Light is one of the most important factors growing any crop. But, how can you be sure that your plants are receiving enough light? There is a way to measure the light intensity received by your plants.
New DLI maps have been created from an updated database that includes data from 1998 to 2009.
Daily light integral (DLI) is the amount of photosynthetically active radiation (PAR) received each day as a function of light intensity and duration. DLI maps display the ambient light delivered daily during each month across the entire United States. The original maps released in 2002 were researched and developed by Jim Faust at Clemson University and Joanne Logan at the University of Tennessee.
The use of LED grow lights to provide specific light wavelengths could allow growers to increase nutritional values of edible crops, enhance the intensity of foliage and flower color and improve the postharvest longevity of ornamental and edible crops.
intensities of LEDs allowed researchers to study the direct effects of narrow wave bands of light on plant physiology.
“LEDs are now available to deliver all blue, all red, all green, all yellow light or mixtures,” said University of Tennessee plant sciences professor Dean Kopsell. “White LEDs are almost a broad spectrum light source. White LEDs are actually mostly blue light with a little bit of red, yellow and green light with a white phosphor over them.”
Kopsell and his colleagues at the University of Tennessee are studying the impact individual types of light can have on the nutritional qualities of edible crops. Their work is focusing on crops that can be produced relatively quickly in 25-35 days, including microgreens and baby greens. They have also begun looking at some herbal crops including basil, tarragon and chives.
|Researchers at the University of Tennessee are finding that exposing plants like brassicas to blue light is having a significant effect on their nutritional values. Photos courtesy of Dean Kopsell, Univ. of Tenn.|
“Some of the unique things we are finding are when we change the light quality environment, going away from broad band light sources like fluorescent, incandescent and HIDs, and exposing plants to narrow band wavelengths of red and blue light, many things are changing in the plants. These narrow bands of light are having an effect on several plant quality parameters from a metabolic standpoint.”
Potential of specific light wavelengths
University of Tennessee researchers have found that exposing plants to narrow wavelengths of the light spectrum has resulted in the increased production of antioxidants and anti-carcinogenic compounds within the plants.
“What is even more interesting is some of the primary metabolites like the mineral nutrients are also increasing,” Kopsell said. “We are shifting the light ratios and putting more blue light into the mix. Blue light is close to the ultraviolet (UV) range and has higher energy values than red light. Because of the higher energy level associated with blue light, the more blue light we are exposing the plants to, it seems the more significant the results are on nutritional values.
“We haven’t got hard data yet, but everything that we can see, smell and taste, these blue lights not only affect nutrient uptake, and anti-oxidant metabolism, but they also affect aromatic compounds and flavor compounds. They make them more intense.”
Although researchers have only recently begun to study the impact of narrow light wavelengths on plant physiology, Kopsell said this will be the major use of LEDs in future applications.
“Not only is a grower going to be able to select the type of light and intensity from the LED manufacturer, but eventually the grower will know when is the critical time to apply a specific amount of light to a crop. One of the things that we have seen with these short term crops is using the light as a finishing-off treatment. The crops are grown under regular light conditions like any grower would have the ability to do and then just before harvest the plants would receive a specific type of light for a certain period of time. This light treatment would stimulate the plant physiology uptake and metabolism right before the plants go to the retail market.”
Kopsell said research exposing leafy brassicas to blue light prior to harvest has intensified pigments and green leaf color.
“We increased the green pigments in the leaves so that they looked more vibrant,” he said. “Other research has shown that UV light increases the anthocyanin compounds in leaf lettuce. Providing a little UV light, which is blocked out in most greenhouse environments, at the right time, a grower can get a crop to color up quickly before the plants are shipped out. What we have done with leafy greens to intensify the color of the leaves can also be done with petal tissue. By changing the light quality a grower could get more vibrant flower colors.”
Need for fine tune management
Kopsell said whether plants are grown outdoors, in a greenhouse or in a closed controlled environment with artificial light, the plants are using specific wavelengths from the available light source.
