very interesting idea, i have no clue as to what would happen but id like to find out. you might run into problems with there not being enough plant above ground. the size of the plant is in step with the size of the rootmass, too much of either one and the plant dies.

(roundup works by making the roots grow super fast and the rest of the plant can't keep up so it dies)

if you can get past that issue, i'd think you could get some faster growth...anyone else?