Articles on sustainable and profitable growing with Plant Empowerment

A new quality indicator for lighting: RNP (2)

By Jan O. Voogt, Plant Empowerment Academy, April 2025

The RNP stands for Ratio Net radiation to PAR light, or the ratio between net radiation and PAR light. Plants need PAR light for photosynthesis and nutrients and heat for optimal growth, and net radiation plays a key role here. Therefore, the RNP indicates which ratio assimilates and nutrients are available, especially at the head of the crop. Article (1) explains that the RNP of LED lamps is considerably lower than that of natural sunlight. This has negative consequences for the three plant balances: energy, water and assimilates, which leads to various cultivation problems. In this follow-up article, we will discuss multiple options to correct the disturbed RNP ratio in the right direction under full-LED.

It also discusses how the microclimate in the head of the crop and the vertical temperature profile can be improved despite low net radiation. The options below assume a situation with a full-LED installation.

1. Reducing the heat emission of the crop

The effective net absorbed radiation in the head of the crop is the result of the irradiation and the heat emission, both in (W/m2). The first and easiest step in increasing the RNP under LED lamps is to reduce the crop’s heat emission to a cold greenhouse cover or screen. This can be done in several steps:
1. Closing a single (energy) screen to 80-90% significantly reduces the heat emission. Note: Closing one screen completely (100%) saves more energy, but then the screen temperature will be lower than the greenhouse air temperature, increasing the heat emission again. Maintaining a small moisture gap, for example, 5%, is not an option because this can result in cold air falling down into the crop (cold fall) and cooling the plants even more.
2. Closing multiple (energy) screens completely ensures high energy savings. At the same time, the temperature of the lower screen will also remain close to the greenhouse air temperature, in favor of lower heat emission. If the moisture permeability of a double closed screen becomes too low, making a moisture gap necessary, the moisture gap should be applied in the lower screen. The upper screen remains completely closed. Warm air flows between the screens through the gap in the lower screen, causing the temperature of the lower screen to rise.
3. If an upper tube is present, it can be used to increase the temperature of the (lower) screen, causing the screen to contribute to the net radiation positively. Note: A combination with vertical air movement is highly desirable to prevent a top tube from leading to a “warm blanket” at the top of the greenhouse and therefore stagnation of the moisture transport from bottom to top. Monitoring the temperature just below the screen and above the lighting fixtures and/or upper tube provides insight into the temperature profile in the greenhouse.

NB: By measuring the net radiation at crop level, the effect of the above measures on the net radiation can be made directly visible. Depending on the solar radiation and the outside temperature, it can be determined at any time whether it is more beneficial to leave the screen open or to close it. This also depends on the properties of the screen in question.

2. Switching/dimming the LED lamps based on realised RNP

If a crop grows under composite lighting, that is, under a combination of LED light and natural sunlight, this will result in an average or summed RNP between 0.225 and 0.5. See article (1). Therefore, the realised RNP in the greenhouse is strongly influenced by the share of LED light and sunlight in the total light sum.
Controlling the share of LED light by switching or dimming based on the realised RNP value can prevent an RNP that is too low. This way, the LED installation can be used sustainably and flexibly for optimal growth.

3. LED lamps with adjustable spectrum

If the LED lamps have an adjustable spectrum, meaning not only the standard red (95%) and blue (5%), then the RNP also varies with the colour. Colours with a short wavelength, for example green and blue, contain more energy with the same PAR contribution. This results in higher net radiation, albeit at higher electricity costs. With more sunlight, the RNP comes closer to the natural ratio, allowing for lighting with the most energy-efficient spectrum.

4. LED lamps in combination with IR radiators

LEDs are energy-efficient because the emitted spectrum contains a lot of PAR light with a wavelength between 400 and 700 nm, but much less heat radiation with a longer wavelength. A recent development is that LED lamps are combined with IR radiators.
In a preferred embodiment, the PAR intensity (µmol/m2.s) and the IR intensity (W/m2) can be controlled separately so that the most favourable RNP value can be achieved in terms of growth and energy consumption in combination with natural light.
Of course, options 3 and 4 cost extra electrical energy per mole of PAR light. However, this is offset by the improved light utilisation efficiency (LUE) and distribution of assimilates in the plant.

