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Who still believes in the pre-night drop and morning dip?

Introduced about two and a half decades ago, the idea of using a sharp temperature drop to encourage fruit production is still a source of contention.

As a teacher of the “HNT (Het Nieuwe Telen) Learning Groups”, an educational program where Dutch growers are taught the principles of Next Generation Growing / Plant Empowerment, I am frequently asked whether I am in favour of the pre-night drop (PND) and morning dip (MD).

As I recall, the idea of the pre-night drop in tomato cultivation popped up in The Netherlands about 25 years ago. At the end of the afternoon, the temperature in the greenhouse had to be lowered considerably in one rapid motion – the faster the drop in temperature, the better. It was believed that this pushes the assimilates towards warmer fruits, so they become bigger. Besides, it was assumed to promote strong plant heads and trusses.

FIGURE 1 - This graph shows an example of a typical pre-night drop strategy. Temperatures are decreased swiftly in the afternoon and after sunset (black arrows). (Red = greenhouse temperature; Blue = RH % (left hand axis, value x 10); Brown = outdoor temperature; Yellow = solar radiation W/m2) 

However, there have been questions from the beginning as to whether this hypothesis is correct. I still remember how one renowned crop adviser argued, that after a sunny day, you should allow the plant to process the assimilates produced during the day; however, the pre-night drop would make this more difficult. This adviser published yield figures of growers who used the pre-night drop and of growers who didn’t apply it. Although there was no noticeable effect of PND, it did not help to support his case. It was like crying in a desert.

More recently, the idea of the so-called morning dip arose, which is very commonly applied as well. After a night under a closed or non-closed energy screen, you should theoretically let the greenhouse temperature make a short dip. This is supposed to promote strong, short trusses. Both the PND and the MD are widely considered to be generative actions and form the basic tools of any serious tomato grower and crop consultant. There have been no discussions about this for a long period of time.

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