Over the years we have noticed that they die when their growth bud
actually freezes. They survive (poorly) in freezing water.

We have a 4 x 8 veggie filter pond on our berm. We use on-end cinder
blocks as end supports and run a 2 x 6 the length of the filter pond.
The 2 x 6 sticks into the open cells of the cinderblocks and is the
ridgepole of our 'tent'. We put a clear/translucent plastic sheet
over the frame and close the ends. That gives us a closed, tented,
well lit area over the hyacinth. When the outside temp is cold enough
to freeze the pond, the atmosphere in there is slightly warmed by the
32 degree water flow, so we have little or no ice forming under the
tent. Most of the hyacinth survive. Those exposed directly to the
atmosphere, i.e. floating out in the open, freeze and die.

Jim