Like the other species of the former genus Aconogonon (all now classified in Koenigia), the hybrid is a perennial, growing from short rhizomes.

It differs from Koenigia weyrichii in having less persistent ochreas (sheaths surrounding the stems), less pointed leaves with fewer hairs on the underside, pedicels that are jointed in the upper part, and larger tepals.

The genus Koenigia is placed in the tribe Persicarieae, whose taxonomic history has been described as "exceptionally convoluted, even by Polygonaceae standards".

[8] The generic boundaries within the tribe have varied considerably; Koenigia × fennica has also been placed in Persicaria and Polygonum.

[17][18][19] Neither P. polymorpha nor Aconogonon speciosum exist as valid taxonomic names, these are horticultural inventions,[20][21] whereas Polygonum polymorphum is a synonym of the related Koenigia alpina.

[23] The cultivar 'Johanniswolke' is supposed to be somewhat smaller than the nominate form, although it grows just as tall as the plants circulating under the name P. polymorpha.

It is furthermore said to be sterile, flowers which become dark pink after anthesis, and to have non-invasive rhizomes which stay short and compact.

[16] P. polymorpha is said to sometimes send up a shoot less than a meter from the main clump after many years, have a rhizome which is able to grow 50 cm downward to escape a barrier, and to rarely produce black berries.