My policy on amending heavy clay is to always "work it in" for the first few years, to establish deep aeration as soon as possible. Grit, sand, compost and fine bark get turned under as much as 18" deep, but at least 12", for the first few years.
Many smarter people are patient enough to just sheet compost on top of the soil, or add lots of top-dress mulch that breaks down quickly, and wait a few years for worms and frosts and decomposition and "perking" to drag the organic matter down to where it is needed most. I'm not patient.
Even just adding mulch every year will EVENTUALLY soften the underlying clay as it slowly decomposes and drips organic matter and "humic acids" down through the clay. Or so they tell me.
>> I have Pennisetum growing in that one bed, that's why I'm aking.
I'm sorry, I know nothing about that. Hopefully someone will chime in. I don't know what forum would specialize in grasses!
"Fountain Grass"?
"Pearl Millet"?
P.S. I experimented with using Daikon radishes to penetrate and break up clay soil. I think I sowed at the wrong time of year, into amended soil, and they made almost no roots, just lots of pods and seeds.
Some of those scattered onto totally unimproved clay, and seemed to like it better there. A few volunteered the next year. But either they could NOT push all the way down into the clay, or these were trying to escape!
