BTW: The DHT11 is a very inaccurate sensor. I would use a better sensor for controlling a green house as and error of 10%RH is a bit to much for such a controller.
You should run the DS3231 on 5V if the I2C bus is run on 5V (as it seems to be in your wiring diagram, internal pull-ups). Providing higher voltages on the I2C bus than the Vcc level may damage the chip.
In your wiring there's no pump, lights and the like as you have in your code. Is that code part just fake?
I still have the impression that you might run out of memory as the serial prints are using to much RAM and you're using the RAM wasting String object (although this is definitely not necessary). I'd guess that using the F() macro might at least make the setup resetting less often. It's worth a try.