In this first article of a four-part series, researchers from Michigan State University share science-based information about indoor production of leafy greens and herbs.

Before light-emitting diodes (LEDs) became common in the horticulture industry, indoor farmers used fluorescent tubes that emitted a cool- or warm-white light. Nowadays, some indoor farmers are using pink or purple lights, which are comprised of blue (B; 400–500 nm) and red (R; 600–700 nm) LEDs. Other growers are using full-spectrum (or broad-spectrum) lights, which are white LEDs with or without supplemental B, R, and/or far-red (FR; 700–800 nm) LEDs. White LEDs are B LEDs with a phosphor coating, which converts some energetic B photons to longer wavelengths, such as green (G; 500–600 nm) and R. Therefore, full-spectrum LEDs emit more G (and usually FR) light than B+R LEDs.

Plants grow just fine under B+R LEDs, so what does G or FR light bring to the table? Although chlorophylls absorb mainly B+R light, leaves also absorb 70-80% G light. While the upper surface of a leaf absorbs most of the B+R light, G light penetrates deeper into the leaf and gets scattered more to drive photosynthesis. Moreover, G and FR light reach deeper into a canopy than B+R light and can elicit shade-avoidance responses such as increased leaf expansion and upward leaf orientation. As a result, larger leaves intercept more light energy to increase photosynthesis and thus, growth. Green and FR light also contribute to photosynthesis directly.

Although these phenomena indicate positive roles of G light in plant growth, effects of G light have been inconsistent in recent studies. In addition, if G and/or FR light are to supplement B+R light, how G light compares with FR light is unclear. We performed an indoor experiment to determine how partly or completely substituting B light with G and/or FR light affected lettuce and kale growth.

Experimental protocol

We grew green butterhead lettuce ‘Rex,’ red oakleaf lettuce ‘Rouxai,’ and kale ‘Siberian’ in the Controlled-Environment Lighting Laboratory (goo.gl/zM4bDh). Seeds were sown in a rockwool substrate, placed on plastic trays, irrigated with a nutrient solution, and germinated at 68° F under a 24-hour photoperiod from warm-white LEDs at 180 µmol·m^-2·s^-1 (400–800 nm). On days 9 to 11, seedlings were transplanted into a deep-flow hydroponic system with three vertically stacked layers per rack. Plants were grown at 68° F under a 20-hour photoperiod from each of eight LED lighting treatments at 180 µmol·m^-2·s^-1 (400–800 nm). The lighting treatments were delivered with the PHYTOFY RL LED fixtures (OSRAM) with seven programmable color channels and software control (Fig. 1). Red light was delivered at 120 µmol·m^-2·s^-1 in all treatments while the remaining 60 µmol·m^-2·s^-1 was comprised of B, G, and/or FR light. Blue light at 60 µmol·m^-2·s^-1 was substituted with G or FR light at 0, 20, 40, or 60 µmol·m^-2·s^-1. In addition, B, G, and FR light were delivered equally at 20 µmol·m^-2·s^-1 each (B20G20R20FR20, where numbers indicate photon flux densities in µmol·m^-2·s^-1). Plants were harvested approximately 30 days after seed sow.

Shoot mass

Substituting G or FR light for B light increased shoot fresh mass of all crops (Fig. 2). Compared with the B60R120 treatment, G60R120 increased fresh mass of lettuce ‘Rex’ by 72%, lettuce ‘Rouxai’ by 79%, and kale ‘Siberian’ by 50%. Compared with B60R120, R120FR60 increased fresh mass of lettuce ‘Rex’ by 37%, lettuce ‘Rouxai’ by 91%, and kale ‘Siberian’ by 43%. Trends for shoot dry mass were generally similar. Shoot mass of all crops under B20G20R120FR20 was comparable to that under B20R120FR40.

