In 1979, botany professor Avinoam Danin from Israel distinguished and described six different species from the Portulaca oleracea aggregate there, based on their seeds. The seed have different surface structures, allowing species to be distinguished by these. Five, partly different, species have also been found in Germany. In Portulaca oleracea s.str., the surface structure consists of smooth, small, flat stars with relatively long and pointed arms that interlock seamlessly; this pattern is sometimes called 'jigsaw puzzle'-like. (Other species additionally have relatively large raised tubercles on the flat central part of the stars and/or smaller raised papillae on the arms.) For a long time it was indeterminate which of the many microspecies worldwide corresponds to Linnaeus's species Portulaca oleracea. It was only in 2023 that it was determined that this is Danin's Portulaca stellata, thus making this name a synonym.
In 2013, F. W. Bomble from Aachen presented Funde von Portulaca granulatostellulata, P. nitida und P. papillatostellulata in Nordrhein-Westfalen, ('Findings of ... in North Rhine-Westphalia'). However, he overlooked the fact that two of these species have been reported by Danin to be tetraploid, while no tetraploid Portulaca had been found in Central Europe at least until 2023, only approximately hexaploid species. Therefore, P. granulatostellulata and P. nitida as identified by Bomble, should not be present in Germany. Furthermore, Danin's original material was subsequently also diagnosed as approximately hexaploid, so there are now serious doubts whether these two species even exist at all.
The authoritative article on Portulaca species occurring in Germany from 2023 onwards is: Morphologisch unterscheidbare Sippen von Portulaca oleracea s.l. - bestimmt anhand von Lackabdrücken der Samen (morphologically distinguishable taxa of Portulaca oleracea s.l. - determined by lacquer impressions of the seeds) by Hans Reichert from Saarbrücken. Impressions of the seeds in nail polish and their microscopic examination under transmitted light are not strictly necessary; species identification can also be achieved by reflected light microscopy of the seeds (although my device belongs to the very lowest category and is rather a toy). Even more up-to-date is the Portulaca chapter in 'Flora Germanica, Volume 3 Kritische Gattungen' (Critical Genera) pages 754-752 (2024). The internet site Flora Germanica shows the current species inventary, too. The diameter of the seeds of the wild species occurring in Germany varies between approximately 0.6 and 1 mm and is not suitable for species identification. The two other species, which I have already seen, are the frequent Portulaca papillatostellulata and the rare Portulaca trituberculata.