Ask Mr. Smarty Plants

From our website on Nassella tenuissima (Mexican feathergrass):

"It is native in North America only to mountains in west Texas and adjacent New Mexico south to central Mexico, but it has become widely used throughout hospitable areas of the US and elsewhere."

We could find not specific lengths of the roots of this native plant, but generally speaking, grasses have long fibrous roots, the length of which can range from 3 ft. to 40 ft. As noted in some literature we have seen, Mexican Feathergrass can even become invasive in an area that suits it very well. But, it will never be as invasive and obnoxious as the non-native bermudagrass, which is considered one of the most invasive weeds in the South. Here is a similar previous Mr. Smarty Plants question to yours which also deals with the problem of a neighbor's bermudagrass.

On the use of the Nassella tenuissima (Mexican feathergrass) as a barrier, it's not a bad idea. It's also pretty aggressive, it's tough and attractive, low water use, etc. We still don't think it will totally block out the neighbor's grass, because the bermudagrass has two weapons in its armory: above-ground stolons and below-ground stolon. Even if you block the grass from the yard next door, there are already below-ground stolons waiting to pop up on your side of the Mexican feathergrass wall. One suggestion that we have that could help is that bermudagrass does not do well in shade; if you can find a way to shade out the invading grass, including with the taller feathergrass, you could make some progress. See this previous Mr. Smarty Plants answer on using solarization to deal with bermudagrass.