Leaf and petiole size

Leaf size increased as B light was substituted with G or FR light. For example, the fifth mature leaves of lettuce and kale were 23–39% longer and 27–34% wider under G60R120 than B60R120 (Fig. 3). In comparison, they were 31–94% longer and 22–50% wider under R120FR60 than B60R120. Leaves were generally longer under FR light than under the same photon flux density of G light for lettuce, but not kale. Substituting 60 µmol·m^-2·s^-1 of G light for B light increased plant diameter by 38% for lettuce ‘Rex’ and by 86% for lettuce ‘Rouxai.’ In comparison, substituting 60 µmol·m^-2·s^-1 of FR light for B light increased plant diameter by 32% for lettuce ‘Rex’ and by 63% for lettuce ‘Rouxai.’ Total leaf area and petiole length of kale were measured. Responses of total leaf area were similar to leaf length and width. Although increasing G or FR light (while decreasing B light) increased petiole length, this increase was more pronounced under FR light. Petioles of kale were 26%, 61%, and 68% longer under FR light than G light at 20, 40, and 60 µmol·m^-2·s^-1, respectively.

Foliage color

Increasing G or FR light (while decreasing B light) decreased an index for chlorophyll concentrations in all crops. At the same photon flux density, FR light decreased it more than G light did. As a result, leaves appeared yellower with increasing G or FR light (and decreasing B light). In addition, the spectrum influenced red foliage coloration of lettuce ‘Rouxai’ (Fig. 2). For example, its foliage became less red and brighter as B light was substituted with G light.

Summary

A partial or complete substitution of B light (as in B60R120) with G or FR light increased harvestable mass and leaf size of lettuce and kale. Increased yields can at least partly be attributed to increased leaf expansion and consequently, light capture for photosynthesis. However, substituting G light for B light decreased foliage redness of red oakleaf lettuce ‘Rouxai.’ Although this study showed apparent benefits of G light in crop growth, it was unclear whether these responses resulted from increasing G light, decreasing B light, or both, which will be addressed in part II of this series. The effects of the light spectrum on other quality characteristics also merit consideration.

Qingwu (William) (qwmeng93@gmail.com) was a Ph.D. graduate research assistant and Erik (runkleer@msu.edu) is a professor in the Department of Horticulture at Michigan State University (MSU). They thank David Hamby, Rodrigo Pereyra, Charles Brunault, Alan Sarkisian, and Dorian Spero from OSRAM Innovation for lighting support; Steve Brooks for technical assistance; Randy Beaudry, Dan Brainard, Roberto Lopez, and Emily Merewitz for instruments; Nathan Kelly, Yujin Park, and Nate DuRussel for experimental assistance; and Michigan State University AgBioResearch Project GREEEN for funding. A YouTube video of this project is available to watch here

Growers are curious to know whether they would save money in regard to greenhouse heating during wintertime upon using a high-pressure sodium (HPS) lighting system rather than a light-emitting diode (LED) system. We explain that it is not feasible to provide more lighting as a useful heating source with the increased amount of electricity power for lighting, so using HPS fixtures, rather than high-efficient LEDs, is wasting energy within the greenhouse/indoor farm. HPS might also deliver some heat stress and cause abnormal fruit growth at a higher intensity than 200 µmol·m^-2·s^-1.

The greenhouse industry across Canada, including Alberta, has been continuously challenged by the market to provide high-quality, off-season products at good prices, despite climatic obstacles — primarily an inadequate amount of sunlight during winter months. That is why currently, vegetable production in Canada is being supplemented by imports from the United States and Mexico, adding up transportation costs.

Artificial light is an essential energy source for vegetable production in greenhouses at high latitudes where lack of natural sunlight severely limits plant growth from mid-autumn to early spring. For instance, during fall and winter months in North America, vegetables such as cucumber and tomatoes are commonly grown under HPS supplemental lighting. Supplemental lighting with HPS lamps is commonly practiced in Canada to increase crop productivity during off-season production. However, HPS lamps waste a large portion of energy and can be a safety concern due to high-voltage requirements. Moreover, the use of HPS lamps can produce symptoms of edema in ornamental and vegetable crops.

