American LED Alliance

Best Practices Report

Indoor Hydroponic Lettuce Farming – Q1 2019 – 2019.02.11

Table of Contents:

  1. Grow Light Wattage Reduction Relative to Photosynthesis
  2. Reducing Wattage Consumption Beyond Man-made Lighting
  3. Reducing Amps by Increasing Volts
  4. Lessons Learned from the Cannabis Industry
  5. Benchmark Watts Per Sq Ft for Indoor Lettuce Growers
  6. Target PPFD and DLI for Indoor Lettuce Growers
  7. Daily Hour Tests to Accelerate Harvests for Indoor Lettuce Growers       
  8. Future Direct Current (DC) Opportunity to Save Energy for Indoor Lettuce Growers
  9. Lessons Learned from University Research for Indoor Lettuce Growers
  10. Key Abbreviations and Units Relative to Indoor Agriculture
  • 1: Grow Light Wattage Reduction Relative to Photosynthesis:

For plants to grow well they need to have the appropriate daily light integral (DLI) relative to their specific species. As an example, in the case of certain types of lettuce, the DLI may need to be between 15 and 18. DLI is calculated based on the amount of Photosynthetic Photon Flux Density (PPFD) at the plant canopy and the number of daily hours of illumination. This DLI and PPFD relationship is true for plants growing outside in natural light, in greenhouses, or within indoor grow operations. For operations using man-made light, the PPFD is typically directly related to the electricity (watts) drawn by the fixture. Photosynthesis relies on electrons and photons to produce plant growth, so more watts per fixture typically equate to more growth. Lower watts, and the correlated lower PPFD reduces the DLI, which extends the time from seedling to harvest. Extending the run time (either per day or adding days) typically means that the same amount of electricity is used and the same cost is incurred. Extending time to harvest has the negative impact of reducing revenue for indoor farmers given the reduced number of crop harvests per year. consumption. Raising the lights to extend the beam angle or reducing the number of fixtures to “stretch” the wattage across a larger grow area also reduces the PPFD which in turn means that growers need to extend the run time of the lights.

Light Emitting Diode (LED) fixtures can deliver the same level of PPFD with typical energy savings of 60% over traditional metal halide (MH) and high pressure sodium (HPS) grow lights. LEDs offer a key advantage for growers relative to the cost of electricity and harvest time cycles. This report includes considerations for indoor growers to optimize their operations, and it specifically focuses on leafy green production.

  • 2: Reducing Wattage Consumption Beyond Man-made Lighting:

To reduce wattage, growers can explore options such as:

A:  Integration of natural light through:

A-1: Skylights

A-2: Sun tunnels / Sun Tubes

A-3: Clearstory lighting

B: Reuse of light through:

B-1: Reflectors to reduce light that is “wasted” on the outside of the planting beds.

B-2: Reflective surfaces at the grow “rafts” for Deep Water Culture (DWC)

and at the top or “roof” of the fixtures

C: Light Sharing though:

C-1: Shelving racks systems that are “Rolling” to reduce the light wasted in service aisles

  • 3: Reducing Amps by Increasing Volts:

As growers seek to expand operations, many are facing challenges with the available supply of electricity service from their respective utility provider. In short, there amp requirements exceed the available supply. Amps x Volts = Watts. In turn, Watts / Volts = Amps. So, to lower the amps, many growers are asking their utility providers to increase the volts from 120v to 240v. This option reduces the amps by 50%. Growers need to check with their respective LED fixture manufacturers to ensure that the LEDs will accept 240v service.

  • 4: Lessons Learned from the Cannabis Industry:

The cannabis industry has pioneered many innovations in indoor growing given the lucrative nature of the market.

Source: https://cannabismaven.io/theweedblog/growing/how-does-lighting-affect-your-marijuana-yield-FlvxskU64Ui6gMxd833cgg/

“You have to pay close attention when setting up your lights – just hanging them up simply won’t do. Follow the below criteria to maximize your light use and results. First, make some decisions about how much light to use. For plants less than thirty inches tall, you can calculate accordingly: multiply together the length and width to get the total square footage). Then take this amount and divide it by your lamp’s wattage. For instance, if you have a five-by-five room, it’s 25 square feet. If your lamp is 1000 watts, divide that by 25 square feet, and you’ll get 40 watts per square foot. Pay attention to the result: anything more than 35 watts per square foot is fine for plants of this size.

