Typically found in moist, riparian habitats and in wetland zones that have undergone anthropogenic disturbance, Persicaria pensylvanica is an ecologically important annual. Commonly known as Pennsylvania smartweed or pinkweed, it has notable physical and chemical properties. This member of the Polygonaceae or knotweed family has a fairly extensive natural range, largely due to its moderate persistence and its vigorous seed production. It is naturally found in Alaska, several Canadian provinces, and throughout the US.
With morphologically variable stems that can grow up to 6.6 feet (2 meters) tall, Pennsylvania smartweed may be quite competitive as a wildflower. Depending on ambient conditions and soil depths, its ribbed stems may maintain an erect or a spreading habit. Its alternate leaves, which are borne on short petioles, tend to be narrow, lance-shaped, and bright green. These may have pigmented margins and purple markings on their central vein. A brown-tinged ochrea, or leaf sheath, is found where the petioles emerge from the stem.
In summer, this showy annual produces eye-catching inflorescences with densely arranged rose-pink blooms. Depending on sun exposure and access to nutrients, the flower spikes may measure anywhere between 1 – 3 inches (2.5 – 7.6 cm). Once these are pollinated by insects, they develop into nutritious seeds with immense wildlife value.
Facts, Benefits & Uses of Pennsylvania Smartweed
Pennsylvania smartweed would be a fantastic option for adding seasonal color and complexity to moist zones in a garden. Its tender stands are likely to thrive along the borders of garden ponds, rain gardens, and irrigated plots in urban zones. Though it does have the tendency to be competitive, it is not known for being particularly invasive. Its annual nature prevents it from depriving hardy perennials of space and winter nutrients.
In areas with mild climate conditions, the spiky inflorescences of this species may repeatedly appear from March to December. If you’re after attracting a diversity of friendly pollinators and birds to your property, consider planting well-maintained summer borders of this plant. Its seeds attract dozens of species of birds, including waterfowl, pheasants, doves, and geese. Along with the leaves and stems, they also attract herbivorous mammals.
Pennsylvania Smartweed Growth, Hardiness & Climate
Pennsylvania smartweed struggles to become well-established in dry substrates or in zones that experience prolonged droughts through summer. Once it does take root, however, substantial moisture promotes its rapid growth and spread. As long as water is readily accessed by the roots, it is able to tolerate a wide range of soil and climate conditions. This species is known for thriving in wet, open, and disturbed locations, such as ditches, fence rows, roadsides, parking lots, and lakeshores in developed areas.
Loamy substrates, generous moisture, and full sun are a trifecta of environmental conditions for this showy herb. High concentrations of nitrogen and phosphorus may also stimulate the rapid elongation of its stems. Equipped with a single taproot and a network of finer, lateral roots, it is able to survive in a few inches of standing water during periods of considerable rainfall or persistent flooding. Its weedy growth may occur on the shores of seasonal basins.
How to Plant Pennsylvania Smartweed
As it is an annual, Pennsylvania smartweed is usually planted using its seeds. In protected areas or in regions with warm winters, the seeds can be planted as early as fall. If you intend to germinate them directly outdoors, you may wait for the final frosts of early spring to pass. The seeds can be buried in about 1 – 2 inches (2.5 – 5 cm) of soil in drawdown zones or in patches with supplementary irrigation. Keep in mind that consistent moisture is necessary for germination.
To facilitate the development of a uniform Pennsylvania smartweed stand, this species may need to be re-planted annually. Harrowing the soil can aid in the natural re-establishment of its previously grown stands. For purposes associated with supporting wildlife, this species’ seeds can be distributed around the drawdown zones of water features or in mudflats. Once the seedlings reach a height of around 6 inches (15 cm), they can be re-flooded. The duration of each flooding period would depend on the maturity of the smartweed stands, with mature stands tolerating longer exposure periods to shallow water.
Over time, mature smartweed stands should begin to generously produce seeds, which will naturally become lodged into bottom substrates and grow dormant through winter. A fair portion, particularly those that are closer to exposed topsoil, are likely to germinate as soon as temperatures rise and waters recede.
How to Care for Pennsylvania Smartweed
Pennsylvania smartweed seedlings usually take around 60 – 80 days to mature. During their growth period, they will require at least 6 hours of full sun exposure each day. Partial shade may be tolerated, but do note that this may delay shoot maturation and reduce flowering rates. Moreover, optimal conditions coupled with an absence of waterfowl (as seed consumers) may cause explosive recolonization rates of smartweed. If specimens of this species are packed too densely, their plots will benefit from being thinned out.
The provision of fertilizer, particularly one that contains 16 parts each of nitrogen, phosphorus, and potassium (i.e. 16-16-16), should benefit seeds that are planted early on in the season. To prevent seedlings from dying back in the increasingly warm temperatures of spring, make sure the substrate is kept moist. Remove weeds and pathogen-infected plants from their plots.
How to Winter Pennsylvania Smartweed
As Pennsylvania smartweed naturally occurs as an annual plant, it does not tolerate cool temperatures in winter. Its stands are most likely to die back once they have released hundreds to thousands of seeds. The seeds will remain dormant through winter, after which they will germinate if spring conditions are suitable. The seeds can be left in the ground and will not adversely be affected by being submerged or buried through winter.
Is Pennsylvania Smartweed Invasive or Toxic?
Though Pennsylvania smartweed may grow quickly in moist areas, it is not known for being an invasive plant. In fact, it may struggle to compete with weeds even if its seeds from the previous season are abundantly distributed. The right set of conditions is crucial for its proper establishment during the warmer months of the year.
An excellent plant for wildlife, it is not known for being particularly dangerous to humans or herbivores. However, as it does contain a complex cocktail of phytochemicals, it may cause unpleasant digestive symptoms when ingested in large amounts.
Is Pennsylvania Smartweed Edible? Do Animals Eat It?
The seeds of P. pensylvanica are consumed by birds even if they are not wholly digestible. A fair portion of the partly digested seeds are actually dispersed via bird droppings, making these animals an important vector for this species’ spread. Small herbivorous and omnivorous mammals may occasionally feed on the shoots and seeds, though these are unlikely to form a major part of their diet. Raccoons, muskrats, and mice may consume this smartweed’s leaves and stems if other choice treats are not readily available.
Where to Buy Pennsylvania Smartweed & Seeds? (UK & US)
Persicaria pensylvanica (syn. Polygonum pensylvanicum) can be purchased in plant nurseries and garden centers throughout its native range. Its scarified (ready-to-sow) seeds are usually available in small to large quantities, especially through its peak growth period. Seedlings or plug plants are more difficult to find via online plant portals.
I’m interested if any readers here have planted this one before? If so, please do share your experiences and knowledge in the comments!