DLI Measurements: A Valuable Addition to Your Toolbox By Jim Faust

Over the past five years, with the support of FloricultureIndustry Research and Scholarship Trust (FIRST), we conducted a series ofexperiments to develop guidelines for using the daily light integral (DLI) togauge the light environment in greenhouses. DLI is a term that refers to thetotal quantity of light delivered to a surface. An analogy will help to clarifythe technique and usefulness of measuring DLI. If you wanted to know how muchrain fell yesterday, you would place a bucket outdoors to capture all theraindrops delivered to that particular spot. Similarly, if you wanted to knowhow much light was available for plant growth yesterday, you would place alight sensor on the greenhouse bench and capture all the light particles(photons) delivered to that particular spot. This value would be the lightquantity, or DLI.

Current methods

We have survived to this point with basically two methods ofmeasuring light. Most commonly, growers use foot-candle meters to estimate themaximum light delivered to a crop inside a greenhouse or to estimate thegreenhouse light transmission by comparing outside and inside measurements.While there is nothing wrong with this, its usefulness — its ability todescribe the greenhouse light environment — is very limited. A singlelight intensity measurement cannot take the day length (or the number of hoursof sunlight) into account. For example, if a crop is said to receive 2,000foot-candles in December and 2,000 foot-candles in June, the total amount oflight (the DLI) delivered to the June crop is considerably higher than theDecember crop, since the June crop receives 15-16 hours of sunlight, while theDecember crop receives only 8-9 hours.

More recently, growers have used outdoor weather stationsconnected to climate control computers. These stations can report the DLIdelivered outside the greenhouse; however, they cannot estimate the lightdelivered to the greenhouse crop, especially if a retractable shade curtain isused.

Our dilemma is that we never know how much light is actuallybeing delivered to the greenhouse crop. DLI is not a new concept for plantscientists; however, few growers have ever used this measurement because theequipment for measuring DLI has been cost prohibitive. That has changed, thuswe are writing this article now, because DLI measurements can be a reality forcommercial greenhouse growers. But first, let’s discuss the value and use forDLI measurements.

Using DLI

Plants are “light counters.” Photosynthesis isdriven by the amount of light intercepted by the plant canopy and the amount ofcarbon dioxide that can be absorbed through the stomata. Since DLI is the sumof the light delivered during the course of one day, there is an excellentrelationship between plant growth and DLI. So, if you measure the DLI, you havean excellent estimate of the plant growth to be expected. ç

The FIRST Web site (www.firstinfloriculture.org/research_reports.htm and click on Light Management in Greenhouses) contains a detailedreport on the use of DLI, so only a brief overview will be presented here. Thefirst challenge in using a new unit of measure is getting used to the numbersand units. The unit of measure for DLI is “moles per day” or (moretechnically correct) “molám-2ád-1,” which means molesper square meter per day.

Measuring DLI

The value of DLI measurements has been evident for many years;however, commercial growers never had the ability to measure DLI due to thehigh equipment cost. In 2003, Spectrum Technologies, Plainfield, Ill.,developed a portable datalogger (Greenhouse Weather Tracker) that measures DLI.The meter can be easily positioned within any greenhouse crop. A digitaldisplay provides the current DLI for that particular day as well as a 30-dayarchive for previous days. The display also shows the current light intensity(micromoles per second or µmolám-2ás-1) and current airtemperature. The minimum and maximum daily temperatures are also archived. DIFmeasurements will be displayed in future versions. ç

We have been making DLI measurements for several years inresearch and commercial greenhouses. The measurements never cease to amaze me.We usually rely so heavily on our eyes to evaluate the greenhouse lightenvironment; however, the human eye is a lousy light sensor. This is becauseour eyes are so effective at adjusting to the ambient light conditions. It isremarkable that we can see well under a full moon (0.01µmolám-2ás-1) or during a sunny day (2,000µmolám-2ás-1). The following are some of the conditionsthat provide surprising DLI measurements (although they seem like common senseonce you’ve made the measurement):

*ÊÊÊ Shadowscast during a sunny day are much darker than the same shadow cast on anovercast day. Similarly, if you have a sensor positioned in the shadow of agreenhouse wall on a sunny day, the light intensity may actually increase as acloud moves overhead, since the cloud reflects the light into the shaded area.

*ÊÊÊ Ineast/west-oriented greenhouses, the benches immediately north of the gutterscan have very low DLI on sunny days (less than 5 moles per day in SouthCarolina in October).

*ÊÊÊ TheDLI measured at the truss (where a hanging basket would be positioned) is muchhigher (20-50 percent) than the DLI measured on the floor or bench, evenwithout any hanging baskets.

*ÊÊÊ Theoutdoor DLI increases 100 percent (or doubles in magnitude) from December toJune.

*ÊÊÊ TheDLI delivered to the bench inside the greenhouse is often only 50 percent ofthe outdoor DLI, even without hanging baskets or shade curtains.

*ÊÊÊ TheDLI delivered to the greenhouse bench may be lower in May than in March if alot of shade is applied to the greenhouse.

*ÊÊÊ Excessivehanging baskets or excessively large hanging baskets can reduce the DLIdelivered to the greenhouse bench by more than 40 percent.

*ÊÊÊ Whitehanging baskets intercept 10-30 percent less light than green hanging baskets.

*ÊÊÊ Eventhough the glazing material may allow 90-percent light transmission, thecomplete greenhouse structure may allow only ç 35-70 percent, dependingon the angle of the sunlight, the amount of infrastructure, overhead curtains,dust, condensation, etc.

*ÊÊÊ Eventhough the shade curtain may allow 50 percent transmission, the entire shadestructure may allow only 40 percent transmission when the retractable curtainsare pulled.

*ÊÊÊ TheDLI delivered inside a retractable roof greenhouse is less than the outside DLIdue to shadows cast by the structure, even when the roof is open. This isparticularly true in hinged roof systems, compared to accordion-styleretractable roofs.

As you can see, there is a lot to be learned about the lightenvironment inside a greenhouse. Everything seems straight forward until youmake some measurements, then you realize there’s more variation and differencesthan you’ve ever imagined.

The next step

EC and pH measurements became commonplace in the 1980s. Thistool has dramatically improved a grower’s ability to reliably produce cropafter crop. Graphical tracking became a commonplace in the 1990s. This tooltook much of the guesswork out of height control. DLI meters have the potentialto be the next major tool for growers to add to their repertoire. DLImeasurements made inside the greenhouse will provide growers with furtherinsight into greenhouse crop management. It will take time, but I believe thatover the next decade growers will become familiar with the DLI concept. Thiswill improve our ability to accurately communicate to each other about lightand will improve our ability to reliably grow quality crops.

Author’s Note: The DLI work discussed in this article is aresult of a project funded by FIRST. A more detailed discussion of DLI can befound in the Research Report provided on the FIRST website(www.firstinfloriculture.org).

Jim Faust

Jim Faust is assistant professor of Floriculture at Clemson University, Clemson, S.C. He can be contacted by E-mail at [email protected]

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