Frost again

Sometimes after a frosty night a spike of ice appears like a shark's fin above the water in the windowbox pond. At other times the water just freezes flat in the normal way. I have no idea why this is, but wonder if it is to do with the speed at which the water freezes.

The long, chilly winter is holding things back. The only signs of life are the two tiny seedlings, both of which have minute green central blobs as true leaves start to form between the cotyledons. Constant freezing and thawing does not affect them at all.