“Horticulture, floriculture and agronomic researchers know how much light is needed in order to produce crops with broad spectrum light,” he said. “The million dollar question that hasn’t been answered is how much light is needed from LEDs to achieve that same level of production? It is going to be less than the daily light integral (DLI) from a broad spectrum light source. But, right now we can’t tell you how much less it’s going to be.
|University of Tennessee studies have shown LED grow lights provide a less stressful light environment for plants.|
“Providing specific types of red and blue light, the amount of stress on plants is reduced because the plants don’t have to tolerate the light not being used for metabolism and physiology,” he said. “We have data that shows LEDs provide a less stressful light environment for plants. So we have to determine how much less light is needed. It is going to require an extra level of management to know what kind of light, how much light and when to apply it. Growers are going to be able to use LEDs to fine tune the light environment. It’s going to depend on the crop, how it’s being grown, where it’s being grown and how the crop will be used. Is it an ornamental, edible or medicinal crop? It’s not going to be as easy as sticking a seed or cutting into a substrate and letting Mother Nature take control. It’s really going to take some fine tune management. But the future looks bright so far.”
For more: Dean Kopsell, University of Tennessee, Plant Sciences Department, Institute of Agriculture, Knoxville, TN 37996-4561; (865) 974-1145; email@example.com.
Greenhouse lettuce can be a successful container or
hydroponic crop for ornamental plant growers looking to give edibles a try.
greenhouse crop may want to try lettuce. Neil Mattson, associate horticulture
professor at Cornell University, said lettuce is a plant with moderate
fertility needs than a greenhouse tomato crop,”
|Ornamental plant growers interested in growing edible
crops may want to try lettuce. It can be produced in
containers with a growing medium or hydroponically
in troughs or a float system (pictured).
Photos courtesy of Cornell University
Mattson said. “Grown as a
container crop, lettuce is relatively similar to petunia. However, lettuce has somewhat
greater calcium needs. Growers can produce a relatively good crop of lettuce in
containers, if they use a complete fertilizer at a moderate strength of 150
parts per million nitrogen.”
to a bedding plant crop. The seed would be planted into a plug tray for three
to four weeks. Transplanting the plugs into larger containers, the crop could
be finished in four to six weeks depending on light and temperature levels.
is directly sown into the growing medium and grown for three to four weeks
until plants reach suitable size.
Leaf tipburn is a physiological disorder that can occur
when growing greenhouse lettuce. It can greatly impact the salability of a
growing too fast under high light,” Mattson said. “For lettuce, the target
daily light integral is 17 moles per square meter per day. The light level should
be lower if there is poor air flow. If the light level goes higher than 17
moles, the rapid growth of young leaves is affected. There may be an inadequate
calcium supply, especially as the lettuce heads begin to mature and close. If
there is not enough air flow and not enough transpiration by the young leaves,
then not enough calcium can reach the leaves through the xylem sap. This can
cause tipburn to occur. It’s a case of pushing the plants too fast.”
tipburn in lettuce is not a result of a lack of calcium
supplied to the plants,
but an inability of the plants to
transport enough calcium to the young leaves.
lack of calcium supplied to the plants, but an inability of the plants to
transport enough calcium to the young leaves.
calcium if the growing medium has a lime charge and if the fertilizer water
solution contains more than 50 ppm calcium,” he said. “Many common bedding
plant fertilizers, including 20-20-20, 20-20-20 and 21-5-20, do not contain
calcium. These fertilizers are typically used with tap water sources that
contain moderate alkalinity. In many cases, these tap water sources also
contain sufficient calcium.”
water sources to make sure adequate calcium is being supplied, either from the
water source or added into the fertility program. If calcium needs to be added,
calcium nitrate is most commonly used. However, calcium nitrate is not
compatible with most complete fertilizers.
using a separate stock tank or a separate injector,” Mattson said. “One
strategy is to use a separate injector for the calcium nitrate in a series with
a 20-10-20 fertilizer that is being added with a second injector. Adding 50 ppm
calcium from calcium nitrate should be sufficient.
grower has only one injector is to rotate between two separate stock tanks, one
for calcium nitrate and one for the bedding plant fertilizer. A grower would then
rotate between the two fertilizers. For example, for two days he would use the
20-10-20 fertilizer and on the third day he would use the calcium nitrate
applied at 150 ppm.”
Mattson has been able to grow a relatively good crop of
container-grown lettuce using granular organic fertilizers incorporated into
the growing medium.
