This weed has alternate leaves with jagged edges and a red/dark stem. The few I've left to grow are about 2 get tall.

I've had this weed coming up in my back yard for some time. I had been pulling out as it showed up, but one day I saw a little green caterpillar munching away. If it's a larval foodplant, I'll leave a patch of it, but I'd like to know what it is.