The first long term measurement I made, to test the behaviour of the sensor over a longer time range was a failure. After the five days with the device introduced in this post, the readings made absolute no sense.
The sensor was not moved in the flower pot and the plant was once watered at the begin of the measurement. While it looked promising at the begin, the frequency suddenly went down again, which was very irritating. I am still investigating how this could happen.
To get closer to the real measurement of the final plant watering sensor, I started a new approach.
I soldered a header to one of the LED pads on a fully assembled plant sensor. Next I changed the device for the measurements.