I didn’t recognize this plant when I first found it growing behind some greenhouses in southeast Michigan. When going about identification, the first trait I focused on was the fact that the stems were four-sided in profile:
While four-sided stems occur in a few families, they’re most commonly found in the mint family (Lamiaceae). Many mints are also aromatic, and the crushed leaves of this plant had a definite odor. However unlike many mints, I noticed the leaves were lobed. The upper leaves had three pointed lobes, while the lower leaves had five coarsely-toothed lobes:
Following this lead, I located a key to the mint family. After following a few steps describing the flowers, I came to one genus described as having “deeply and sharply palmately lobed” leaves: Leonurus, the motherworts.
According to the USDA, only two species of Leonurus occur in North America (both introduced from Eurasia). The honeyweed or Siberian motherwort (L. sibiricus) has leaves that are very deeply lobed, not like the plant shown here. The other possibility, common motherwort (L. cardiaca) was a perfect match.
These weedy plants were apparently introduced for their folk medicinal value, to treat “female conditions” (hence the name motherwort). While I don’t hold much stock in folksy remedies, the attractive bilateral purple-pink flowers and distinct odor made this plant notable to me.