We think that this may be green kyllinga. See more on our website https://extension.umd.edu/hgic/topics/green-kyllinga
Yes, I believe you are absolutely correct - had never seen it before. I've manually removed all the kyllinga I've seen - it comes up in clumps fairly easily, especially when ground is wet. It certainly behaves as a "weed" and spreads quickly.
I don't understand how it came to grow in a linear fashion along a line where I had planted Candy Tuft in the Spring, behind a slight gully with new river rock over plastic.
Thank you very much.
It's possible that when you were preparing the planting bed and installing your candytuft, that you created a perfect environment for the kyllinga seeds, too. They took the opportunity to flourish.
Also possible that the seeds or a small kyllinga weed came in the candytuft soil. Many weeds get spread this way.
The important thing now is to keep the kyllinga from spreading all over your landscape. Keep a keen eye out for it in that area for the next few years.