Some plants have extremely sensitive biological thermometers  For example, the leaves of rhododendrons curl when the temperature of the air around them is below 0 degree C (Celsius)  Similarly, mature crocus blossoms open in temperatures above 2 degree C
So someone who simultaneously observed rhododendrons with uncurled leaves, crocuses with mature but unopened blossoms, and a thermometer showing 1 degree C could determine that the thermometer' s reading was accurate to within plus or minus 1 degree C
The climate and soil conditions that favor the growth of rhododendrons are also favorable to the growth of crocuses.