I am going to use Roundup to spray my invasive Japanese Silver grass (eulalia ???). It has already grown to large clumps some 4 feet tall. It's a real nuisance!!

I read somewhere that I should wait till it's about to flower for the best results.

I'm afraid I'll have too much of it over a large area if I wait till then as it flowers in August or Sept here in Korea. Should I go ahead now?

2nd (related) question. Should I cut down the leaves with my bushwhacker before spraying with Roundup? Or is it better to spray the complete undisturbed plant?

Thanks for your advice!