The focus of energy savings usually lies in technology. But the crop itself offers numerous opportunities to economise on energy consumption. Many research results are still waiting to be translated into practice.
Significant differences in energy consumption per square metre or per kilo of product regularly exist between nurseries with similar greenhouses and the same crop. These can be attributed to different views on production among the growers. One grower likes to play it safe, the other looks more into the possibilities of the plant. Within the scope of The Next Generation Growing other ways to deal with the characteristics of the crop is gaining more attention. Previous research results form the basis for this.
Transpiration is the driving force behind essential processes such as mineral uptake, transport in the plant, maintaining cell tension and development of fruits. But the plant exaggerates: In the greenhouse it often transpires much too much. This ‘luxury’ transpiration brings an excess of moisture into the air, which then needs to be removed. And draining off moisture always costs energy. It would be very advantageous to be able to slow down transpiration. Various studies have shown that it can be reduced by 30 to 35% in tomatoes without any cost to production.
Less minimum heat
Yet in practise, growers are still reluctant to slow down the transpiration. They want an ‘active crop’ and are afraid that root growth will lag behind. But ‘active’ means that the crop fully assimilates; this can also be done with less transpiration. And you can stimulate root growth better with a lower greenhouse temperature.
Over the last few years commercial growers have been assessing much more critically the use of the minimum heating pipe to reduce air humidity. Actually, raising the temperature encourages the crop to transpire even more. And then the windows have to be opened to lower the air humidity.
One step further than economical use of the minimum pipe is dehumidification with external air in combination with more screening. Then you control the air humidity independently of the window position. But this of course requires extra technology.
A very effective means to drastically reduce crop transpiration is to pick leaves on a major scale. With a leaf area index (LAI = m2 leaf surface area per m2 ground) of 3 to 4 you already have sufficient light interception. Any number above that means you have a superfluous amount of leaf in the greenhouse.
It’s normal to pick the leaves of tomato plants but it would also be a good idea for sweet peppers too. The lower leaves only transpire and don’t contribute any more to photosynthesis. Picking could also be an option for different ornamental crops. Of course you need to consider whether the extra labour outweighs the energy savings.
In practise there are many fixed views about the necessary temperature gradient during the day. Tomato production is definitely a crop that is very dependent on the temperature strategy. But some of these opinions lead to very high energy consumption. If you heat before sunrise, when the outside temperature is at its lowest point, it costs a lot of gas. If you want to achieve a sharp drop in temperature at the end of the day, and therefore open the windows, all the heat that you’ve just put in is simply lost.
Retain the heat
The question therefore is whether the temperature gradient during the day needs to be so precise. To find out, a study compared three regimes: Heat up quickly in the morning and cool down quickly in the evening; heat up and cool down slowly; and a middle road in which the house was heated slowly and cooled down quickly. The researchers followed the crop for an entire season, critically observed by a growers group. What happened? Looking at the crop you couldn’t tell which treatment had taken place and yield hardly differed. However, the steady strategy did save energy.
For the growers group it was a question of ‘seeing is believing’. They applied the regime to their own nurseries. Seen from a plant perspective the results were not surprising: The plant responds sooner to the mean 24-hour temperature than to a specific gradient during the day. So as a grower of fruit vegetables you can easily heat the house adapted to the amount of light and keep the heat in at the end of the day. You achieve the same 24-hour temperature with less energy.
Another point is that at the end of the day leaves and fruit cool off at different speeds. The leaf temperature follows the greenhouse temperature; the fruit temperature lingers behind. The effect of this could be that the fruit attracts more assimilates. The differences are so small, that it’s hardly noticeable. Research has shown no differences in fruit weight between the different cooling strategies.
In pot plants, where the shape of the total crop is important, phenomena like DIF (the difference between day and night temperatures) and DROP (a sudden drop in temperature) can indeed affect the elongation or the compactness. Then it’s worth having a temperature regime during the day.
Light and lighting
If you look at light from an energy point of view, you arrive at two questions: How do you best utilise the natural light and when does it pay to use assimilation lighting? The answer to the first question was always: Ensure that the greenhouse has the highest light transmittance possible. Based on the research over recent years we can now add: Diffuse light almost always pays off. This light penetrates much deeper into the crop, the horizontal distribution of light is more uniform and both result in more assimilation.
The answer to the second question requires some more explanation. With respect to temperature, the plant responds to the average over the day, or over a few days. The latter forms the basis for temperature integration. With light however, there is an immediate response. At the same time, there are reasons why the plant, despite a lot of light, assimilates very little, for example, because the stomata are closed for one reason or another. It is therefore very useful to know the reason why. Then you know when the assimilation lights have an effect.
A grower can already determine the photosynthetic activity himself with instruments such as the Plantivity, but these measure just a very small piece of leaf. New methods are being developed that measure the photosynthesis (actually the fluorescence) of a square metre of leaf surface area.
A better understanding of photosynthesis can save energy because then the grower can adjust the lighting and CO2-dosing according to the activity of the crop.
A different growing strategy is a potential key to saving energy. An important part of this is to slow down transpiration. Furthermore, the precise temperature gradient over the course of the day is often not that important. The plant responds more to the average for the day (or several days). This response also offers a basis for saving energy. Finally, better utilisation of natural and assimilation light is possible.
Text and images: Ep Heuvelink (Wageningen University), Anja Dieleman (Wageningen UR Greenhouse Horticulture) and Tijs Kierkels.