When is it sure to put the plants in the biological swimming pool ?

The robust plants (read water, lotus, floating heart, etc...) can usually survive the winter at the bottom of the natural swimming pool. Tropical plants such as the water hyacinth and water lettuce can be placed in the natural swimming pool once that the threat of freezing passed. These plants prefer the temperatures above 0ºC. Tropical lilies should not be placed in water until the temperature is not cnstamment above 20ºC.