Farm Operations Management

LEDs and PPFD in the Vertical Farm: From the Basics of Photosynthesis to Designing the Light Environment

Articles for Farm Operations Managers

LED management in the vertical farm is not simply a matter of making things brighter. Light pushes growth up, and at the same time it brings along electricity costs, photoinhibition, CO2 shortage, and disturbances in temperature and humidity.

What you need to watch is neither light quality alone nor PPFD alone. It is whether the amount of light the crop receives, the rhythm of light and dark, CO2, temperature, humidity, and airflow are holding together as a single system.

In this article, I will work through, in order, the basics of photosynthesis, LED selection, how to think about PPFD, lighting schedules, and environmental design that does not waste light.

What you should know about photosynthesis before you grow a crop

What matters for growing crops well in the vertical farm is understanding photosynthesis, the engine of growth.

LED selection and PPFD (photosynthetic photon flux density) settings for shaping the light environment are hard to make effective unless you have a grip on this basic process called photosynthesis.

Put simply, photosynthesis is the mechanism by which a plant uses light to produce its own “food” (sugar). Once you understand this mechanism, the practical answer to “what kind of light, and how much of it, should I give the crop to grow it well?” starts to come into view.

Photosynthesis proceeds, broadly speaking, in two stages.

1. Photochemical reaction (light reaction)

This is, literally, the “light-using” stage.

Chlorophyll contained in the leaves of the crop absorbs light and uses that energy to split water. Oxygen is released as a by-product, and ATP (adenosine triphosphate) and NADPH — the “energy substances” — are produced.

2. The Calvin cycle (dark reaction)

In this step, the energy substances just produced (ATP and NADPH) are used to synthesize sugar from carbon dioxide in the air. The name “dark reaction” does not mean it takes place where light is absent; it means the reaction itself proceeds without directly using light.

CO2 enrichment (adding carbon dioxide) in the vertical farm is meant to promote this stage. No matter how well you optimize the light, photosynthesis does not happen without CO2.

Once you understand this two-stage process, it becomes clear that “you need to manage not only the LED light, but CO2 concentration at the same time.”

To maximize crop growth, what matters is building an environment that supports the whole process of photosynthesis.

Light changes the shape of a vegetable: points you can make use of

The goal of growing a crop is not simply “growing it big,” but producing a “quality crop” in terms of appearance, taste, and nutritional value.

Light is not only an energy source for growth. It also plays a role in determining the shape and quality of the crop. If you understand light quality, you can manage it in line with the goal of your cultivation.

In an environment with a lot of blue light, leaves grow thick and compact, and their color becomes deeper. In an environment with a lot of red light, stems tend to elongate and leaf expansion is promoted. Applying this, you can give red-leaf lettuce cultivars more blue light to bring out more vivid leaf color, or use a specific wavelength balance on basil to enhance its aroma.

That said, under light conditions that prioritize color or aroma, crop enlargement can be suppressed as the price you pay. You cannot maximize everything at the same time.

Is too much light counterproductive? Know the limits of photosynthesis and grow efficiently

It is easy to assume that “the more light, the better a crop grows,” but it is not that simple.

When you weigh the cost of lighting against the growth you get, there is a “right amount” of light. By understanding the ceiling beyond which growth plateaus even if you add more light, you can cut wasted cost and still get the maximum harvest.

What happens when there is too much light: photoinhibition

If you keep hitting a crop with light that is too strong, you get a phenomenon called “photoinhibition,” in which the photosynthetic system is damaged.

Symptoms include leaves turning yellow or looking brown and scorched (the leaf edges are especially vulnerable to damage), slower growth than expected, and yield that does not increase in proportion to the electricity bill. When these overlap, suspect photoinhibition.

For example, leafy greens such as lettuce grow sufficiently at a PPFD (light intensity) of 200 to 300 μmol/m²/s. Giving them more light than that barely changes growth. It wastes electricity and raises the risk of photoinhibition.

By knowing each crop’s “light saturation point” (the point beyond which more light does not increase photosynthesis), you can design lighting without waste.

Another pitfall: photorespiration

Photorespiration is a phenomenon that occurs when CO2 runs short under strong light.

The enzyme that should be combining with CO2 reacts with oxygen instead, and the energy hard-won through photosynthesis is wasted.

To prevent this, when you apply strong light, you must also raise the CO2 concentration (around 800 to 1,200 ppm). If you only crank up the light without adding CO2, you will not get a yield increase that justifies the investment.

