These so-called thugs are plants that cover the ground so fast that if left to their own devices they eventually crowd out every other plant in a bed.
I just can’t seem to get enough of these dark-colored plants in my garden.
Liriope muscari has grassy foliage and spikes of blue or white flowers in the heat of late summer. It is especially accommodating as an edging or groundcover and there are some varieties with handsome variegated leaves (eg ‘Silvery Sunproof” ).
A good edging plant, and one that thrives in window boxes, hanging baskets and places that need a summer groundcover, is "Portulaca grandiflora."