What plant is that?

Happy New Year!

Our January mystery plant might be a challenging ID but will be found in Newcomb’s with no name change. This low herb grows along pond edges and in moist soils. It has squarish stems and opposite leaves that are coarsely toothed (but not lobed) and taper at both ends. The leaves do not have a distinctive scent. The tiny white flowers appearing in summer are tightly clustered or whorled around each node. Each irregular flower flares open at the end of the tubular corolla. The plant also grows a small edible tuber (which I vow to taste in 2026!)