I am sorry that you have inherited this issue, it is a good example of why I advise folks against planting bamboo, especially the runnering types. This is a large established plot of mature bamboo, and there will be no easy solution. It will take repeated treatments over a few years, both chemical and non-chemical, to remove it.

For non-chemical options, cutting down and mowing can control bamboo over time. If you chop down the culms (or canes) and then continually mow back any new shoots, then eventually the rhizomes will be depleted of nutrients and will stop putting up new shoots. But it takes regular and frequent mowing, and 2.5 acres is a large area to cover, plus you need the right equipment to chop down the culms (ex. chainsaw, string trimmer with metal circular blade, etc.). There are root barriers that can be put down, but again, this is such a large plot of land that it would be difficult and costly to surround the perimeter with a root guard. However, your neighbor might want to consider a root barrier on their side of the property.

With regards to herbicides, glyphosate (ex. Roundup and other brands) is one of the best options, but it is more effective against the young shoots, not tall established mature stands of bamboo. Imazapyr (ex. Arsenal and other brands) is another herbicide that is effective against young bamboo shoots. You shouldn’t attempt to apply any pesticides to the stand until you chop down all the culms. Both herbicides are more effective against young shoots, and itt is not safe to apply herbicides to such large stands (it would require a boom or air blast sprayer to get it up high enough into the bamboo canopy, and would expose you as the applicator. Plus, it would require a lot of herbicide to spray a mature stand of this size). You will need to chop down and/or mow the bamboo before applying an herbicide. Once you have chopped down the culms/canes, allow it to send up new shoots, and then apply the herbicide once the new shoots are about 3 feet tall and new leaves are fully expanded. It will take repeated applications over a few years to bring it under control. Glyphosate and Imazapyr will kill other woody plants and grasses, so be careful spraying it around trees and shrubs you wish to keep.

As noted, this is a large plot of established mature bamboo that will require specialized equipment and repeated treatments to bring it under control. You might want to consider hiring a local landscaping company to do some of the work for you. I know a grower that worked with Glass Tree Care and Spray Service to bring some invasive weeds under control and was happy with the service. You can reach them at<personal data hidden> (contact Dustin Boyer).

For more information on bamboo removal, both chemical and non-chemical, refer to: https://extension.umd.edu/resource/containing-and-removing-bamboo
and https://hgic.clemson.edu/factsheet/bamboo-control/.