With Nymphaea tetragona there was quite a confusion in naming untill recently. There were some subspecies of Nymphaea tetragona, and then there was Nymphaea pygmaea which was supposed to be a just a synonym. It was known that some of the Nymphaea tetragonas in cultivation grew only in cooler water while other did well in warmer water.  Meanwhile the botanist have cleaned up the mess and today we believe it to be like this:

Nymphaea tetragona got no subspecies at all and occurs around the polar circle all over on the northern hemisphere. It needs cold water to do well and is completely hardy, even if frozen solid in ice.

Nymphaea pygmaea is no synonym att and occurs from India to Japan. It needs remarkably warmer water to do well, and it won`t survive if frozen solid. Almost all plants that were sold as Nymphaea tetragona are in fact Nymphaea pygmaea.

To make things real complicated, there is also a variety by Latour-Marliac that is called Nymphaea `Pygmaea Alba`  - it is no relative of Nymphaea pygmaea or Nymphaea tetragona. Since some nurseries ignor the rules of correct nomenclature this variety is often spelled without capital P and without quotation marks, so it can be mistaken for a species by the reader. 

What we sell here is true Nymphaea pygmaea - the small species for warmer water. Its flowers are slightly larger than those of Nymphaea tetragona, but it is hard to tell without direct comparison. It is blooming from summer into autumn without rest. It is hardy here if the ice won`t reach the rhizome.