From what I can find, the Marigolds would have to be planted as a cover crop to do much good against nematodes, and planted densely to prevent weeds. So it does not sound practical as a method to be used in the garden, except in places where they could be grown as a cover crop when not growing other things.
I have tried planting regular Marigolds for insect prevention, just spaced out among my tomatoes etc. I could never tell they did any good, however, I still plant them among my vegetables and other plants, just because I like Marigolds. Not that familiar with the Mexican Marigolds, I did see it was listed as medicinal weed.