An alternative to the HPS lamp is the high-intensity LEDs, which have higher Photosynthetic Photon Flux (PPF) efficiency (indicator of light intensity), longevity and a lower safety concern. If LEDs are designed and operated successfully, the greenhouse growers will be able to save up to 46 percent on energy costs (data will be shown in a future issue of Produce Grower). The Canadian greenhouse industry could benefit from new, energy-efficient and cost-effective LED lighting approaches to obtain maximum year-round production.

Cucumbers and tomatoes are the most common greenhouse vegetables produced in Canada, specifically Albertan greenhouses, and as they require more light intensity, they have been selected for this research project. Available evidence comparing HPS and LED lighting systems has been controversial, prompting further studies to provide more accurate information for growers. In small-scale studies, HPS lamps were shown to consume 40 percent more electricity than LED lights to achieve the same PPF over the canopy, suggesting that HPS lights are energy-inefficient_1,2. However, in a larger-scale study, HPS lights were shown to save 44 percent more energy when compared to older generations of LED lights₃. Further studies have revealed that artificial lighting systems, including LED lights, can create problems with crop growth and physiology, possibly due to insufficient tuning of crop cultivation under these lit conditions₄. These problems seem to focus on plant phenology, plant load and the influence of LED light quantity and quality on microclimate and plant morphogenesis.

In the current study, we have compared HPS and LED lighting systems side-by-side to address the aforementioned issues arising in large-scale commercial greenhouses under Alberta’s climatic condition (Fig. 1). To assess the effect of light supply on leaf, flower and fruit microenvironment as well as plant phenology, we recorded sunlight intensity received at plant canopy levels in calendar weeks under glass and poly glazing materials. To determine the microenvironment and percentage of photosynthetically active radiation (PAR) available inside the greenhouses, we measured light intensity from sunlight, HPS, LED and their combinations (hybrid overhead and intra-lighting), using WatchDog Plant Growth Stations (10-second interval) with 16 Li-Cor quantum sensors. Also, we recorded temperature, relative humidity and dew point (15-second interval) with Onset Temp/RH loggers inside the plant canopy for each experimental unit (Fig. 2).

Part 1. Plant microenvironment

One basic question for many greenhouse growers is whether temperatures rise within the plant canopy upon using lighting systems; HPS lighting systems compared to LED or no supplemental light might significantly save energy for them or adversely affect their crops. They need to determine if this heat would be useful, and whether there would be any savings for the cost of greenhouse heating during the wintertime when using an HPS rather than an LED system.

HPS lamps release plenty of direct heat due to a low energy efficiency for converting electricity to photons at the PAR range. As demonstrated in Fig. 3A, heat is mostly produced by a high peak in the HPS spectrum within the far-red band of 820 nm. Far-red photons at 820 nm are not activating chlorophylls (chl a, b, c) and other phytochromes to stimulate photosynthetic activity and plant yield improvement. Far-red photons also contain a large amount of heat₅. The question remains: Would the far-red photons produced by HPS lamps provide useful heat to be a heating source during winter months and save energy within the greenhouse?

We tried to understand the actual distribution of heat at different locations of the plant canopy in side-by-side comparisons of HPS, LED, HPS plus LED and control conditions in commercial scale greenhouses. In Fig. 4, it has been illustrated how the temperature and humidity of the plant canopy are greatly influenced by supplemental lighting, particularly during the winter months, when day lengths are short and vents generally stay closed. We collected data for the top, middle and bottom of the plant canopy with four replications.

The recorded minimum and maximum temperatures under sunlight (control) were 16.8^o C in the night and 22^o C during the day. Under the same condition, the minimum and maximum temperatures were 16.5^o C in the night and 24^o C in the day using HPS alone and HPS plus LED treatments. The greater temperature fluctuation under the HPS lights might be because of far-red radiation produced by the HPS lamps. These temperature fluctuations may cause a thermal stress for the plant (due to sharp consecutive rise and fall of temperature at upper plant leaves). It also means that non-uniform leaf temperatures increase the likelihood of fruiting disorders caused by high temperatures.

When temperatures were compared between the HPS and LED treatment groups, the HPS lights were found to warm up plant tissue more than the LED lights only at the top of canopy (top five leaves). Temperature, and consequently humidity and dew point, were quite similar in HPS, LED and HPS plus LED treatments in the middle and bottom of the plant canopy (Fig. 4).

