This graph is minimum temperature of January of Hirakata by the Meteorological Agency data.
There seem to have been many days when a temperature became 0 degrees or less.
The place where the eucalyptus is put lowers from 1 to 2 degrees more than the temperature of the Meteorological Agency.
On the day of a blue line, the plant pot is sure to have frozen.
For your information, the 31st Jan. was -6 degrees.