Is there an herb, or another low-growing plant, that I can plant to choke out mint and another unidentified spreading weed?
Before you can plant another low-growing plant to do battle with your mint and the ubiquitous “USW” (unidentified spreading weed), you need to weed out, with a spading fork and cultivator, the existing mint and weed root systems. Mint not only spreads through seed dispersal but also by sending out underground stems into the surrounding area, popping up a new plant where you least expect it. You simply can’t send in a new contender to the garden bed and hope it wins if the mint and USW are that well-established.
Your mission this season, if you decide to accept it, is to clean the invaded area of the noxious mint and USW, finding and removing as much of the roots as possible. If you had not done so last year, you might want to shear the mint carefully and dispose of spent flower heads. This will help prevent latent seeds from accidentally landing in the cleaned area as you remove the bulk of the plant.
A vigorous herb and low-growing plant to fill in the area would be any of the thyme varieties (Thymus). There are some wonderful lemon-scented and walkable cultivars now available. Depending on the size of the area, you could plant several varieties together, enjoying what each has to offer.
Since the mint is growing too well, the site is probably full sun and well-drained. Thyme loves those conditions as well. It will rot quickly in wet areas and suffer meltdown in hot, humid summers.
The next phase of your mission is vigilance. If you want the new pockets of thyme to thrive, you need to be on the lookout for mint and USW seedlings, pulling or cultivating them out before they can take hold and put down a root network. Look around your property, as well as your neighbors’ borders, for breeder plants that could be supplying seed for wind and animal dispersal. Taking a zero-tolerance approach this season to the former mint and USW area will give the thyme varieties a chance to establish themselves. They will cover the ground thoroughly, providing fragrance and flavor for your kitchen as well.