[Talisker]

And for the record, before someone posts it, Yamamoto never said invading America was impossible because there would be a rifle behind every blade of grass.

Why because Wikipedia says so? There's a bastion of valid American research for you. In fact, 5 seconds on Google found at least this, though the author is obviously loathe to admit it. Nevertheless, it appeared he did indeed say it in an interview, and therefore probably off the record as well.

There is some poorly documented evidence that Yamamoto, in an alleged interview with a small New England newspaper, when asked if the Japanese intended to invade the US responded with the "blade of grass" quote.

[Hugin]

Why because Wikipedia says so?

Because there's no evidence or documentation that he said it. No date. No name of the supposed paper he said it to. Moreover he was at Harvard between 1919-21, and served as an embassy attache from 1923-36. It's unlikely that he would have been interviewed as either a student or an attache, and doubtful if anyone in that period would have asked about Japan invading America.

It's impossible to prove that anyone didn't say anything. But unless someone can actually cite a source, including where he said it, and the date, it's not a credible quote.