In general, the majority (>98 percent) of electrical energy that enters an indoor farm or greenhouse via a lighting system will ultimately be converted to heat, regardless of the energy use efficiency of the lighting fixture. Heat can be released directly from the light fixture, such as in the case of an HPS lamp, or indirectly, when light is reflected by the plant tissue or other objective within the greenhouse; less than 2 percent of the energy from light is absorbed by chlorophyll and converted into hydrocarbons (sugars) through photosynthesis. Therefore, it is not feasible to provide more light as a useful heating source with the greater amount of electricity power which is used for the lighting of additional lamps. This leads to energy wasting within the greenhouse when using an HPS fixture compared to an LED fixture. All growers should know that the electricity power is not a cost-effective choice for heating a greenhouse. Moreover, in the cloudy days of spring or summer while the indoor temperature rises to above the set point (hot but dark days), it may be necessary to continuously run the supplemental lighting. On those days, growers are challenged to eliminate the heat released by HPS lighting. As a result, some growers must install additional air fans with extra electricity consumption in order to reduce the heat stress on their plants. Though, if they select the right LED fixture with significantly lower power consumption then there should not be any heat stress concerns. For more details on energy saving in your greenhouses, follow the second chapter of this article in a future issue of Produce Grower.

Saeid Mobini, Ph.D in Horticulture, was a greenhouse specialist, Crop Research and Extension Division in the Alberta Ministry of Agriculture and Forestry in Canada. He is currently the master cultivator at SugarBud Craft Growers. mobinisaeid@gmail.com

References:
1. Wojciechowska, R., et al. (2015) Effects of LED supplemental lighting on yield and some quality parameters of lamb’s lettuce grown in two winter cycles. Scientia Horticulturae, 187, 80-86. 2. Nelson, J. A., & Bugbee, B. (2014). Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS One, 9(6), 99010. 3. Pinho, P., et al. (2013). Dynamic control of supplemental lighting intensity in a greenhouse environment. Lighting Res. Tech. 45:295–304. 4. Dueck, T., et al. (2011). Growth of tomatoes under hybrid LED and HPS lighting. Paper presented at the International Symposium on Advanced Technologies and Management Towards Sustainable Greenhouse Ecosystems: Greensys2011 952. 5. Nelson, J. A., & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS One, 10(10), 0138930.

For container-grown ornamental plants, managing micronutrients can be a challenge for producers. We generally classify annual and perennial bedding plant, potted plant and cut flower species into different groups, depending on their micronutrient requirement. First, there is the “general” group that doesn’t require too much special treatment during production, and the average amount of micronutrients available to them is sufficient to grow a high-quality crop. Next, we have those groups of plants that do require special attention: 1) micronutrient-efficient crops and 2) micronutrient-inefficient crops. The micronutrient-efficient group requires less-than-average amounts of micronutrients, while the micronutrient-inefficient group requires more-than-average amounts of micronutrients.

Producers of herbaceous annuals and perennials and woody plants typically manage micronutrients in containerized crop production by managing the root-zone pH. For those species classified in the “general” group, no special accommodations are made; a standard substrate pH of 5.8 to 6.2 is acceptable for these species. Alternatively, the root-zone pH is typically increased into the range of 6.0 to 6.4 for micronutrient-efficient plants (also called the “geranium group” or “marigold group”), while the pH for micronutrient inefficient plants are usually lowered to 5.6 to 6.0. By adjusting the pH of the root-zone, we alter the availability of micronutrients, but not the actual concentration or amount. The availability of micronutrients decreases as pH increases, whereas availability increases as pH decreases. While adjusting pH to make micronutrients more or less available in container-grown crops in soilless substrates, hydroponic crops require a different approach.

For food crops grown hydroponically, the variety in micronutrient requirements is going to be the same as substrate-based systems, with some species requiring higher or lower amounts of micronutrients than the average crop. For example, basil is a crop that generally requires more iron than other herbs and chlorosis can be problematic when it is deficient, while rosemary should be provided with lower amounts of micronutrients (Fig. 1). However, it is more difficult to use pH to control micronutrient availability. Rather, instead of using a low pH to make more micronutrients available or a high pH to restrict them, it is more effective to regulate the actual concentrations of micronutrients in your solution.