If the level of light is upwards of 65 watts per square foot, you should look into increasing the size of your growing area, then add a new light to the new growing area. This is better than simply adding an extra light to the original growing area because it instead doubles both the growing area and the yield. If you’re using more than 65 watts per square foot, then you’re wasting light and should add more plants and expand your growing area.”

  • 5: Benchmark Watts Per Sq Ft for Indoor Lettuce Growers:

Since leafy greens like lettuce and basil require less PPFD than flowering plants, such as cannabis, the wattage levels per sq ft for leafy greens are often 60% of the 35 watts per sq ft minimum target for cannabis growers. 60% of 35w = 21 watts / sq ft. For a frame of reference, ASHRAE standards for a commercial office building is 1 watt per sq ft, and indoor farming to “replicate the sun” is often 20 times or more the light output off typical building illumination. If leafy green growers are using more than 21 watts / sq ft they may be either overloading their plants with PPFD or not using LEDs that have been optimized for their specific plant growth.

  • 6: Target PPFD and DLI for Indoor Lettuce Growers:

“lettuce plant needs ~80 µmol/m2/s (PPFD) during seedling and ~150 µmol/m2/s (PPFD) during vegetative phase”                                                                                                                                                                                                                        

Source: http://www.valoya.com/light-planning-3-steps-ensure-efficient-plant-growth-leds/                                                                                                                                            

“Lower-light crops like lettuce require a DLI of 12-14 mol/m-2/day for maximum growth rates, and higher-light crops such as tomatoes require at least an average DLI of 22 mol/m-2/day or up to 30 mol/m-2/day to reach light saturation at maturity.

Daylight reference: A dull winter’s day with heavy cloud cover and a short day-length may see a DLI inside a greenhouse as low as 3 mol/m-2/day. On the other hand, a bright, clear, sunny mid-summer’s day with a long day length of 18 hours can create an average DLI of 35 mol/m-2/day inside a greenhouse.”                                                                                                                                                                                                                                                                                     

Source: https://www.maximumyield.com/hydroponic-illumination-the-daily-light-integral/2/1450                                                                                                                                                         

Cornell Study: Lighting uniform light distribution is required in the Pond Growing Area. A supplemental light intensity within the range of 100-200 μmol/m2/s (for a total of 17 mol/m2/d1 of both natural and supplemental lighting) at the plant level is recommended. It should be noted that 17 mol/m2/d (PPFD) is the light integral that worked best for the particular cultivar of Boston bibb lettuce that we used. For some cultivars, 15 or mol/m2/d is the maximum amount of light that can be used before the physiological condition called tip burn occurs.       

http://cea.cals.cornell.edu/attachments/Cornell%20CEA%20Lettuce%20Handbook%20.pdf

PPFD (µmol/m2/s) micromoles per meter squared per second 

DLI (Daily Light Integral referred to as mol/m2/d ) = PPFD x 3,600 x hours of daily illumination / 1MM

e.g. 250 PPFD x 3,600 x 16 hours = 14.4MM / 1MM = 14.4″                                                                                                                                                                                                            

The test in this video (https://youtu.be/gHWf4Hfi3hA) shows that the “winning” setup for the lettuce that they planted is PPFD 278 at 13″ mounting height with 16 hours per day yielding DLI of 16. Since our lights are more powerful than the ones tested, we were right to raise them up.

The Cornell study and the other studies all recommend a DLI between 15 and 17 for lettuce. 15 to 16 looks like a “magic” number for lettuce. At 17″ to 18″ mounting height the PPFD on our iLED lights averages around 270, so we are right in the sweet spot. If we lower them the PPFD naturally goes up.                                                                                                                                                                                                       

Refresher: DLI = Daily Light Integral                                                                                                                                                                                                                                                                                                                                         

PPFD x 3,600 x Daily Hours / 1MM                                                                                                                                                                                                                                                                                                                                                           

270 x 3,600 x 16 = 15.55 DLI                                                                                                                                                                                                                                                                                                                                                       

Lettuce Growth Performance = target 270 PPFD range and DLI 15 to 17

Muir lettuce reports: 300 PPFD at 17″ mounting height and 16h/day delivered excellent Muir Lettuce. 300 PPFD x 3,600 x 16 / 1MM = 17.28 DLI                                                                                                                                                                                                                       

Summary: Certain lettuce types thrive at between 270 and 300 PPFD, which makes sense since the Cornell study references “supplemental” PPFD between 100 and 200 PPFD when they are using a glass house with natural light. All studies and practical experience indicate that for lettuce, the DLI is best between 15 and 18.

