Honestly don't remember but my oldest plants have to be at least 7 years old. They are the biggest in regards to width. Easily 5 to 6 foot wide.
It's main purpose is use as a biofuel that will produce much more than switchgrass. Everything I read on the biofuel information gives it a 20 year life. That may just be 20 years of producing top tonnage for biofuel. It will probably continue be useful longer to us habitat guys.
3 applications of gly will kill it. Spray June, July and August and it's done. I didn't say this but I would think 1 application of gly and diesel would get it done.