Thank you Millet for the precise information.
I knew that kumquat need high temperatures, but I didn't know how many degrees.
I got fed up of kumquats, I removed all of my kumquats and I grafted them with other citrus.
They never flowered or sometimes a few scattered flowers which never made into fruit.
My climate is oceanic, mild winters with nearly no frosts and mild summers, we grow decent lemons, oranges and mandarins, but not kumquats due to mild temperatures in summer.
I disagree when people recommends kumquat like an easy citrus. It is easy for growing, but not for fruiting, at least in mild summers climates.