Can I write a note directly from a planting?

Yes, and it is already linked, so the note shows up where you will look for it. Open the planting from the Game Plan or the Crop Map, go to its Notes tab and click Create note.

From a crop in the Crop Library, the note is linked to the crop instead, and comes back on every planting of it.

See Link a note to a planting, crop or block.