  • 7: Daily Hour Tests to Accelerate Harvests for Indoor Lettuce Growers:                                                                                                      

This video is about accelerating lettuce growth to 22 days, with 14 days of illumination at 16 hours per day and 8 days of 24 hours of illumination per day. Growers may want to experiment themselves to replicate the results. Time lapse of 22 day growth video: https://youtu.be/8jojU-eDZc

Given that summer sun in a greenhouse with 18 hours of daylight yields up to 35 DLI, this could be why the test worked. E.g. 270 PPFD x 3,600 x 24 = 23 DLI (possibly worth testing)

  • 8: Future Direct Current (DC) Opportunity to Save Energy for Indoor Lettuce Growers:

As of 2019, growers have increasingly started using LED grow lights that are more efficient than traditional lights at generating PPFD. Current LED grow fixtures include LED drivers that are powered by Alternating Current (AC), at typically 120 volts or 240 volts, any the drivers have efficiency loss levels that are 12% to 20%. New developments into Direct Current (DC) systems offer higher efficiency with loss levels that are only 3% to 6%. The added efficiency with DC has the potential to save between 9% to 17% of electricity consumption for indoor growers. Beyond saving money on monthly operations, this is a potential “mission critical” path given that some growers have limits on the available electricity from their utility service providers, at their specific property locations. The cost of supplemental power (e.g. generators) or renewable energy (e.g. Solar, Wind, Anaerobic Digestion, etc.) are often high enough that they reduce the profitability for the growers and limit the success of a given business model. To date, some manufacturers have developed “Direct Current Microgrid LED” systems for building illumination. As of Q1 2019, the technology has not been launched for indoor agriculture. When it is available, DC Microgrid LED technology may create significant energy reduction advantages for indoor leafy green growers as well as other growers across the marketplace.                                            

  • 9: Lessons Learned from University Research for Indoor Lettuce Growers:

Cornell Controlled Environment Agriculture – Hydroponic Lettuce Handbook

Overview: The hydroponic greenhouse production system was designed for small operations to provide local production of head lettuce as well as employment to the proprietors. Our research group has experimented with many forms of hydroponics but have found this floating system to be the most robust and forgiving of the available systems. This system is built around consistent production 365 days of the year. This requires a high degree of environmental control including supplemental lighting and moveable shade to provide a target amount of light which, in turn, results in a predictable amount of daily growth.

by Dr. Melissa Brechner, Dr. A.J. Both, CEA Staff

Cornell Study: http://cea.cals.cornell.edu/attachments/Cornell%20CEA%20Lettuce%20Handbook%20.pdf

PPFD and DLI:

Lighting uniform light distribution is required in the Pond Growing Area. A supplemental light intensity within the range of 100-200 μmol/m2/s (for a total of 17 mol/m2/d1 of both natural and supplemental lighting) at the plant level is recommended. It should be noted that 17 mol/m2/d (PPFD) is the light integral that worked best for the particular cultivar of Boston bibb lettuce that we used. For some cultivars, 15 or mol/m2/d is the maximum amount of light that can be used before the physiological condition called tip burn occurs.

Beyond lighting, the following information is key for growers to succeed with their indoor farming enterprises:

Nutrients:

For Deep Water Culture (DWC): “Pond Solution – Equal portions of Stock Solutions A and B (see formulas in appendix) are added to reverse-

osmosis RO water to achieve an EC of 1200 µS/cm or 1.2 dS/cm.” – page #19

Appendix  – Stock Solutions 

Two stock solutions are prepared which will be added separately to RO water and will supply nutrients to the lettuce plants while in the pond area. 

Two separate stock solutions are prepared to prevent certain chemical reactions. These chemical reactions will cause some of the chemicals to form a precipitate and become inactive. The precipitates will not form if mixed one after another with a large volume of RO water. 