Points for growing efficiently

To avoid photoinhibition and photorespiration and grow crops most efficiently:

1. Know the right light intensity for each crop:
   • Leafy greens grow sufficiently even under relatively weak light (200 to 300 μmol/m²/s).
   • Fruiting vegetables such as tomatoes and strawberries become more productive under stronger light (400 to 600 μmol/m²/s).
   • Choosing the “just-right light intensity” for the crop is the first step to saving on electricity.
2. Balance CO2 concentration with light intensity:
   • When applying strong light, always raise CO2 concentration as well.
   • This prevents the vicious cycle of strong light → higher CO2 consumption → CO2 shortage → photorespiration.
3. Appropriate temperature management:
   • Generally, 20 to 25 °C is the optimum temperature range for photosynthesis.
   • When temperature is too high, photorespiration is promoted and photosynthetic efficiency drops.
4. Acclimate seedlings to the light environment gradually:
   • Hitting them with strong light all at once easily causes photoinhibition, so it is important to raise light intensity in stages.
   • Be especially careful of abrupt changes in the light environment when moving seedlings to final planting.

By designing the light environment on the basis of this kind of knowledge, you can achieve cultivation that is both “optimal for the plant” and “efficient for the business.”

Choose LEDs based on the plant’s characteristics

Having understood the mechanism of photosynthesis, the next question is “what kind of LEDs should I choose?” I will explain how to put the physiological characteristics of plants to work in actual LED selection.

First, about “light quality” — the wavelength of light

The range of light wavelengths that is effective for photosynthesis in plants is 400 to 700 nm. This is called photosynthetically active radiation (PAR).

Within PAR, the wavelengths with especially high photosynthetic efficiency are:

• Red light (600 to 700 nm): Promotes the light reaction of photosynthesis and contributes greatly to sugar production.
• Blue light (400 to 500 nm): In addition to promoting the dark reaction of photosynthesis, it also influences chloroplast development, the opening and closing of stomata, and plant morphogenesis (height, leaf thickness, and so on).

By changing the composition of the materials used, LEDs can emit light at a variety of wavelengths. The fact that they can supply light in the wavelength range most effective for photosynthesis in plants is a strength of LEDs that neither sunlight nor earlier artificial light sources had.

• Green light (500 to 600 nm): Photosynthetic efficiency is lower than for red or blue light, but green light reaches deep into the leaf and plays a supporting role for photosynthesis. Because green light is also easier for the human eye to perceive, it makes visual inspection easier for workers.
• Far-red light (700 to 800 nm): Involved in plant morphogenesis (such as stem elongation) and flower bud formation.
• Ultraviolet (UV-A: 315 to 400 nm): Promotes pigment synthesis in plants, giving them vivid color and improving aroma and flavor. UV-B (280 to 315 nm) is also said to enhance disease resistance.

With LEDs, you can mix these wavelength ranges in the proportions you need and adjust the color balance.

In the past, two-wavelength LEDs combining red and blue were widely used, but in recent years the view that other wavelengths also matter has become mainstream. In this article I recommend choosing LEDs with a spectrum close to sunlight, or white LEDs.

What is PPFD — the intensity of light

The PAR (photosynthetically active radiation) explained above indicates the wavelength range of light that a plant can use. On the other hand, the metric that shows how strongly that light reaches the plant is PPFD (photosynthetic photon flux density).

PPFD represents the number of photons (particles of light) effective for photosynthesis reaching the plant per unit area per unit time.

Put simply, it is a numerical expression of “the intensity of light reaching the plant.”

When choosing lighting, PPFD (the amount of light) is just as important a criterion as wavelength (light quality).

As explained earlier, PPFD above the light saturation point is wasted, and the optimal PPFD value differs by crop.

• LEDs for leafy greens
  • Leafy greens light-saturate at around 300 to 400 μmol/m²/s, so a lighting design that pushes PPFD higher than that easily becomes a waste of electricity.
• LEDs for fruiting vegetables
  • Because a higher PPFD of around 400 to 600 μmol/m²/s is required, you need to choose a high-output type.

Rather than picking the output of an individual LED, what you actually do is adjust PPFD through the number of LEDs installed and their layout. Plan this in line with the crop you are growing.

