Persicaria pensylvanica aka Pennsylvania Smartweed

Persicaria pensylvanica is a wildflower that lives for one year. It belongs to the knotweed family, Polygonaceae. Common names include Pennsylvania smartweed, pink smartweed, and pink knotweed. It is native to North America, Cuba, and Hispaniola. It grows in temperate regions, where the climate is mild. It is not native to Britain or Ireland. Its older name is Polygonum pensylvanicum. Both names refer to the same species.

The plant grows dense and upright, with slim stems. These can bend in a zigzag between their joints. Stems may be green or have a red tint. Their swollen joints, called nodes, are red. A thin sheath called an ocrea wraps each node. No fringe grows along the edges of these sheaths. Accounts describe both smooth stem parts and sticky hairs. Some plants have small barbs on their stems too. The shiny green leaves grow in an alternate pattern. This means they take turns along the stem. They are lanceolate, or shaped like a lance head. Tiny hairs line their edges. Leaves reach about 0.33–0.5 feet (0.10–0.15 metres) long, sometimes longer.

Small flowers form dense clusters called racemes, which look like spikes. Each cluster is about 0.08–0.17 feet (0.025–0.05 metres) long. The flowers are rose-pink, though some are white or red. The sources list summer as the main bloom season. Bees, butterflies, and moths come for nectar. So do wasps, flies, and beetles. The shiny black seeds are flat and almost round. Songbirds and waterfowl eat them. These birds include cardinals, sparrows, geese, and mallards. Some seeds pass through birds' guts and still survive. Bird droppings then spread these seeds. Birds also use the plant to nest.

This smartweed grows in many open spots with moist ground. It grows in fields, roadside ditches, prairies, and swamps. It also grows at pond edges and beside fences. Old parking lots are another place it grows. Grow it in full sun and moist soil. It does best in dark loam but accepts sand and clay. It copes with ground that gets wet at times. Its listed USDA hardiness range is zones 3–10. It grows at a medium rate. The sources advise using seed to grow new plants.

This plant suits native gardens and sites left to grow wild. Its blooms add color in season; its seeds feed wildlife. NC State also lists its use as a specimen plant. That source calls it a weed too. Think about its spread when you choose a site. The sources also note that it does not catch fire easily. They do not say whether people can safely eat it. Nor do they state whether it is toxic. They list no specific pest or disease problems.