However, for plants grown hydroponically and especially in recirculating water culture systems such as nutrient-film technique (NFT) and deep-flow technique (DFT), the relationship between pH and micronutrients is a little different. The micronutrients are much more available to plants from the nutrient solution. Therefore, producers need to manage the concentration of these elements in the nutrient solution.

Fertilizer concentrations are commonly measured using the electrical conductivity (EC) of nutrient solution in hydroponic production systems. The EC doesn’t measure the amount of individual mineral nutrients; rather, it reflects the total concentration of all nutrients. As the name implies, micronutrients are required in such small quantities that the amount commonly present in nutrient solutions has a negligible effect on EC. As a result, EC values represent primary and secondary macronutrients, those elements in greater quantity in the nutrient solution. Additionally, the speed at which nutrients are taken up varies. Some are taken up rapidly, while others are taken up slowly, and micronutrients are no exception. Some micronutrients such as manganese are taken up very quickly, whereas boron is taken up slowly. Therefore, you’ll want to measure the concentrations of micronutrients in nutrient solutions and/or plant tissue to better manage them during production. Over time, you will have a better understanding of the dynamics of micronutrients in your solutions.

The nutrients added to your nutrient solution throughout production may contain different concentrations of micronutrients than the initial or original nutrient solution. Accommodating specific, individual micronutrient concentrations will only be possible for those producers who are mixing their own fertilizer. For those using a one or two-bag fertilizer, the proportions of micronutrients are fixed; for these fertilizers, watching for concentrations that are out of balance (i.e. deficient or toxic concentrations) will be especially important.

Christopher (ccurrey@iastate.edu) is an Assistant Professor in the Department of Horticulture at Iowa State University.

Growing root crops and leafy greens during winter’s whiteness could be the ticket to putting a little more green in your pocket over the winter months. However, there is a learning curve to growing a crop in a hoophouse after the snow begins to fall.

“There are unique problems in winter growing situations,” says Zachary Grant, extension educator with the University of Illinois Extension. “Even if you’re a farmer [who has been] growing for decades, if you have never had the experience of growing in a high tunnel in the winter it can be a whole new learning curve in terms of planting dates, how you’re direct seeding or transplanting.” For example, he says irrigation is “totally different,” as is ventilation, the latter of which has to be done more carefully to allow for fresh air exchange while not freezing out your plants.

With that said, growing produce in the winter has the obvious advantage of being able to provide fresh vegetables to markets over the winter.

Growers are using various types of structures to grow crops under cover, including high tunnels and low tunnels. Both are interchangeably referred to as “hoophouses.” Call them what you want, the main point is to consider shape, location and the quality of the covering.

Grant says gothic and gable-style hoophouses are a safer bet in areas that get a lot of snowfall. With Quonset-style hoophouses, the snow will fall down the sides and build up, causing the structure to cave in. This is different from gothic and gable-style hoophouses, where the snow falls off. He says for a walk-in style hoophouse, the sides should be angled at 45 degrees to shed snow. Snow melt can also be an issue.

“Snow load is definitely a potential problem,” Grant says. “You need to clear the perimeter of snow buildup. If you allow snow accumulation on the sides and it melts, you’re dealing with rainwater — you need to have good drainage.”

Locate with winter in mind

Wind can certainly be a factor, but Grant is not worried about winter winds where he’s located, about 30 miles south of Chicago. However, he does stress the importance of locating the structures away from trees and buildings that could shade your crop, especially with the sun at such a low angle in the winter.

“You can put it up in June and say, ‘Hey, no problem,’ but then here you are in the middle of winter and a building or tree could be casting shadows,” Grant says. “You can run into some unique problems in site location and design if you’re not careful.”

Grant suggests growers reach out to their county extension agents for information on how to locate and set up these structures. He says you should also weigh the pros and cons of setting up a large structure yourself or hiring someone to do it for you. Hire someone and it could go up in a day, he says, as opposed to three to four weekends if you’re doing it yourself.

