High temperatures cause screen dilemma

The peppers of the Maranello variety grow well in the 2SaveEnergieKas greenhouse. According to the growers, the crop is doing fine. A good, not too high plant load, large fruits and sufficient growth in the top. The setting could be a bit more. So for the supervising group of growers and researchers, there are few points to worry about. You would think so, but you always have to stay sharp, as Arie de Gelder from Wageningen University and Research writes. A sunny day and high night temperature require attention for the screen and ventilation control and causes discussion and dilemmas.

On a sunny day we effectively close the energy screen for 87% to prevent burning of fruit. That works fine because we do not see burnt fruit and the screen ensures that the moisture in the greenhouse remains at an acceptable level. The plants keep evaporating with well-opened stomata and can therefore absorb CO₂. But what happens when the radiation drops and the screen opens? Then the moisture easily exits the greenhouse and it can happen that the VPD plant rises, the stomata closes and that you just lose a bit of growth. Then the windows have to be closed further to limit the moisture loss. The transition from a screen to reduce radiation and prevent burning of fruit to no more screening against burning therefore requires extra attention.

Another point of focus of screening against fruit burning is that the fruit temperature on average remains lower. After all, the fruit receives less (direct) radiation and our diffuse greenhouse roof certainly plays a role in this. As a result, the growth time of the fruit can increase slightly. In doing so, we have to keep some reservation because there is not much to be found in the literature on this topic. Or as expressed in the supervisory committee with growers (BCO): the ripening is slow, which is not favorable for setting. There we have a dilemma. We do not want the fruit to burn, that is why we are screening, but this is unfavorable for the ripening. In this case we choose for screening and accept a little more growing time. Good point to measure the fruit temperature during screening, but what is good or bad here?

If the radiation total is lagging a bit due to clouds, the 24-hour temperature will soon be 1 to 2°C too high in relation to the sum of light. The warm nights at the end of May do not really cause cooling. What should be done then with the screens? Are you going to screen against radiation? We opt for screening with 70%, even though the radiation of the crop to the roof is not high, because of the double roof. Only when the greenhouse temperature approaches the heating temperature by a few tenths, above that the energy screen will be closed down completely. Up till now, we have already had 2°C above the heating temperature, but now we've brought that limit closer together.

A final discussion point is the CO₂ concentration or dosage. We dose 100 kg/(ha.hour) and in the test we often measure a CO₂ concentration in the middle of the day that is close to the outside value. That means that we have little ventilation loss and that the crop has to absorb almost everything. Growth should be better if the concentration is higher (about 600 ppm), but the CO₂ loss is also greater in that case. If we start to dose more pure CO₂ (this means purchase), then a large part of this will be lost immediately. A point of discussion here is whether it is more important to look at the amount of dosed CO₂ or at the concentration. The first is supported by practice, while the second is the proposition from research. A nice dilemma.

A warm month of May provides sufficient food for thought about the best cultivation strategy for peppers. In the 'Pieken zonder Pieken' (Peaks without Peaks) project, it continues to peak in all respects, except in gas use, because the pipe rail has not been used for heat for a week.