Optimizing the light environment of a vertical farm: practice

Building on the mechanism of photosynthesis and the basics of light quality and light quantity, I will explain practical methods for the vertical farm in actual operation.

Setting the light period and dark period, and how plants respond

Plants carry out different physiological activities during the light period (when light is on) and the dark period (when it is off). Because the vertical farm lets you set the timing of light freely, understanding this rhythm gives you a practical lever for management.

What plants do during the light period

• Activation of photosynthesis: Chloroplasts absorb light energy and synthesize sugar from CO2 and water.
• Promotion of transpiration: Stomata open, and water, along with nutrients, is taken up from the roots and moved throughout the plant.
• Photomorphogenesis: The direction of elongation and the thickness of stems and leaves are adjusted in response to the direction and intensity of light.

What plants do during the dark period

• Activation of respiration: The sugar produced during the light period is used to generate energy.
• Promotion of cell division: Active cell division takes place, especially at the plant’s growing points.
• Synthesis of secondary metabolites: Functional compounds such as aroma components and antioxidants are produced.
• Adjustment of plant hormones: The balance of hormones that control growth is adjusted.
• Translocation and storage of carbohydrates: The sugar produced during the day is moved to the roots and stems and stored as starch.

These physiological activities form the plant’s “internal clock.” When this internal clock is thrown off, it can lead to abnormal growth and reduced resistance to pests and diseases. This is why you should not change the lighting schedule frequently.

The design philosophy of the lighting schedule

Having understood the importance of the light-dark rhythm, we move on to the actual design of the lighting schedule. In the vertical farm, scheduling has to take both plant physiology and economics into account.

Basic tips for schedule design

• The basics of the light-dark ratio
  • Leafy greens (lettuce, mizuna (a Japanese leafy green), and so on): 16 hours light period / 8 hours dark period is typical.
  • Depending on the cultivation goal, you may switch to 14 hours light period / 10 hours dark period to suppress physiological disorders or enhance flavor.
• Points for setting the light period start time
  • For the plant, “how long the light and dark periods are” matters more than “what time the light period begins.”
  • Because of this, schedules that make effective use of cheaper electricity time slots are possible.
  • Concrete example: if you set 22:00 to 14:00 the next day as the light period, you can make the most of off-peak nighttime electricity rates (a cheaper tariff offered by Japanese utilities for nighttime use).

Tips for lighting schedule design with economics in mind

• Use the time-of-use unit price of electricity
  • With many Japanese utilities, the unit price of electricity is lower at night (22:00 to 08:00 the next day).
  • By setting the bulk of the light period within this time slot, you can cut lighting costs.
  • Example: for a 16-hour light period, setting 22:00 to 14:00 the next day lets you reduce electricity costs.
• Staggered switching-on as a form of demand management (a response to the contracted peak-demand cap set by Japanese utility contracts)
  • In large facilities, rather than turning on all lights at once, stagger them and switch them on in sequence.
  • This holds down peak electricity (the demand value) and makes it possible to reduce the basic fee.
  • For example, if you want a 16-hour light period, split the farm into three zones and stagger the start time of each zone’s light period by 8 hours; this spreads the switching-on load.

The lighting schedule is not just a matter of setting ON/OFF times. It is an important management lever for achieving both lower electricity costs and higher quality at the same time. Once you also take crop growth speed and the suppression of physiological disorders into account, the design elements differ from site to site.

172 Tips for Raising the Profitability of a Vertical Farm

Various techniques for improving light efficiency

I will introduce techniques for improving light efficiency that hold electricity costs down while raising yield.

Improving light utilization with reflectors

These are measures to use the light from LEDs without waste.

• Choice of reflector material:
  • High-reflectance aluminum panels: Initial cost is high, but they reflect more than 95% of light and hold that performance for a long time.
  • White-painted surfaces: Relatively cheap, with reflectance of around 80 to 90%, though they can yellow over time.
  • Specialty reflective film: Thin, lightweight, and easy to install, but prone to scratching.
• Effective placement:
  • Mount reflectors on the sides and ceiling of the cultivation racks to minimize light escape.
  • Light also reaches areas that would otherwise sit in the shadow of leaves.
  • The edges of the cultivation area, in particular, tend to receive less light, so angling reflectors inward to direct light back onto the canopy helps.

When reflectors are introduced properly, you can raise yield by 10 to 15% for the same LED power consumption. Relative to the equipment cost, it is a cost-effective improvement.

