There are a number of different species of Polygonum known as smartweeds or knotweeds (because their stems have swollen nodes). Most of the ones growing around here are considered weeds even when they are native, like Polygonum pennsylvanicum.  Even though they are somewhat weedy, we like to see them on the trail because bees like the flowers and because the small seeds (fruits actually) are important food for birds, mice and other animals.  If you have ever eaten anything made with buckwheat flour, or kasha, you have also eaten the fruits of a member of the smartweed family.