Lot of this around the perimeter of back yard. Stem is hollow with red root....

We think it is the wildflower called spotted jewelweed, also called touch-me-not. Most people like this plant - its flower is attractive and it is a bee and hummingbird attractor. It is an annual, so the plants won't come back next year. But it seeds prolifically. If you don't want it, pull it as it comes up. Here is a reference:
http://www.friendsofthewildflowergarden.org/pages/plants/spottedjewelweed.html