Hal wrote:

Water hyacinth can tolerate a frost on the leaves, but they will look
UUgly until they grow back out (new leaves) after the water warms up.
They grow better in water over 70*F.

I am keeping my water hyacinth in tubs in a greenhouse. The air
temperature gets down to 40F and the water temperature gets down to 50F.
They are doing fine but basically just sitting there. I expect to see
growth now that the water temperature and minimum air temperature is
rising.

In my spring fed pond they prosper in water that never gets above 60F.
I won't put them back into the pond until the date for planting tomatoes
which is May 15th here, and then I will check the weather report first
to make sure that there is not chance of frost.
--
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18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA