Mugwort plants spread by rhizomes. At least that's what I was taught.  But mugwort can, occasionally, produce viable seeds.  When the conditions are right, the plants produce copious amounts of viable seeds. I've only seen them 3 times in the last 35 years but 2022 seems to have produced a bumper crop. Seedlings emerge in early spring. The small seedlings are unremarkable but if you had mugwort last year, look for the seedlings now.  Control them before they establish and spread. For more information about this nuisance weed, and more photos of the seedlings, check out the fact sheet on mugwort