8-4-4) into the growing medium at a rate of 8 pounds per cubic yard for both
the seed germination and transplant growing mixes,” he said. “That provided
good fertility, but for optimum yields I would also suggest making some liquid
organic fertilizer applications, maybe two to three times a week as the plants
temperature-dependent and is broken down by soil microbes. Sustane 8-4-4 has a
45-day release period, but under very warm greenhouse temperatures Mattson has
noticed quicker release rates. He said there are other slow release organic
fertilizers with different release periods. For example, Verdanta EcoVita lists
a 75-100 day release period.
electrical conductivity and pH
One strategy that Mattson recommends growers do periodically
is to monitor the electrical conductivity (EC) and pH levels.
are receiving sufficient fertility,” he said. “If a grower is incorporating a
slow release fertilizer, this is a good indicator of when additional fertilizer
needs to be added. An under-fertilized plant will show yellow lower leaves from
electrical conductivity (EC) can help avoid
under fertilizing lettuce plants,
which show yellow
lower leaves caused by nitrogen deficiency.
nutrient availability. He said lettuce isn’t commonly susceptible to iron
deficiency, but it will start to show up when the pH starts to increase above
hydroponics,” he said. “A good grower who is producing his crop in a growing
medium in containers will monitor the pH every week or two. The pH may change
over the course of a week by maybe one unit.
the pH every day and make adjustments. Depending on the type of fertilizer and
the quality of the water, the pH in a hydroponic set up could change two units
in a day.”
Mattson said light and temperature are going to be the
drivers for how long it takes to finish a lettuce crop. Whether a grower is
producing the crop in containers with growing medium or hydroponically
shouldn’t have any effect on the length of production.
lettuce head. If plants are grown in small containers and spaced pot-to-pot,
the lettuce heads may not reach full size.
developed a hydroponic production model that enables growers to produce a
lettuce crop from seeding to harvest in 35 days if temperature and light
intensity are at optimum levels.
take more than 100 days from seeding to harvest,” Mattson said. “High pressure
sodium lamps would be the best lamps to use if a grower is looking to provide
supplemental light in a greenhouse to increase the daily light integral. For
the Cornell model we adjust the amount of light in the greenhouse based on the
amount of outdoor light. Seventeen moles per square meter per day is the daily
light integral we are aiming for with the model. The optimum temperature for
plant development is about 75ºF
during the day and 65ºF
For more: Neil
Mattson, Cornell University, School of Integrative Plant Science; (607)
Worth, Texas; firstname.lastname@example.org.
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Researchers at Purdue University are finding LEDs can
have positive effects on both ornamentals and leafy vegetables.
By David Kuack
(LEDs), scientists are discovering new ways to use the lights on ornamental and edible
plants. Researchers at Purdue University have done extensive studies on annual
bedding plants, comparing the growth of seedling plugs and vegetative cutting liners
we are finding new and exciting results, especially with the indoor production
of young plants and microgreens,” said associate horticulture professor Roberto
Lopez. “Some of the work that we have been doing has shown the benefits of
try to produce plugs indoors and not in a greenhouse, I would have said no. If
you would have asked me five years ago if I would be working on greens or
vegetables, I would have said no. Now I am doing both of those things with
|Purdue University graduate student Joshua Craven
and associate horticulture professor Roberto Lopez are
studying the effects LED
lights have on ornamental
plants and leafy vegetables.
Photo by Tom Campbell, Purdue University
No need for
Lopez and former graduate student Wesley Randall found
that greenhouse-grown seedling plugs of impatiens, marigold, petunia, vinca and
zonal geranium did as well or better when supplemented with LEDs compared to
plugs supplemented with light from high pressure sodium lamps. What Lopez found
surprising was the quality of the plugs produced in a growth room with LEDs as
the only light source.
than when they are grown in a greenhouse with sunlight supplemented with light
from LEDs or high pressure sodium lamps,” Lopez said. “It is amazing how good
the plugs look grown in an indoor multilayer production system with LEDs. The
plugs are compact, sturdier and greener with a similar root and shoot dry mass
to greenhouse-grown plants supplemented with light from LEDs or high pressure
LEDs is petunias.
indoor grow room equipped with red and blue LEDs, encountered a slight delay in
flowering in the greenhouse,” he said. “We are going to see if exposing the
plants to far-red LED light prior to moving them into the greenhouse will
induce them to flower.”
