Page 14 - Griffin Gazette Issue 2 - 2018
P. 14


            flowers                                                   TO

                                            Step 1: Find your location on the map (on the next page) and determine the lowest
                                            outdoor average DLI for your location. For example, a grower in Michigan would see that
        By Tami Van Gaal, CEA Division Leader   they can have as little as 5-10 mol/m /d in December, their darkest time of the year.
        A quick way to estimate
        supplemental lighting needs for     Step 2: Estimate the amount of ambient light reaching your crop. Remember
        year round lettuce production       that ambient outside light of 5-10 mol/m /day in December does not mean that you have
                                            that same amount of light at crop level in the greenhouse. Total transmission to the crop
        When planning your supplemental     will be lower due to the angle of the sun, your greenhouse coverings and the greenhouse
                                            structure itself. If you have a light meter that measures phostosynthetically active radiation
        lighting needs for year round       (PAR) in µmol/m /s, you can use this meter to measure intensity outside the greenhouse
        production, you need to plan for    and inside the greenhouse at crop level. Divide the inside reading by the outside reading
        adequate light to cover the winter   to get your transmission level. Then, multiply the DLI value from Dr. Faust’s charts by this
                                            transmission level to estimate your DLI at crop level. If you don’t have the proper light
        months. The challenge is knowing    meter (LUX and footcandle meters are not appropriate; it must measure PAR), you can
        how much light you have in the      estimate transmission by multiplying your outside DLI by 0.65. For our Michigan scenario,
        greenhouse in the winter when you   multiplying the 5-10 mol/m /d by 0.65 gives us 3.25-6.5 mol/m /d.
        aren’t under those conditions now,
        but want to make a lighting decision.   Step 3: Compare your DLI value to Dr. Neil Mattson’s target of 17 mol/m /d. The
                                            difference between 17 and your number is the amount of light you need to provide to grow
        Fortunately, Dr. Jim Faust did some   a consistent, high quality crop year round. In our Michigan example, we need to provide an
        great work on ambient daily light   additional 10.5 to 13.75 mol/m /d. Over a 16 hour photoperiod, we would need to provide
        integrals (DLI) around the country.   0.85 mol/m /hr. Divide 0.85 by 3,600 to convert to seconds and then multiple by 1,000,000
                                            to get to µmol/m /sec, the units we generally use to measure instantaneous light intensity.
        This data can serve as a valuable   Once again using the Michigan example, we would need to hit 182-238 µmol/m /sec at
        tool to get you started.            crop level for 16 hours a day to provide a DLI of 17 mol/m /day. This is very achievable with
                                            LEDs or HPS. Go with LEDs, and you will also be able to use the lights to supplement under
                                            warmer conditions without increasing the heat load on the crop.
          Bonus       Digital Content
   9   10   11   12   13   14   15   16   17   18   19