Hi Petra,
In my collection (west of London, UK) the earliest Mamms to flower are usually M. weingartiana, M. lasiacantha and its friends like M. magallani, M. egregia, and M. hyaline (wohlschlageri). M. senilis can be quite early as well. Some of the Dolicothele subgenus also flower quite early, M. surculosa for example, but later than M. lasiacantha.
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Chris43, moderator