5. Improving temperature and microclimate in the head of the crop

Other common problems under full LED are temperature being too low and an unfavorable microclimate in the head of the crop. Radiant heat from above causes an increased temperature in the upper layers of the crop. In ornamental crops, this also means an increased bud/flower temperature. This results in the attraction of more assimilates and a higher supply of nutrients, and also prevents moisture accumulation. Too low a temperature can even lead to condensation in the upper crop layer or bud/flower, with all the consequences. There are several options to improve this situation.
1. Screens against heat emission as explained above.
2. Installing an (extra) growth tube near the head of the crop.
3. Vertical air movement with fans in combination with the introduction of dry air into the greenhouse to ensure sufficient moisture drainage.

Note: To assess the effect of these measures, measuring the microclimate in the head of the crop is highly desirable, if not indispensable.

6. Improving the vertical temperature profile of the crop

In a vegetable crop, it is beneficial for the zone with ripening fruits to be slightly warmer than the head. This results in the natural relationship between the development speed at the bottom and top, and a good distribution of assimilates over the vegetative and generative parts of the plant. Net radiation from above can still contribute to this if the crop is sufficiently open. Still, in the case of full LED lighting, this higher fruit temperature must be achieved entirely differently.
1. A proven measure is an independently controlled growth tube at the height of the ripening fruits. This also promotes a subtle upward movement of air in favour of the microclimate.

Note: To assess the effect of this measure, it is desirable to measure the fruit temperature with a thermal camera, for example.

7. Increasing the greenhouse temperature

In practice, the greenhouse temperature is often increased in response to the abnormal development of the crop under full LED due to the lack of sufficient radiant heat. However, if this is done with bottom heating, a vertical temperature profile is created that differs essentially from the natural situation in which the radiant energy comes from above. This results in a different assimilate distribution, more moisture production and higher gas consumption. Moreover, this does not solve the problem at the head of the crop. Evaporation is increased at the bottom of the crop, while evaporation is the limiting factor for nutrient supply in the head. Heat is also brought in at the bottom, while this is the limiting factor in the head of the crop for the development speed and optimal assimilate transport from the leaf to the growth point, flower or fruit. This leads to incomplete use of assimilates, so a too low LUE.
Therefore, an increase in the greenhouse temperature must always be combined with one or more of the measures mentioned above to achieve the correct vertical temperature profile. Note: If the installation of full LED lighting also involves working with a higher light intensity, this will lead to higher realised light sums. In that case, the daily temperature should be increased, following the RTR to keep the assimilation balance in equilibrium.

8. Stimulate evaporation with (extra) dehumidification

One of the consequences of a low RNP is lower radiation evaporation and therefore less absorption of nutrients. Convection evaporation can be stimulated by extra dehumidification, and consequently, a lower RH or higher HD. But here too, this is not explicitly aimed at evaporation and nutrient supply, in the head of the crop. With dehumidification with hoses at the bottom of the crop, like with a higher under-pipe temperature, evaporation is stimulated at the bottom, instead of at the top of the crop.
Points of attention are:
1. Convection evaporation only occurs if there is sufficient air movement due to natural convection through a warm tube or fans.
2. The extra absorbed nutrients are automatically transported to plant parts that evaporate more. Therefore, stimulating evaporation at the bottom of the crop makes little sense if the nutrients are mainly needed in the head.

An integrated approach and monitoring are required to achieve the best result

The low RNP of LED lamps has consequences for both the plant balances and the greenhouse balances. Several measures have been discussed above to compensate for one or more of these consequences, either in whole or in part. However, each of these measures also influences other factors.
Therefore, in the improvement process, it is essential to always look at the whole and carefully investigate which combination of measures for a specific crop and cultivation system leads to the best possible result.
Step 1 in the process is to investigate what can be achieved with the existing instruments.
Step 2 is to determine which improvements to the installation are desirable/necessary for a better result and whether this investment would be profitable.

Measuring is knowing
Naturally, reliable measurements and objective observations are necessary to monitor whether the measures taken lead to the intended result. This includes the following factors:
1. The achieved RNP.
2. The realized temperature of the different layers in the crop (head, bunch, etc.).
3. The realized crop evaporation in the critical crop layer (head crop).
4. The microclimate in the critical crop layer.

The RNP as a quality and control factor for the growth light
The RNP that a lighting installation emits depends on the type of LED fixture, the spectrum and the possible presence of additional IR radiators. It is therefore crucial that when the installation is delivered, not only is the light distribution, light intensity and light spectrum checked, but the value of the “growth quality” and therefore the RNP is also visualised. The RNP is thus an essential part of the specification of a lighting installation. In addition, the RNP is a new instrument for controlling and automating the “growth quality” via the climate computers.

