Centaurea montana Here is a link that might be useful: Centaurea montana

Try a pre-emergent. Round-up only kills the plant it covers, and it needs sunshine, so if the snapdragons are shaded, round-up is less effective. It's also not very effective in cold weather. There might just be some old seeds sitting around. The But if you do go the plastic route, get a landscape grade that can handle a little abuse. It will kill the nutrients in the soil so I'd advise against it, and the seeds can probably outlast the plastic. Maybe layered newspaper under a bark mulch might help. From the Minisota Dept of Agriculture "For existing infestations, develop a long-term, site-specific management plan. The seedbank can persist for up to 10 years so management practices must be implemented repeatedly as the infestation regenerates from the seedbank." Here is a link that might be useful: MN Dalmatian Toadflax

I live in zone 7b and have 4 horrida which get 5 feet tall and 6 feet or wider. They do not take much water and I never fertilize it; can take the heat of Texas. It dies back after the first frost in the Fall and I trim it back to about 6 to 12 inches, come Spring it is back. Mockingbirds love the berries (they are poisonous) and spread the plant in our neighborhood. Hummingbirds and butterflies really like this plant...mine is always busy. Peggy

the mullien will produce a seed pod with about a billion seeds.. all of which could sprout in your garden next year .. if left.. you will have a couple hundred more next year ... get rid of it in a formal garden ... if you have a wild flower field of a few acres ...... intended to feed birds ... then it could stay ... because it would fit within the definition of the garden you grow ... ergo.. for most of us.. its an aggressive reseeder.. not welcome in the garden .. no matter how cute and fuzzy it is .... ken https://www.bing.com/images/search?q=mullien+seed+pod&FORM=HDRSC2