Perilla frutescens (shiso). Spreads (and it will!) by seeds, of which each plant has hundreds. Seedlings are easy to pull, but once the plants get some size on them, they're tough customers to get out. It's used as an herb in Asian cooking, as well as an ornamental. The green form supposedly has a clove/cinnamon taste, the purple is more anise/licorice. It has many common names such as beefsteak plant, wild basil, Chinese basil...please call it perilla, lol. I happen to love it, but it can be a pest if let go.

Yes, it has invaded some natural areas as well.