By measuring the RNP of the LED lighting with the right sensors, the grower gains insight into the growth quality of the assimilation lighting. If the RNP is too low, problems can be expected, and additional measures are necessary. This follow-up article explains various options to influence the effective RNP or compensate for an too low RNP. An integral approach and reliable monitoring are required to achieve the best result in each case. It is recommended to read Article (1) in advance.

Would you like to know more about the Plant Empowerment concept, or how to accurately measure PAR and Net radiation inside the greenhouse? You can order the books “Plant Empowerment the basic principles” and “Plant Empowerment Digitaal Growing”. Check the books page. for more information.

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A new quality indicator for lighting: RNP (1)

By Jan O. Voogt , Plant Empowerment Academy, March 2025

RNP stands for Ratio Net radiation to PAR light. Plants need PAR light for photosynthesis. The PAR light sum indicates the availability of assimilates (sugars) in the head of the crop. Net radiation plays a key role as an energy source for evaporation and, thus, for the uptake and supply of nutrients. The RNP indicates which ratio of assimilates and nutrients is available, especially at the head of the crop. The proper ratio is essential for optimal growth. In addition, net radiation also ensures the necessary heating to promote growth.

All the more reason to investigate and determine the minimum ratio between PAR light and heat radiation a specific crop needs for proper growth and development. That is why the Plant Empowerment Academy introduced a new quality indicator for growth light: the RNP: Ratio of Net radiation to PAR light. By monitoring and adjusting the RNP, the RNP can be influenced for optimal growth and development of the crop.

Full LED light has an unnaturally low RNP

With the introduction of LED lamps, an RNP was automatically introduced, which deviates significantly from natural sunlight and HPS lamps. See the table below. LEDs are energy-efficient because they emit a lot of PAR but much less heat radiation. The result, however, is that under LEDs, especially in the head of the crop, evaporation and, therefore, the availability of nutrients, as well as the temperature of the plant, is much lower than under sunlight. This means photosynthesis, assimilate distribution, and processing do not proceed optimally. Furthermore, it has appeared that a higher PAR level does not result in proportionally higher production. Under full LED, all three plant balances (the energy, water, and assimilate balances) shift due to a lower RNP. Therefore, it is high time to monitor and control the RNP.

Controlling LED light in a targeted and energy-efficient manner with the RNP

The net radiation that an LED installation delivers depends on the LED type and the spectrum but is often insufficient for a favourable RNP. The share of natural sunlight in the total light sum positively influences RNP in the greenhouse.
In a follow-up article, we will discuss various options for regulating and controlling the effective RNP in more detail.

How can we measure and calculate the RNP?

With a net radiation meter, we can measure the net radiation at the crop level of each light source. This gives a value in Watts per square meter [W/m2].
We measure the PAR intensity with a PAR sensor, which gives a value in micromoles per square meter per second: [ micromoles/m2.s]
The ratio between these values is found by dividing them:
• Calculation of RNP (instantaneous radiation and PAR intensity)
W/m2 / µmol/m2.s = J/m2.s / µmol/m2.s = J/µmol
In the same way, we find the RNP over a certain period, for example, a day:
• Calculation of RNP (radiation sum and PAR sum)
MJ/m2 / mol/m2 = MJ/mol

The RNP of known light sources

The table below shows the radiation heat of the most known light sources at 200 micromoles/m2.s PAR. (Source: the book Plant Empowerment the basic principles). We can use this table to determine the RNP of these light sources.

RNP direct sunlight = 100/200 = 0.5 J/µmol
RNP diffuse sunlight = 75/200 = 0.375 J/µmol
RNP HPS light = 95/200 = 0.475 J/µmol
RNP LED light = 45/200 = 0.225 J/µmol
We find the same RNP values when the radiation and PAR are added up to a radiation sum and light sum.

HPS lamps have approximately the same RNP as natural sunlight. However, the RNP of an LED installation is more than 60% lower than the RNP of sunlight. Lighting with full LED, therefore, significantly affects the plant’s energy, water, and assimilate balances and the microclimate.

The RNP as a quality and control factor for growth light

The RNP that a lighting system emits depends on the type of LEDs, the spectrum and the presence of additional IR radiators. It is, therefore, essential that when the system is delivered, the light distribution, light intensity and spectrum are checked, and the value of the “growth quality”, i.e. the RNP, is also visualised. This makes the RNP an essential part of the specifications of a lighting system. In addition, the RNP is a new parameter to control and automate the “growth quality” via climate computers.

