Ask Mr. Smarty Plants

Your description doesn't bring to mind any native plant in your area and the fact that the plants are growing in a planter suggests to me a cultivated nursery plant (not a native plant) introduced from elsewhere as an ornamental.  Our focus and expertise here at the Wildflower Center are with plants native to North America so you really need to ask someone else about your plant's identity.  The best way to determine its identity is take digital photos of it—the entire plant, a closeup of the stem to show how the leaves are attached and a closeup of the leaves to show their shape.  Next, you should visit our Plant Identification page where you will find the link to several plant identification forums that will accept photos of plants for identification.  Once you have learned its identity, you can search the internet to learn how to control it.

Alternatively, if you don't really care what it is but just want to get rid of it, you can:

1.  Dig up the roots and dispose of them in the garbage; or

2.  Cut the stems off near the bottom of each stem and, using a small foam brush, immediately paint the cut surface connected to the roots with an herbicide (such as RoundUp).  It is important to paint the cut area immediately because the plant's defense system will quickly seal the cut surfaces making it harder for the herbicide to penetrate.  You may have to do this several times before the roots die.  Be careful not to get the herbicide on plants you want to keep and read and follow the personal safety instructions on the label of the herbicide.