IP65 Capacitive Soil Moisture Sensor for ESP32

I am developing a solution which works far from any infrastructure in my garden. For powering this sensor I am using a LiPo and a solar panel. The board is a Firebeetle ESP32 C6 from DFRobot.

To save as much energy as possible I am using deep sleep mode and in addition I power off the sensor.. In opposite to my testing I see that the soil moisture values are… not so serious.. Therefore the question: how long before measuring I have to power on the sensor?

Second question: It seems that the 3,3 Volt from my battery is too less for the sensor? Could that be?
Third question: There are two black cables on this sensor.. Is it really obligatory to connect both black with ground? 

Many thanks in advance
Karl, Vienna