Page 14 - Griffin Gazette Issue 2 - 2018
P. 14
MOVING FROM
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.
2
A quick way to estimate
supplemental lighting needs for Step 2: Estimate the amount of ambient light reaching your crop. Remember
2
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
2
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,
2
2
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,
2
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
2
2
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
2
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
2
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
2
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
