If you send a photo I could give you a better guess but from your description of a square stem I will say you have a member of the mint family. These tough plants become invasive especially in moist soil. Keep pulling, cutting and covering the plants with cardboard or sheets of newspaper with a mulch on top. Test to see if this invasive weed will respond to an application of RoundUp. Spring is the time of year to get control of any invasive weeds before they go to seed or spread more via underground roots. Good Luck, Marianne Binetti