This is a weed that originally was sold as an ornamental plant for flower gardens.  Commelina communis is also known as Asiatic Dayflower.  Identifying characteristics are the leaves clasping the main stem with a papery sheath.  ( It gives the plant a look similar to Japanese Knotweed or Bamboo)  It is as difficult to get rid of as the Bamboo though.  When you go after this weed you need to be persistent and knock it down when it tries to spring back on you.    When you cut the plant off at the base pour a little Glyphosate (Roundup) into the cut stem.  This is neater than trying to smother the plants with black plastic which will kill the surrounding grasses.

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