intensify leaf, flower color
Lopez said many of the annual spring bedding plants grown
in greenhouses in northern climates are produced under low light levels. The
result is that some plants don’t produce the same intense foliage colors that
they would if they were grown outdoors.
sun’s ultraviolet light because it is blocked by the glass,” he said. “The
result is that crops like zonal geraniums and purple fountain grass (Pennisetum setaceum ‘Rubrum’) don’t
“color up” like they would outdoors. One of the things we noticed with zonal
geraniums was the dark patterns on the leaves stood out much more when the
amount of blue light was increased. We hypothesized and found it was a result
of an increase in anthocyanin production. We have also looked at geraniums that
have very dark foliage and found not only does leaf color darken, but flower
color can be made darker by exposing market-ready plants to red:blue LEDs.”
production was also dramatic for purple fountain grass.
species produced by many growers,” Lopez said. “Grown in the greenhouse, the
leaves appear to be dull green and not very purple. We found that putting the
plants under a combination of red and blue LEDs for one to two weeks of what we
are calling “end-of-production lighting” resulted in an attractive purple
color. UV light is what stimulates anthocyanin synthesis.”
leaves exposed to the LED lights change color. Those leaves not exposed to the
LED light remain green.
to vegetable crops
Seeing the positive results that occurred with LEDs and
purple fountain grass, Lopez and PhD student W. Garrett Owen expanded the
research to red leaf lettuce to see if they could produce a similar response.
greenhouse growers if they are producing crops under low daily light integrals
(DLIs),” Lopez said. “Growers producing red leaf lettuce under low DLIs are
essentially producing green lettuce.
used for purple fountain grass and the plants colored up in three to five days.
Based on our research, red leaf lettuce and purple fountain grass can be placed
under a 50-50 red and blue LED combination prior to harvesting or shipping
triggering anthocyanin formation.”
formation, Lopez said the studies may be expanded to look at the impact of LED
light on ornamental cabbage and kale. “Growers, especially those in the South,
have a hard time coloring up ornamental cabbage and kale,” he said. “It is
primarily a temperature response, as the night temperatures get cooler the
plants start to color up.”
and kale under LEDs that resulted in a minimal color change. When
greenhouse-grown plants were grown under cool night temperatures and exposed to
LEDs, they exhibited the most intense color.
who have access to coolers, is to use a cool temperature/LED treatment,” he
said. “We will be conducting this study next fall. Smaller container sizes like
4-inch pots, could be rolled on carts into a cooler and exposed to cool
temperatures and LED lights for three to four days prior to shipping enabling
the plants to color up.”
Another study conducted by graduate students Joshua
Gerovac and Joshua Craver looked at the effect of LEDs on the growth of three
different microgreen species (kohlrabi, mustard and mizuna) in an indoor
multilayer production system. The study included three different light
qualities and three different DLIs (light quantity).
is for three microgreen species we trialed, the length of the hypocotyl,
basically the height of the microgreen, decreases,” Lopez said. “The more light
the plants are provided, the more compact they are. If the plants received 6
moles of light, they were much taller than if they received 18 moles of light.
Depending on the growers’ market, some customers might want microgreens that
are a little leggier or they might want plants that are more compact. That will
depend on market preference.”
Lopez said the ideal vertical LED light module would
contain all of the wavelength colors.
would enable growers to turn them on when they need them and off when they
don’t, depending on the stage of plant growth,” he said. “Once flowering begins
a grower doesn’t want stem elongation. Far-red light works for flowering so the
far-red would be turned on for the minimum amount of time required for
flowering. If the grower wants to increase the amount of anthocyanin in the
leaves or flowers, he can turn on the red and blue light near the end of the
crop. To be able to turn on specific colors when a growers needs them, that is
something I envision happening with LEDs.”
Roberto Lopez, Purdue University, Department of Horticulture and Landscape
Architecture; (765) 496-3425; email@example.com;
Worth, Texas; firstname.lastname@example.org.
