Water Hyssop is a low growing herb.  They love water or moist places, and grow in a thick blanket that covers the ground.  The entire plant is of a lighter green color, and appear to be very tender.  They are considered weeds in many places becuase they can spread very fast.  They are perennial and will grow back years after years.  This herb taste a little bitter and doesn't really have much of a scent when crushed.  They are very popular in Asian cuisines.