Irises and Mondo Grass with little to no soil

I'm building a new garden area. It WAS full of weeds, so I sprayed it with Round Up in February to wipe it all out. This week I covered all of the dead weeds with cardboard (brown corrugated shipping boxes, no wax or tape), then covered that with mulch.

I'm mostly filling it with container plants this year, then hope to plant things permanently next year.

But I'm thinking about my irises. I'm not 100% sure if they're bearded irises, but they're planted with practically no digging at all! I usually just sit them on the ground and then cover the bulb with mulch.

I have literally hundreds of these, and the quantity doubles every year! I give them away by the bucket full, but I still have so many left over that I end up throwing them in the woods. And I swear, most of those come up, too! LOL

Since I have so many, and they seem to be virtually indestructible, do you think that I could plant these on top of the cardboard and just cover with mulch? With little to no actual soil?

Or would I need to cut out a little hole in the cardboard so that the roots actually touch the ground?

Same goes for my mondo grass. I usually plant it in bunches along walkways, and I could use it to emphasize the edge here. Could it be planted between cardboard and mulch, or do I need to cut a hole for it?

I'm asking because, realistically, if I spread out irises and mondo grass and need to cut holes then it'll be about 60 holes :-O If I can just cover them in mulch then I can have it all done in a few hours; if I'm cutting holes then it'll take at least a week.

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