Winter cover

Grant recommends standard greenhouse film or woven fabric to cover any type of winter growing structure. There is also a need for another protective layer inside. He says some growers use another layer of poly inside, however, there can be issues with poly.

“The problem is it doesn’t breathe, and it can heat up pretty quickly,” Grant says. “If you can’t get out there to ventilate, it can cause problems.” He says what a lot of growers are turning to is a 20 percent fabric row cover. He says it works as well as poly in terms of providing protection, but it’s breathable, adding that this fabric is one step up in thickness from the row covers growers use as an insect barrier during the summer months. Too thick of a fabric will inhibit light too much.

“Managing that inner layer is really important,” he says. “If you have a week where the sun doesn’t come out at all it is still critical that you remove that inner layer and allow circulation.”

No rest from the wicked

Just because it’s cold enough outside to numb your hands doesn’t mean there isn’t disease pressure and even weeds inside these structures. Grant says downy mildew on lettuce and spinach can be an issue in a winter hoophouse, and he’s even seen chickweed that had to be plucked. Good ventilation is the key to preventing fungus problems, he says, as are organic-based fungicides.

Feeding the student population

The University of Illinois has a production and learning facility for produce growing and marketing on campus. Matt Turino manages the Sustainable Student Farm at the University of Illinois Urbana-Champaign. The facility consists of three 30-foot-by-96-foot greenhouses run by students, workers and volunteers. Turino says they can produce fresh greens and tomatoes for nine to 10 months out of the year. Winter growing can be a challenge, but it’s worth it as it means fresh produce for students in the dining hall on campus.

“Growing in winter can be significantly more challenging than growing in the season they like to grow,” Turino says. “There is more labor per product, but you can get a better price during winter, and if you’ve got workers you want to keep year-round, it’s a good way to dial in your winter production.”

The farm starts winter greens, including spinach, baby bok choy, baby lettuces and kale in the fall and are able to bring several harvests of the greens into the dining hall over the winter months. Turino says they don’t start greens during the winter months mostly because they don’t have a frost-free water line close to the houses. The winter crop is over in March, at which time they start tomatoes and other spring crops.

“A good place to start is with spinach [because] it’s the most hardy,” Turino says. “It’s a little bit harder to establish, doesn’t compete against weeds as well, but if you can take care of it, it will keep chugging. That and baby kale are the true workhouses.”

Like Grant, Turino emphasizes the importance and challenge of timing winter crops, something that is a little tricky when there still might be summer crops in the greenhouses in late fall or you need to get ready for spring and need to move a winter crop out.

The routine at the farm is to start a different crop in each of the three greenhouses: one in the late fall for winter harvest, one devoted entirely to hardy spinach and one with a mix of greens.

Instead of using organic matter, such as compost that many hoophouse growers use, they rely solely on cover crops to provide nutrients and replenish the soil. Having moveable greenhouses allows this system to work reasonably well.  

“I really like the moveable aspect of the greenhouses,” Turino says. With this method they’re able to start a crop on one side, and plant a cover crop consisting of oats, buckwheat and sorghum sudangrass, on the other. If they can time it right, they’ll sow rye and vetch in the fall for a winter cover. They do this on a yearly basis starting in the late fall. Turino says he thinks being able to move the structures back and forth mainly helps with disease control, but also allows them to flush out salts with rainwater on the uncovered growing plots.

Their biggest challenge to a successful crop, however, is ventilation. It’s finding a balance between controlling moisture buildup and freezing out a crop.

“The big thing is balancing so there is enough air flow for moisture reduction and making sure the plants stay warm enough, which is often counter to each other,” Turnino says. “Putting fans in tunnels helps. Don’t leave anything covered too long — provide good air flow and keep the plants warm.”

Keeping the plants warm and cozy over a long Midwest winter is helping to provide fresh produce to the students at the University of Illinois while teaching the student body how to grow under cover.

Neil is a horticulturist and owner of Green Industry Marketing. greenindustrywriter.com