Commercial growers should always conduct small-scale trials to see how plants respond in their own facilities before adopting new practices.

Overall, microgreens can be a profitable crop, and they may be more practical (in terms of energy cost) for indoor farming without sunlight, as they require lower DLI than head lettuce and fruiting crops.

Jonathan Allred is a Ph.D. student at Cornell University. Neil Mattson is a researcher and professor in the School of Integrative Plant Science at Cornell. This article is used with permission from e-GRO Edible Alert (e-gro.org).