Little Bluestem is a wonderful grass. However, given good conditions it will flop! This grass actually prefers soil that is on the dry side. So, the sprinklers may be contributing to the problem. The other thing that it likes is, soil that is low on nutrients. So, your new landscape with lovely rich soil is most likely the main cause of the flopping.
As for blooming ~ probably the reason your grass is flopping now is because it is setting up to bloom. Did you notice it getting taller in the last few weeks? My bluestems in zone 5 are all at this stage now. No bloom yet but getting ready. AND, the only clump growing on 'good garden soil' is already flopping.
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