You can do what you like, sunshine, but if you want your new tree or hedge to establish and thrive, you got to keep the grass away from its root zone.
Kill, or “strip and flip” the turf, and maintain an edge to protect your investment.

Videos that show planting bareroot whips through grass are misleading to some, because people think that the grass will remain as the tree or hedge grows.
Whether or not the grass in fact remains, it shouldn’t as far as the new tree or hedge is concerned.

Three point five Slit Planting Techniques

Liz Zorab shows the cross slit technique, which takes 3 spade movements and opens a bit more soil:

Linda Gilsenan shows the T-shape slit technique (it could also be an L-shape), which takes 2 spade movements and still opens plenty of soil:

Teignbridge District Council shows the same single slit technique that we promote in our creaking old country hedge planting film, which we honestly will update this quarter century, promise.

Austin from The View From The Clouds shows a double dip variant on the “single” slit method, which gives you a bit more space for the roots and closes it all up smoothly with minimum footwork when you firm the ground down at the end.

Those fine videos make it seem as though the new trees and hedges are going to grow up cheerfully alongside the grass, like best buds.

But that, dear reader, ain’t gonna happen. Grass will kill your new plants if irrgiation is poor, and if irrigation is adequate it will still throttle your plants’ growth.

A tree or hedge plant trying to grow new roots into soil already dominated by grass will grow much slower than one surrounded by open soil that’s covered in mulch to maintain moisture and therefore soil fertility.

So how do people slit plant into grass, if grass is so bad for establisment?

Weedkiller.
Spray the planting area with a systemic herbicide like glyphosate based RoundUp, and wait a short while, according to the instructions.
When you plant your whips into it, the grass will still look hale and hearty, but really it’s dying.

Alternatively, you could lay down carpets of mulch to smother the grass.

• Start with a layer of fibrous mulch e.g. cardboard, newspaper, wool, any natural fabric to pin down the grass.
• Cover it with normal mulchy-mulch e.g. compost, grass clippings, old straw, mowed-over leaves & twigs.
• The fibrous mulch should hold the grass down long enough to die, rather than grow up through the mulchy-mulch.

Mulching this way is practical at a reasonably small scale in the garden; not so much for doing miles of farm hedge on the clock. With that being said, farmers often use cardboard mulch instead of polypropylene fabric