This volunteered in my tomato bed last March. It grew rapidly, and is now almost 7ft tall. It has several upward-reaching branches, with oval green leaves along each branch. In August the end of each main branch was covered in tight purple flowers (visible on upper branches), now turned to grape-sized purple fruits (visible on the central lower branch). (The red spheres at lower left are tomatoes...)
It looks like some variety of Polygonum (Smartweed). You would have to see which ones grow in your area.
I've groped through pictures of many varieties of polygonum, and also of red sorghum, without finding a good match for my plant. Further suggestions (or leads to a matching picture) will be welcome! Here's a closer view of the fruits.
possible ID I believe the plant is Phytolacca americana (Pokeweed). it might be the other specie or variety (there is currently debate in the literature) known as Phytolacca rigida or Phytolacca americana var. rigida http://plants.usda.gov/java/profile?symbol=PHAM4 When the plant was younger, did the raceme every have white flowers? I think the 'purple' flowers you are describing are the purple sepals. ~Meg
Meg mafief-- After studying as many pictures as I have been able to find, I think you are correct: phytolacca americana or maybe phytolacca octandra. I did not notice white flowers as precursors of the very purple later sepals-- but maybe I wasn't paying attention at the right time. Most of the pictures I've found show (within the raceme) individual berries each on a stalk that seems 1 or 2 cm long, whereas my sample has the fruit closely hugging the stem (on individual stalks of no more than 1 or 2 mm). Many of the pictures showed the stem bent over so that the fruited tip is hanging down, whereas my sample has the fruit at the end of stems that are very definitely pointing to the sky. Despite these differences, your identification seems to me probably correct. After reading about the toxicology of pytholacca, I'm glad I didn't eat more than one of them!
