Hi..

I've never seen them that big, you must live in a warmer
climate.

Mine died in spring: I suppose it was too cold outdoors.

But I've never seen a 18" WH, too.

I had a terrible probelm with String Algae last summer, it
was practically September before I got it under control.
The Algae was sucking all the nutrients out of the water
and the WH were sickly. Once I started adding nutrients
the WH slowly recovered and covered the surface.

Had similar trouble with water lettuce. In may I cleaned and
refilled one summer pond with a mixture of tap water and
rain water. The water lettuce I had troublesome
overwintered at the souterrain window didn't start growing
and were in danger to die while the hair algae multiplied.
But some flower fertiliser saved that critical situation..
--
cu
Marco