Um, although they have a robust root system, they mostly spread by seed. Enthusiastically.

What sort of mallows are they? Any chance of a picture?

if you have so many.. dig one up and FIND OUT ...

it should be easy to see a rhizomatic root system sending out runners rather than a self contained root mass that shows no indication of such ..

ken

i though i made up that term.. lol... hmmm.. go figure. . lol ..

https://duckduckgo.com/?t=ffcm&q=rhizomatic+root+system&iax=images&ia=images

Plants adapted to bird dispersal characteristically make some kind of a berry within which are seeds that pass through their gizzards intact. Mallows don't produce berries.

I have no explanation then as to how they arrived then. the seeds resemble millet. they have also since showed up in some beds 100ft. from this patch. a few years ago i had some unexplained turnips sprout up in my front yard. not far from the mallows. the mallows are right along the curb. maybe someone thought they'd expand my garden.

Weed seeds get around in all kinds of ways. Including wind. And if your reference to a curb in this instance indicates a streetside location then that means there is plenty of exposure to human sources of dispersal including pedestrians and gardening company trucks carrying plant parts.

Oho, seeds have a zillion ways of getting to where they need to go - from hitching a ride in animal fur (and woolly cardigans) stuck in the treads of our shoes, carried around by ants (snowdrops are notable for this), blown in on the wind, carried by river drift, moved inadvertently when transplanting...and, of course, via guerilla gardeners who think nothing of tossing a few seedbombs around.

here they are. they're not full height. I've been deadheading them to stop seed spread. this is the 3rd year. started as 6 plants. a pinched nerve has curtailed a lot of my gardening duties.

Perennial hibiscus. No point in cutting the flowers off as these are the main reason for growing the plants. And it does not look like they are encroaching on much else except weeds - why not just let them form a flowering hedge effect each summer?