I have a walkway that's bordered by clumps of either monkey or mondo
grass (I'm not sure which). The clumps usually do pretty well, but
this year several were pretty much taken over by weeds! The weeds grew
in the middle and pushed their way out.

Now that it's gotten a little colder (I'm in zone 7), most of the
weeds have died out. I tried pulling out as much as I could, but I
know that there are still some roots in there. Is there anything I can
do to strengthen the mondo grass now, so that they won't be crowded
out in the spring?

I have a lot of this grass that I can thin out, so I thought about
just digging up the entire clump and replacing it with a clump from
somewhere else. But that's a lot of work and probably won't look good
until spring, so before I do that I want to make sure there's not an
easier / better option.