What is this plant. It seems to be invasive. It has small purple flowers at...
This looks like a common weed called Perilla, which is in the mint family, which can have invasive tendencies. Check and see if it has square stems, as all mints do.
Try to keep it from going to seed by removing and trashing the flower spikes.
If you live on or near farmland, keep it mowed on field edges as it is very poisonous to livestock.