Some type of Euphorbia. Euphorbia cyparissias

https://www.invasiveplantatlas.org/subject.html?sub=3116

Yes. Bit of a thug.

More than just a thug in some locations. The fact that it is a non-native "volunteer" should provide some insight into its tendency to spread. Rampantly!!

Thank you all! I will let my parents know - but I also know they are probably doomed to failure. Too much arthritis for too many hills.

If you just pull them right now, it might not be too much of a battle. If you wait, it will. It does look like the one that bloomed has already set seed and dehisced. So you’ll have some new seedlings to Pull next spring too.

"Just pulling them" definitely doesn't work with this cute little beast.

Even if you carefully dig it up, each and every remaining root fragment will re-grow. I had a small patch, about 18" x 18*', which required 2 years to eradicate.

So start digging and don't stop until you see bare soil -- i.e.: no new sprouts -- for at least a season.