Answer: Some varieties of bearded iris simply have weaker stems than others and need to be staked. This can be especially true with some of the "show" varieties. Another possiblity, if the floppy ones are in a different part of the garden, is that the weather has been wet and cloudy and the stems have grown taller and weaker than normal as a result. Finally, an excess of nitrogen can cause them to have floppy stems.
I am at a loss as to the others blooming so short without any apparent problem with the plants. There are of course "dwarf" types which bloom as low as six inches, as well as "intermediates" and then the "talls" reaching as high as four and a half feet. Perhaps you are growing some of the dwarfs; their flowers can be quite large!
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