Sturdy plants with jointed stems and small white or pink flowers. Some kinds tend to get out of hand and need to be controlled.
P. affinis. Himalayan Knotweed. Zones US, MS, LS; USDA 6-8. Himalayan native. Spreading plant to 1 ft. high, 2 ft. or more wide. Deep green, lance-shaped, finely tooth-edged leaves are mostly basal; they grow 2412 in. long, turn bronze in winter. Dense, erect, 2- to 3-in. spikes of bright rosy red flowers bloom in summer, early fall. Informal border or ground cover. 'Darjeeling Red' forms 3-in.-high foliage mats, has 10-in. spikes of deep pink flowers that age to red; its foliage turns red in fall. 'Dimity' sports pale pink flowers and larger leaves than the species.
P. alpina (P. polymorpha) Alpine Knotweed. Zones US, MS, LS, CS; USDA 6-9. Native to China, Japan. To 46 ft. tall, 4 ft. wide. Forms a bushy clump of rough-textured, lance-shaped, dark green leaves to 8 in. long. In summer, sturdy stems are topped by big, fluffy, creamy white flower plumes reminiscent of astilbe. Not invasive. Tolerates heat and humidity.
P. amplexicaulis. Mountain Fleece. Zones US, MS; USDA 6-7. Himalayan native. Forms a big clumpto 4 ft. tall and wide when plants are in flower. Medium green leaves are pointed ovals up to 10 in. long. Blooms profusely from midsummer to fall, bearing narrow, 4-in. blossom spikes similar to those of lavender (Lavandula) but in a wider range of colorspink, purple, red, white. 'Firetail' has bright scarlet flowers.
P. bistorta. Bistort. Zones US, MS; USDA 6-7. Native to Eurasia. Makes a clump to 212 ft. tall, 3 ft. wide, with broadly oval, medium green, 4- to 12-in.-long leaves. Tight, 2- to 3-in. spikes of pale pink or white flowers bloom from late spring until well into summer. 'Superba' is a good pink selection.
P. capitata. Pink Knotweed. Perennial in Zones US, MS, LS, CS; USDA 6-9; annual elsewhere. Himalayan native. Tough, trailing ground cover grows 36 in. high, spreading indefinitely both by rooting stems and by self-seeding. Oval, 112-in.-long leaves are dark green when new, take on pinkish overtones when mature. Leaves of all ages have a bronzy cast that deepens in cooler weather; they discolor and die below 28F. Stems and small, round flower heads are pink. Blooms almost all year in mild climates. Best in confined spots (where it won't be able to spread) or in uncultivated areas. No watering needed.
P. microcephala. FLEECE FLOWER. Perennial. Zones US, MS, LS, CS; USDA 6-9. 'Red Dragon' is the poster child of the species and a striking addition to a sun- to-part shade bed. Dark burgundy foliage is the reason to grow it, although it does have small white flowers in late summer. It does not spread underground, remaining in a clump unless a stem bends over and puts down roots. Well-behaved to 3 ft. tall.
P. vacciniifolia. CREEPING FLEECE FLOWER. Zones US, MS, LS; USDA 6-8. Himalayan native. Prostrate plant forms a foliage mat to 3 in. high, trailing to 2 ft. or wider. Slender, branching, reddish stems are clothed in oval, 12-in.-long, shiny green leaves that turn red in fall. In late summer, 6- to 9-in. flower stalks bear dense, upright, foxtail-like, 2- to 3-in. spikes of rose-pink blossoms. Excellent as a bank cover or for draping over a boulder in a large rock garden.
P. virginiana. VIRGINIA KNOTWEED. Zones US, MS, LS; USDA 6-8. Native to eastern North America, eastern Asia. To 24 ft. tall, spreading indefinitely by creeping rhizomes. Oval green leaves 310 in. long; insignificant flowers. The species is rarely found in gardens. More commonly seen is 'Painter's Palette', valued for its flashy foliage: Leaves are marbled in green, pale gray- green, and ivory, with a ragged chocolate-maroon V in the center. On new growth, the ivory patches are closer to yellow, and the central V-shaped mark has a pink cast.