STOCK A

These chemicals are added to 300 L of RO water

Calcium Nitrate 29160.0 g

Potassium Nitrate 6132.0 g

Ammonium Nitrate 840.0 g

Sprint 330 Iron – DTPA (10% Iron) 562.0 g

STOCK B

These chemicals are added to 300L of RO water

Potassium Nitrate 20378.0 g

Monopotassium Phosphate 8160.0 g

Potassium Sulfate 655.0 g

Magnesium Sulfate 7380.0 g

Manganese Sulfate*H2O (25% Mn) 25.6 g

Zinc Sulfate*H2O (35% Zn) 34.4 g

Boric Acid (17.5% B) 55.8 g

Copper Sulfate*5H2O (25% Cu) 5.6 g

Sodium Molybdate*2H2O (39% Mo) 3.6 g

Dissolved Oxygen (DO) in the Water:

“Figure 18. Dissolved oxygen (DO) sensor. DO levels should be greater than 4 ppm (parts per million) to prevent growth inhibition. Visible signs of stress may be observed at 3 ppm.”

Page #17: 2.1 Dissolved Oxygen Sensor

Most manufacturers recommend that dissolved oxygen sensors be calibrated daily. Modern 

sensors are fairly stable and will probably not go out of calibration in such a short time period. 

Remember that your data is only as good as your calibration, so be sure to calibrate all sensors 

on a regular basis.

A hand-held sensor (~$600 in 2013) is always an essential trouble-shooting tool and should 

always be available. If the facility is one acre or larger, an in-line sensor may be a worthwhile 

investment. 

Model: Orion 820, hand held, battery operated

Manufacturer: Orion Research Inc., Boston, MA

Some other manufacturers that make this same quality meter are YSI, Oakton and Extech

Air Temperature Range for Lettuce:

Target Air Temperature for lettuce: “24 C Day/19 C Night (75 F/65 F)”

Some facilities lack enough insulation to maintain the target 75 F during the day (max) and 65 F at night (minimum). Enhanced insulation can help reduce the HVAC costs to reduce overall operating costs for indoor growers.

Other Key Factors for Lettuce:

Water Temperature No higher than 25C, cool at 26C, heat at 24C

Relative Humidity minimum 50 and no higher than 70%

Carbon Dioxide 1500 ppm if light is available, ambient (~390 ppm) if not

Light 17 mol m-2 d-1 (PPFD) combination of solar and supplemental light

Cornell Controlled Environment Agriculture – Hydroponic Lettuce Handbook

`

Table of Contents:

Chapter 1: Greenhouse Hardware……………………………………………………………………………………… 6

1.1 Nursery or Seedling production Area………………………………………………………………………… 6

Ebb and Flood Benches …………………………………………………………………………………………….. 6

Solution Tank and Plumbing ……………………………………………………………………………………… 8

Lighting ………………………………………………………………………………………………………………….. 9

1.2 Pond Area……………………………………………………………………………………………………………. 12

Lighting ………………………………………………………………………………………………………………… 13

Lighting Configuration and High Intensity Discharge (HID) Lamps …………………………….. 14

Paddle Fan …………………………………………………………………………………………………………….. 14

Aspirated Box………………………………………………………………………………………………………… 15

System Component Information ………………………………………………………………………………….. 16

2.1 Dissolved Oxygen Sensor ……………………………………………………………………………………… 16

2.3 Compact Submersible Centrifugal Pump…………………………………………………………………. 16

2.4 Flow Meters…………………………………………………………………………………………………………. 16

Chapter 3: Computer Technology and Monitoring…………………………………………………………….. 17

3.1 Biological Significance of Environmental Parameters ………………………………………………. 17

Temperature…………………………………………………………………………………………………………… 17

Relative Humidity…………………………………………………………………………………………………… 17

Carbon Dioxide or CO2 …………………………………………………………………………………………… 17

Lights ……………………………………………………………………………………………………………………. 17

Dissolved Oxygen…………………………………………………………………………………………………… 18

pH ………………………………………………………………………………………………………………………… 18 `

Electrical Conductivity……………………………………………………………………………………………. 18

Monitoring………………………………………………………………………………………………………………… 18

3.3 Set-points…………………………………………………………………………………………………………….. 19

Chapter 4: Lettuce Production ………………………………………………………………………………………… 20

Chapter 5: Packaging and Post-Harvest Storage ……………………………………………………………….. 26

Chapter 6: Crop Health ………………………………………………………………………………………………….. 27

Disease …………………………………………………………………………………………………………………….. 27

Pests…………………………………………………………………………………………………………………………. 27

Chapter 7: References ……………………………………………………………………………………………………. 28

Appendix……………………………………………………………………………………………………………………… 47

Table of Figures

Figure 1.This is a photo of an empty Ebb and Flood bench while the bench is flooding for sub-irrigation………………………………………………………………………………………………………………………… 6

