I barely make low 80's here on the surface.. in a long hot spell we might see 85 but we have 58-64 deg nights so it just doesn't heat up and stay there very long. Last week we had 2 nights in a row below 50..47 and 49 I believe.
I bet that is what is holding them back. I put the plants in my pond as the surface temps got above 70 and they really did not show strong growth until the mid 80's. Now that it's upwards of 90F, they are exploding.
According to Wiki...
"The temperature tolerance of the water hyacinth is the following; its minimum growth temperature is 12 °C (54 °F); its optimum growth temperature is 25–30 °C (77–86 °F); its maximum growth temperature is 33–35 °C (91–95 °F), and its pH tolerance is estimated at 5.0–7.5."
This seems to be matching my observations. I do envy your nighttime lows, we have been seeing low 70's at night and muggy humidity...Stymie first thing in the morning, then the sun comes out!
