Common Weeds 
Agronomy 105's Weed ID
Common smartweed
Common smartweed is an annual plant found growing in low areas and areas that are frequently flooded.  It is a serious problem in these low areas throughout all of Indiana and the neighboring states.  Pennsylvania smartweed is very similar to common smartweed and will be found in the same locations.
Smartweed will have pinkish flowers located at the ends of the stems.  The stems will be smooth and commonly are somewhat reddish in color.
The stems of a mature smartweed may be as long as 4 or 5 feet, however these stems are very weak and generally trail along the soil surface.
The nodes, where the leaves attach to the stem, are swollen and this will help identification.  The leaves are shaped similar to those of a peach tree. 
The purplish-black to black colored seeds are round and flattened with a projection on one side.