Ann Marie:  Thank you for contacting the UVM Extension Master Gardener Helpline with your request in identifying a weed found in your lawn.  

I reached out to our Extension Master Gardener office (Deb Heleba) this morning because I suspected this is not a typical lawn weed.  Deb concurred with my assessment that this is actually poplar seedlings.  Some of these trees (Populas Alba) are considered invasives in parts of the country because of their ability to disperse seeds through wind.  They also can pop up as root suckers from a nearby tree. Do you have a nearby popular tree that could be the source of these shoots?  This may explain why you are not seeing the root of the plant.  You may just be breaking off the plant when pulling.

If this is the case, you may want to mow them down as they appear.

Attached is some information you may find helpful on woody vegetation control from the University of Minnesota and another fact sheet discussing white popular as an invasive.

https://extension.umn.edu/planting-and-growing-guides/woody-vegetation-control

white poplar: Populus alba (Salicales: Salicaceae): Invasive Plant Atlas of the United States

Good luck with your attempts in eradicating this nuisance sapling!