I've been asked to identify another plant, photographed at the Fullerton Arboretum in California.
It has a trimerous laciniate epicalyx, 5 narrow sepals with a stellate indumentum, 5 red petals spirally arranged in a cup-shaped corolla, pentadelphous stamens placed antepetalously, and a gynoecium somewhat obscured by the androecium. The foliage is hairy, and bronzed when young. Obviously not a mallow except in the broadest sense.
Once I had convinced myself it was a dombeyoid I cheated and looked in the collections database on the Fullerton Arboretum website, comparing the 5 species of Dombeya held (it clearly wasn't either of the other two dombeyoids - Pentapetes phoenicea and Trochetiopsis ebenus - held by the institution). It turns out to be Dombeya macrantha, known by the Germans as Madagaskar Linde.