By measuring the RNP of the LED lighting with the right sensors, the grower gains insight into the growth quality of the assimilation lighting. Problems can be expected if the RNP is too low. In a follow-up article, we will discuss various options to influence the effective RNP.

Would you like to know more about the Plant Empowerment concept, or how to accurately measure PAR and Net radiation inside the greenhouse? Order the books “Plant Empowerment the basic principles” and “Plant Empowerment Digitaal Growing”. Check the books page. for more information.

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Plant Empowerment book officially launched

Canadian Greenhouse Conference October 2018

A diverse group of growers, production managers and associates from all sectors of horticulture gathered on Wednesday, October 3rd at the Canadian Greenhouse Conference in Niagara Falls for the official North American launch of the brand new book on Plant Empowerment. Author Jan Voogt represented fellow authors Peter Geelen and Peter van Weel, and started off the afternoon with an introductory lecture on the principles of Plant Empowerment.

The first book to Truly Green Farms

After his interesting sneak preview, Jan Voogt presented the first signed copy of the book to Hilco Tamminga, owner of Truly Green Farms. Hilco was very pleased to receive the first copy: “It is an honour to receive the first book. We endorse the principles of Plant Empowerment. This theory helps us to grow better without forgetting the 3 Ps; People, Planet and Profit. With Plant Empowerment we can achieve all 3!”

* Extraction from a news article in Horti Daily on October 11th 2018 *

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Plant Empowerment knowledge overhaul

Article #1

What if someone told you that plant physiology and physics could change the way you think about greenhouse climate control?

BY GRETA CHIU

In their book Plant Empowerment: The Basic Principles, climate controls systems specialist Jan Voogt and his colleagues, former cut rose consultant Peter Geelen and former researcher at Wageningen University and Research (WUR) Peter van Weel, have pooled their knowledge and experience to show the interplay between climate conditions in the greenhouse. Filled with detailed information on applied theory and best practices, the underlying principle is the need for balance. Crops balanced in energy, water and assimilates are healthier, less vulnerable to diseases and pests, and more likely to produce higher yields and better quality. If the point of every plant process is to maintain balance, then a grower’s job is to ….

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Empower plants through well-balanced irrigation

Article #2

The goal of every greenhouse irrigation strategy should be to maintain water balance in the crop.

BY JAN VOOGT

What is Plant Empowerment? Typically, traditional plant production methods are based on a mixture of blueprints, best practices, common knowledge of plant physiology, as well as the ‘green fingers’ and ‘emotional perception’ of growers. This approach has been successful but also has several limitations. The concept of ‘Growing by Plant Empowerment’ (GPE) brings experience and knowledge together in an integrated approach. Its starting point is the natural behaviour of plants relative to the greenhouse environment, as described by six balances concerning energy, water, CO2 and assimilates. Monitoring these balances with sensors, combined with crop measurements in a coherent framework based on physical and plant physiological knowledge and insights, provides hard facts required to control and improve the cultivation process. In this first article of a multi-part series, we’ll explore the importance of GPE in a greenhouse’s irrigation strategy.

WELL-BALANCED IRRIGATION

Typically, an irrigation strategy consists of a mixture of different methods, based on clock times (such as start time, stop time, and time intervals), radiation intensity (W/m2) and light sum (J/cm2), the measured percentage of drainage water, water content or weight of the slab, and so forth. The goal of the irrigation strategy, however, should be ….

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Empower plants through well-balanced ventilation

Article #3

Rather than aiming for a preset greenhouse temperature, ventilation should be used to support the plant’s water and CO2 needs.

BY JAN VOOGT

In January’s issue of Greenhouse Canada, readers were introduced to the concept of Growing by Plant Empowerment (GPE). Combining grower experience and knowledge of plant physiology, the goal of GPE is to optimize the behaviour of plants in the greenhouse environment by maintaining critical balances involving energy, water, CO2 and assimilates within the plant. These balances can be monitored by sensors, combined with crop measurements, and then interpreted in the context of plant physiology and physics to help finetune and improve the crop. In the first article of this multi-part series on GPE, the importance of a greenhouse’s irrigation strategy, in relation to the water balance of the plant, was explained (see March/April 2019 issue of Greenhouse Canada). In this second article, we will explore the importance of GPE in a greenhouse’s ventilation strategy, focusing on the relationship between ventilation and plant evaporation, and eventually how a well-balanced ventilation strategy can promote photosynthesis and light use efficiency (LUE).

