Lady's Thumb,  Persicaria maculosa
(formerly called Polygonum persicaria)
Polygonaceae or Buckwheat Family

Short, erect plant, 1 to 2 feet tall.  Pink to purplish, tiny, 5-petalled flowers, in dense spikes at top of plant.  Leaves lance-shaped, strongly veined, not opposite, usually with a poir of darker blotches on either side of main vein (faint, but visible in the picture).  Stems jointed, and above each joint they are surrounded by a membrane called a sheath. Sheath is fringed with long hairs.  Blooms in late summer.

Introduced weed of gardens and waste places.  In Wildwood fairly common along the bikeway, especially in mowed areas.

Smartweeds (genus Persicaria) all have the tiny flowers in spikes, and jointed stems with sheaths.  The long fringing hairs of the sheath and the dark leaf blotches identify this one. Mild water pepper (P. hydropiperoides) is very similar, but is more drooping, likes wet places, and lacks the hairs and leaf blotches.