Your water hyacinth's are absolutely beautiful! You certainly showed a lot of ingenuity in the way you are maintaining them.
I had this beautiful plant in my pond for a number of years. After getting tired of the constant over population we removed the last one. In that regard, I hate to be a kill joy but I should mention that due to their rampant regeneration "Water Hyacinths", at least the natural variety, are illegal in some states. Please let me know if your WH is a domesticated strain. Our hyacinths were from the bayou's of Southwest Louisiana.
Another way to keep them over winter is to remove a group from your pond and place them in a large plastic container filled with pond water. It worked for us for several seasons.