Other Latinate binomial: Persicaria pensylvanica, L.
Color-based names: Purple head, Pinkweed, Pink knotweed
Other common names: Glandular persicary, Big-seeded smartweed, Pinweed
Confusing names: Lady’s thumb, (usually applies to very similar Polygonum persicaria, L.), Swamp persicary (usually applies to Swamp smartweed, Polygonum amphibium,L.), and Hearts-ease (usually applies to Pansies—different family of plants altogether)
Geography: Native to many parts of the world including the east and central U.S.
USDA Plant Hardiness Zones: 3-10.
After a long bout of kneeling to eradicate this smart little weed, I have often wished for a replacement set of knees. Appropriately, its Latinate name of Polygonum means “many knees” in Greek. This name derives from the many jointed knee-like “nodes”, or growth cells along its stems. “Nodes” can form adventitious roots, or “stolons”, to steal territory and form new colonies wherever they touch ground.
Over-irrigated or alternately wet and dry situations, even in shade, awaken it to bring nearby plants to their knees. In the wild, in full sun on flood plains, this water-loving annual with viable drought- and flood- resistant seeds can dominate. Flood plains typically dry up in winter, which starves out those water-loving perennials that cannot tolerate a cold drought.
Pennsylvania smartweed reproduces energetically. Roots regenerate if broken. Its nectar attracts pollinators, and its lightweight pollen also travels by wind to fertilize other plants. It can self-fertilize since male stamens and female pistils mature at the same time. Each fertilized plant produces 800-1,500 seeds over a long season. Seeds in protective “achenes”, or hard seedcases, fall to the ground, wait in the soil seedbank for years, and break dormancy after frost, once the soil warms up; or travel in the bellies of feasting birds, insects, and mammals to new terrain where they get deposited with partially digested outer coatings and encased in compost; or float in water, such as irrigation ditches and rivers, to germinate
when they land. Seeds germinate quickly in shade to sun, in sterile to fertile, airy or compacted soil over a long season of repeated annual successions.
In gardens it attracts troublesome Japanese beetles, which devour Roses and other cultivars, then spawn baby beetles, called “grubs”, that eat lawn roots as their main grub. Moles then build tunnels (which disfigure the lawn) to grub out the grubs and feast upon them. Then voles travel through the mole-built tunnels to consume more plant roots and then take shelter in these ready-made tunnels, evade predators, and linger to procreate more root-feeding voles. It creates problems in farms where this hardy annual invades crops, grows quickly, and can host Beet curly top virus.
Its benefits? It feeds and shelters birds, beavers, and butterflies; creates a rapid ground cover in disturbed moist soils and flood plains which helps prevent erosion; absorbs excess rainfall; brings nitrogen up to the soil surface; and some rate it as a pretty wildflower that brightens up shade, roadside, and waste areas. Initially, as a novice gardener, its abundance of flowering stalks enchanted me, till its speedy spread dissipated the spell. In the wild, it grows as an early succession annual native in ever-changing balance with competing native weeds. It eventually cedes territory to second succession weedy perennials, so it doesn’t threaten species diversity. As a Buckwheat, Polygonaceae, family member, it can provide
nitrogen to the soil as a cover crop, however, that use poses future risks from this weedy invader.
The common English name of “Smartweed” for all the Polygonums comes from skin toxins in the leaves of many Polygonum that sting or smart. While this particular subspecies irritates skin less than other family members do, it still causes gardeners’ backs, knees, and egos to smart, especially when it rapidly reappears.
Key potential risks to human health include: photosensitivity if ingested by man or beast—but most often by beast; mild dermatitis if handled; and potential hay fever from voluminous light weight pollen, a bit unusual in a plant that attracts so many pollinators. Native American Indians used it as an anticonvulsive for epilepsy, to stop bleeding, and to ease hemorrhoids, though I find it a pain in the derriere. Its leaves reportedly taste a bit peppery and bitter.
Its almost identical weedy cousin, the hardy annual Lady’s Thumb, Polygonum persicaria, L, differs from Pennsylvania smartweed in having lots of fringy hair on its jointed nodes and its greater likelihood than P. pensylvanicum of containing a purple blotch in the center of the leaf.
It sends a message that the soil lacks nitrates and contains an excess of moisture. It has its greatest competitive advantage when not only nitrogen, but also humus, calcium, phosphorous, and magnesium have washed away, leaving tough, compacted soil with overly high levels of potassium (salts) and sulfur (from lack of aerobic activity). It survives drought that follows flooding or heavy rains. I have often noticed it blooming and spreading in the garden, or even in barren pathways, right after a rainstorm. It germinates in the rain, takes hold, and lasts through subsequent droughts.
TIPS FOR CONTROLLING PENNSYVANIA SMARTWEED
Pull and toss the whole plant since seeds remain viable for a long time. Or add its nitrate rich foliage to the compost pile if all seeds and roots are removed. Read its message to add nitrogen and diminish water in the soil. It resists most herbicides. Plant vigorous competitors nearby in enriched, drained soil.
Pictures 1 and 2: Close-up its florets display “sepals”, pink petal-like structures, that surround 8 male stamens and 1 female pistil, handy for self-pollination and visiting pollinators alike. Clusters of florets along a flower spike resemble Good ‘n Plenty candies
Pictures 3 & 4: An “ocrea”, or sheath, connects leaf and flower spikes to sometimes reddish, lanky, stems in a “knee-like” joint. Its lance-shaped, simple, slim leaves alternate and have smooth edges. Japanese beetles find them yummy and it plays host to this garden pest.