You mean on turf, like lawn? I think the seed needs some direct contact with soil, it depends on how thick the grass is, how much rain there is (thin grass and a lot of rain could result in the buckwheat getting down to soil.
I've had good success on fairly bare soil, I've never tried overseeding with it. Maybe if you rake strongly with a dirt rake. . .
You would need very discouraged and traumatized grass for this.
Here is a recipe to discourage grass greatly:
You can cut 2" thick strips with a sod cutter, flip it over, broadcast buckwheat and daikon (and optionally step on them to work the seed in a little better).
To discourage more, remover the sod cutter strips completely and pile them in potato growing mounds, planting the buckwheat in the newly opened ground.
As for leaving the local paper for the grass to read. I have left different sections of my local paper on the grass for the grass to read at its leisure. I find the political section kills the grass the fastest
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