Sheet Mulching

[ducks4you]

Don't indiscriminately use hay or straw. I can tell you that some will sprout seeds. I wouldn't worry too much about bark over cardboard, just use enough to really cover. I get it--you don't want the weeds and you don't want the cardboard to show. If you poison the grass/weeds you might damage the hydrangea.
Just stick to your plan.

 

[Beekissed]

Would I still have to use cardboard?

Mary

I never do. Anyone who has ever left a hay bale, be it square or round, in place for any length of time can testify that the grass truly dies there and the earth worms have already started making the bottom of the bale into soil for you by the time you come back to move it.

Just put down whole flakes, then spread them a tad to interlock them into a nice, thick mat and you shouldn't have any problems. I'd cut your existing grass pretty short beforehand just to make all of that easier.

IMO, unless you live somewhere it is pretty wet and you get plenty of rains, the cardboard will dry out and the mulch material gets displaced or blown off it, either by foot traffic or wind or a hard shower of rain. I've never liked using cardboard or newspaper or any kind of paper for mulch material...it's always problematic where I live. Could be different where you live, Miss Mary.

Plus, the cardboard is hard to cut through to place plants if you want to plant right into it after placing it down. Hay...easy...just move it to one side, move it back. The rain or your watering will pack it into place and the interlocking fibers of hay will keep it in place.