Making the light distribution uniform

By delivering uniform light to the whole cultivation area, you reduce variation in growth.

When growth varies a lot, more plants fail to meet the shipment criteria, and profitability drops.

• Using diffuser panels:
  • Install a translucent diffuser panel between the LED and the plant to soften direct light.
  • Especially effective when using high-output LEDs.
  • They tend to trap heat, so attention to ventilation is required.
• Devising the LED layout:
  • Grid layout: A more uniform light distribution is achieved (PPFD variation can be held within 5%).
  • Edge-reinforced layout: Install additional LEDs at the edges of the cultivation area to eliminate uneven lighting.
  • Layered layout: Install LEDs at multiple heights in line with plant height (especially for taller crops).

By combining these techniques, variation in harvest volume is also greatly reduced.

As a recent cost-cutting trend, designs that use higher-output fixtures and reduce the number of units installed, widening the spacing, have become widespread in the industry. There are cost-side merits, but as spacing widens, uniformity of light at the canopy level and in the “row gaps (sides of the cultivation racks)” easily breaks down. The more you cut cost, the more demanding uniformity management becomes — on site, design that secures uniform illumination through the combination of mounting height, angle, and reflectors is important.

Get the other parts of the environment in order before the light

Even if you perfect only the light, if the other environmental factors are insufficient, it just wastes electricity.

To maximize productivity in the vertical farm, you need to manage not only “light” but also “temperature,” “humidity,” “CO2 concentration,” and “airflow” as a whole. These factors influence one another, and if even one of them is insufficient, the effect of optimizing the others is limited.

Temperature and humidity in harmony with light

Light intensity and temperature are closely linked. Photosynthesis is a chemical reaction, and the reaction rate changes with temperature.

• Maintaining an appropriate temperature range:
  • For many crops, 20 to 25 °C is optimal for photosynthesis.
  • When light intensity is high, a slightly higher temperature (23 to 26 °C) improves photosynthetic efficiency.
  • Too low, and enzyme activity drops; too high, and photorespiration increases, so efficiency falls.
• Points of humidity management:
  • A relative humidity of 60 to 70% is generally optimal.
  • When humidity is too high, transpiration is suppressed, and the uptake and transport of nutrients is impeded.
  • When light is strong, transpiration is also active, so humidity management is especially important.

Without CO2, the light is wasted

CO2 is the raw material for photosynthesis. Even if there is light energy, photosynthesis cannot proceed if CO2 is lacking.

• Balancing CO2 concentration with light intensity:
  • At ordinary atmospheric CO2 concentration (around 400 ppm), photosynthesis hits its ceiling quickly even when you raise light intensity.
  • When light intensity is high, a CO2 concentration of around 800 to 1,200 ppm is ideal.
  • Setting a high PPFD without CO2 enrichment is simply a waste of electricity.
• Timing of CO2 enrichment:
  • Raise the CO2 concentration from just after the start of the light period.
  • In a closed plant factory, CO2 consumption by the plants is intense during the light period, so constant monitoring and adjustment of supply are necessary.
  • During the dark period, you can stop CO2 supply with no problem (in fact, plants release CO2 through respiration).

Airflow also matters

• Maintaining appropriate airflow:
  • When a layer of still air called the “boundary layer” forms around the leaf, the diffusion of CO2 is impeded.
  • By applying a light breeze (around 0.3 to 0.7 m/s), you break up this boundary layer and improve the efficiency of CO2 uptake.
  • As a result, photosynthesis proceeds more efficiently at the same light intensity.
• Devising airflow circulation:
  • Optimize the position and angle of fans to create uniform airflow.
  • Give attention to planting density and layout so that air also flows between the plants.

Summary

Light management in the vertical farm is not the simple story that “stronger light means higher yield.” PPFD above the light saturation point wastes electricity, and when CO2 is in short supply, strong light causes photorespiration.

In practice, the priorities are clear. First, grasp the light saturation point of each crop and decide the PPFD ceiling, and secure a CO2 concentration that matches that PPFD. Choose white LEDs or LEDs with a spectrum close to sunlight, and raise the utilization efficiency of light with reflective material and uniform layout. Design the lighting schedule to hold down cost with nighttime electricity and staggered switching-on, while not disturbing the light-dark rhythm.

In the end, the story of light is inseparable from temperature, humidity, CO2, and airflow. Getting this whole system in order is the condition for making the investment in LEDs pay off.