MILE-A-MINUTE WEED/ASIATIC TEARTHUMB (Persicaria perfoliata)
Three things to look for: leaf shape, round sheaths, bright blue berries
(also has recurved thorns, barbs, on the stems - hence the name "Tearthumb")
There are several species of Tearthumb, but only P. perfoliata has the bright blue berries.
Grows six inches per day, more with water.
Alien invasive - write-up at:
http://www.nps.gov/plants/alien/fact/pepe1.htm