What is this and why is roundup not working?

Maybe it needs a surfactant along with a higher concentration.

Vince said:
Maybe it needs a surfactant along with a higher concentration.

Was gonna suggest surfactant. I had a few plants that seemed immune to round up in general application. With surfactant added they all die. Gotta be way more careful!

White vinegar with some Dawn soap mixed in. Stronger vinegar is even better. Spray it and walk away. It should be dead within 24 hours.

For me, the problem I run into is that these little clover-like weeds produce a ton of seeds, and they do it very quickly. So it wasn’t so much that I wasn’t killing the weed, it was that a new one was sprouting up in its place right away. So for walkways and such, I use Round-Up 365. It has the Round-Up plus a pre-emergent that keeps new ones from taking over as soon as the old one dies.

Brush sand into the cracks.