Figure 2. Bench for seedlings. ………………………………………………………………………………………….. 7

Figure 3. Seedling area on edge of pond in greenhouse. ………………………………………………………. 7

Figure 4. Breaker on the end of a wand for hand-watering. ………………………………………………….. 7

Figure 5.Humidity cover propped against a sheet of rockwool. …………………………………………….. 8

Figure 6.Nutrient solution reservoir fiberglass tank (A), Pump (B), Piping (C), and Valve (D). The bottom of the germination bench can be seen in (E). …………………………………………………….. 8

Figure 7.Fluorescent (A) and incandescent (B) lighting in the growth room. Fluorescent lighting is used for plant biomass production and incandescent lighting is used for photoperiod control. . 9

Figure 8. High Pressure Sodium (A) and Metal Halide (B) lamps in a growth chamber…………… 9

Figure 9. High Intensity Discharge (HID) luminaire in a greenhouse…………………………………… 10

Figure 10.Aspirated box in a greenhouse. A fan draws air from the bottom of the box over the sensors…………………………………………………………………………………………………………………………. 11

Figure 11. Aspirated box opening on bottom of box………………………………………………………….. 11

Figure 12. Empty pond with liner. …………………………………………………………………………………… 12

Figure 13.Edge of pond detail. The inside edges of two separate ponds made of wood and separated by structural members is shown on left. The right hand picture shows a concrete pond…………………………………………………………………………………………………………………………………….. 13

Figure 14. Paddle fan to increase vertical air movement and therefore evapotranspiration. This is important for the prevention of tipburn. …………………………………………………………………………… 14

Figure 15. Aspirated box with digital output screen in greenhouse. …………………………………….. 15

Figure 16. Model: H-03216-04: 65 mm variable area aluminum flow meter with valve and glass float for O2. Manufacturer: Cole Parmer Instrument Co., Niles, IL …………………………………….. 16

Figure 17. Quantum PAR sensor to measure light available for photosynthesis. Foot-candle sensor and lux meters are inappropriate because they are designed to quantify the sensitivity of the human eye and overestimate (~25%) the light available for photosynthesis…………………….. 19

Figure 18. Dissolved oxygen sensor. DO levels should be greater than 4 ppm to prevent growth inhibition. Visible signs of stress may be observed at 3 ppm………………………………………………. 19

  • 10: Key Abbreviations and Units Relative to Indoor Agriculture

 A         Area – Square feet or square meter.

CEA      Controlled Environment Agriculture Producing plants in a greenhouse or other space.

cm       centimeter – A unit of length

CWF     Cool White Fluorescent – A type of supplemental lighting

DLI       Daily Light Integral – The sum of photosynthetic (PAR) light received by plants in a day.

DO       Dissolved Oxygen – Oxygen concentration in nutrient solution measured in parts per million.

EC        electrical conductivity – An indirect measurement of the strength of a nutrient solution.

HID      High Intensity Discharge – A type of HID supplemental lighting

hp        horsepower – A unit of power

HPS      High Pressure Sodium – A high intensity discharge lamp/luminaire type for supplemental lighting

kPa      kilopascals – A unit of pressure, force per unit area

LED      Light-emitting Diode (LED)  – A type of supplemental light

MH      Metal Halide – A type of HID supplemental lighting

mol      pronounced ‘mole’ – A number of anything equal to 6.02 x 1023 items. We use it to quantify the number of photons between 400-700 nm of PAR light plants receive.

mol/m2/d        moles per square meter per day – Integrated PAR light

mol/m2/s        moles per square meter per second – Instantaneous PAR light

nm       nanometer – Unit of length in SI, one billonth of a meter

PAR      Photosynthetically Active Radiation   The portion of the electromagnetic spectrum between 400-700 nm plants use for photosynthesis

PPFD – Photosynthetic Photon Flux Density PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”. PPFD is a ‘spot’ measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second (μmol/m2/s)

ppm     parts per million – A unit that describes dimensionless quantities such as volume fractions. For describing carbon dioxide concentrations it is a molar basis.

SI         System Internationale – International system of units aka metric system – built around 7 basic units of measurements

μmol/m2/s      micro-mole per square meter per second – Instantaneous PAR light

μS/cm microsiemens per centimeter – A unit of measurement for electrical conductivity