It can be difficult to be able to identify plants, especially herbaceous ones, with out flowers. 

It appears to be something in the pea family, perhapsDesmodium. You can wait to see if it flowers and compare photos with some species on the link we provided, or try sending back a photo to us with the flowers.

If it is unwanted and you don't want to risk it taking over a garden space you can dig and pot it up and wait, or pull it it out as to not let it spread. 

Emily