WELL-BALANCED VENTILATION

Typically, a ventilation strategy consists of a mixture of different methods. First, there may be a minimum window opening needed to avoid a build-up of adverse gases like NOx, ethylene, etc. The release of moisture may be another reason to increase the angle of the window opening, which also depends on weather conditions. Typically, the main driver for large window openings is temperature control. It is often believed that temperature is the key to plant development and that a too-high temperature reading must be avoided at all times. The ultimate goal of the ventilation strategy, however, …

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Empower plants through well-balanced climate control

Article #4

Day or night – how to promote a crop’s self-cooling strategies and improve temperature uniformity under a blackout screen.

BY PETER VAN WEEL

The sun can bring 800 Watts per m2 inside a greenhouse. That is a huge amount of energy, especially considering the total size of greenhouses nowadays. For a 10-ha greenhouse, that equates to 80,000 kW! However, only two per cent is converted by the crop canopy for growth, the other 98 per cent will leave the greenhouse. In that sense, plant production is not very energy efficient. What is true for sunlight is also true for expensive artificial light, however, lamps bring less energy into the greenhouse. For example, to produce 200 μmol/m²·s, high-pressure sodium lamps bring in 114 W/m2 of energy, while LED lamps bring in 74 W/m2. When LED fixtures are water-cooled, this energy is reduced to 48 W/m2. What consequences does this have for the crop and the greenhouse when most of this energy must be released again? This article explains the important role of energy balances in a greenhouse and how we can support the crop and the greenhouse in getting rid of these huge amounts of energy.

WHAT IS ENERGY BALANCE?

Greenhouse production is bound to the rules of physics, and one of the most important rules is that energy never ‘disappears’ but is usually converted into different types of energy. For a greenhouse, energy input and output are always in balance. (For more, refer to the first article of this series in Greenhouse Canada, Mar/Apr 2019). The amount of energy absorbed by the plant is less than the energy entering the greenhouse, as part of the radiation is reflected. The plant can actually control that reflection by turning its leaves or by changing the shine of the leaf. The amount of energy absorbed by the canopy can be measured with a device known as a net radiometer. It measures light energy that …

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Empower plants through the assimilates balance

Article #5

By finding the right balance of sugars created and sugars consumed, you can produce a more healthy and resilient crop while maximizing the plant’s use of available light.

BY PETER GEELEN

This article is part of a series on the concept of Growing by Plant Empowerment (GPE), a global follow-up to a new Dutch growing method called Het Nieuwe Telen (HNT) also known as Next Generation Growing. GPE starts with the natural behaviour of plants, as they are capable of coping with very different circumstances and climate conditions. The plants keep themselves alive by managing three balances: energy, water and assimilates (refer to 2019 Greenhouse Canada Mar/Apr, June, and Oct). In this article, we will focus on balancing the production and consumption of assimilates and how this knowledge can be used to produce higher crop yield and quality.

STARTING WITH PHOTOSYNTHESIS

Photosynthesis is the starting point for optimizing plant growth. In the process of photosynthesis, carbon dioxide (CO2) and water (H2O) are transformed into sugars or assimilates, with the help of energy from sunlight. These sugars can be used as building blocks for plant tissue production and to make new cells, for instance. The biochemical processes responsible for the production of new cells also need energy, which is indirectly supplied by sugars as well. So assimilates have two roles: building material and fuel for plant growth. The plant will always balance its consumption and production of assimilates, as shown in Figure 1. If there is an assimilate shortage, the plant has to cut down on consumption, at the cost of development or quality. Surpluses, however, mean inefficient utilization of available assimilates, which is also undesirable.

SOURCE–SINK RELATIONS

Green plants contain a pigment called chlorophyll. Along with other secondary pigments, chlorophyll absorbs part of the solar spectrum that we call photosynthetically active radiation (PAR). This is light in the wavelength range of 400 to 700 nanometers – essentially the colour spectrum. Leaves that produce more assimilates than they consume are known as “sources” of the plant. All other parts of the plant which consume more assimilates than they produce are called “sinks.” The main sinks are the fruits or flower buds. Young leaves belong in the “sinks” category because they consume more assimilates than they can produce. But mature leaves, …

 

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