Best time for spring Badlands visit?

Years ago I visited Badlands NP and by coincidence hit the peak of the
spring grasses that grow there. But I forget exactly when I was there
(I am thinking late May), and I know those grasses don't last long. I
want to return there next spring and see it all again.
Can someone familiar with that part of South Dakota estimate for me
when the peak of the verdure occurs? Maybe a two-week window, for
example? Thanks in advance, John Fetkovich