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Measuring daily light integral (DLI) provides a more
accurate reading of the amount of photosynthetic light being received by
Do you measure the light in your greenhouse to ensure
your plants are receiving an adequate amount of light? If you are using a
footcandle meter to measure the light intensity you are not getting a true
measurement of the amount of light received by the plants. Measuring the light
in footcandles or micromoles per square meter per second (µmol/m2/s) is
a measurement of instantaneous light, that is, the amount of light at the time
the measurement is made. An instantaneous measurement made under sunny or cloudy
conditions may not provide an accurate evaluation of the total amount of light
perceived by the plants over the course of a day.
light is extremely dynamic,” said assistant horticulture professor Chris Currey
at Iowa State University. “From sunrise to sundown, there are variations in
light. If growers take a single light measurement early in the day, they may be
underestimating the amount of light. Alternatively, if the light measurement is
made later in the day, growers may be overestimating the light level.
Instantaneous light levels change over the course of a day.”
Currey said another issue with measuring light with a
footcandle meter is that it measures the light that is visible to the human
“Photosynthetic light, which is the light plants can use for photosynthesis and
is defined as photosynthetically active radiation (PAR), is the light that
occurs between 400 and 700 nanometers. So if a footcandle meter measures light
that is visible to the human eye, this includes wavelengths outside of PAR.
Consequently, footcandle measurements tend to overestimate the amount of light
for plant growth.”
Currey said it is the total amount of photosynthetic
light that is going to impact how a greenhouse crop is going to grow.
light over the course of a day,” he said. “In production situations where there
is a static light source it is relatively easy to determine the DLI. Examples
of a grower producing a crop under a static source of light include growing
plants under high pressure sodium lamps in a warehouse or tissue culture plantlets
grown under LED lights in a laboratory. In these situations the light levels
are not going to change throughout the day.
throughout the day. So to take one instantaneous measurement to indicate the light
level isn’t the best way to describe the total amount of PAR light available.
It’s better to look at the total amount, which is the DLI.”
measure the light intensity,” he said. The grower would then use these
measurements in some calculations that would give the DLI. Realistically, very
few growers are going to determine DLI this way.”
DLI. Currey said one of the best ways to determine DLI is to use a quantum
sensor that measures PAR. The unit of measurement for DLI is moles per square
meter per day (mol/m2/day).
logger that records the light measurements,” he said. “The quantum sensor can
be attached to a data logger that can record frequent instantaneous light
measurements which can then be integrated into a cumulative total for the day.
The quantum sensor can also be hooked up to a greenhouse environmental control
computer, such as an Argus or Priva, which can calculate the DLI.”
Researchers at Clemson University used light measurements
collected by the National Oceanic Atmospheric Administration to developed
monthly DLI maps for the United States.
Monthly daily light integral maps for the United States
were developed using light measurements collected by
the National Oceanic Atmospheric Administration. The
maps are based on historical averages.
Photo courtesy of Michigan State University
“Poinsettia growers will tell you each growing season is different. Sometimes
they will have bright, sunny Novembers and other years, it’s dark and cloudy.
These maps provide a good indication of light levels, but there are going to be
variations between years. That’s why it is important for growers to measure DLI
so they know what is happening in their greenhouses and be able to react,
including using supplemental lighting to increase DLI.”
DLI necessary for specific crops.
he said. “For African violets, a DLI of 6 mol/m2/day is enough to
produce a good crop. For poinsettias, 10-12 mol/m2/day are needed to
grow an acceptable quality crop. For cut roses, the DLI needs to be above 20
mol/m2/day to produce a good crop.”
Currey said some growers are using DLI, their
environmental controls and supplemental lighting to provide their plants with optimum
“Growers have their lights turn on when the light intensity is below a certain
level and turn off when the light goes above a certain level. This way a grower
is not adding light during the brightest time of the day when the light level
is at or above the light saturation point for photosynthesis. Growers can use
lighting set points so that if they are not achieving the target DLI for a
crop, they can have the lights turn on sooner or turn off later to ensure
plants receive enough PAR. As a crop goes later into the spring, a grower is
likely going to lower that trigger light intensity because there is going to be
more natural sunlight so less supplement light is needed.”
For more: Christopher
Currey, Iowa State University, Department of Horticulture; (515) 294-1917; email@example.com.
Worth, Texas; dkuack@gmail.
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Figure 1. The electromagnetic spectrum. The visible spectrum encompasses approximately 380 nm to 780 nm. Photosynthetically active radiation or PAR is 400 nm to 700 nm.
Figure 2. Image depicting photons of light falling on one square meter.
Figure 3. Map of outdoor DLI by month throughout the United States.
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