Throughout the trial, all knotweeds were observed regularly and appraised for their cultural adaptability to the soil and environmental conditions of the full-sun evaluation garden; susceptibility to diseases and pests; winter hardiness and survivability; and ornamental qualities associated with flowers, foliage, and plant habits. In addition, plants were closely monitored for any reseeding and for rhizome or stolon vigor. Final performance ratings for 26 of the 30 taxa evaluated are shown in Table 2; final ratings are based on foliage and habit quality, flower production and floral display quality, plant health and vigor, and winter survivability. Most taxa were evaluated for a minimum of four years, except for Bistorta amplexicaulis ‘Blackfield’, B. amplexicaulis ‘Inverleith’, Persicaria microcephala ‘Red Dragon’, P. runcinata ‘Purple Fantasy’, P. filiformis ‘Compton’s Form’, and Koenigia weyrichii, which lived for less than four years each. The four Japanese knotweed selections in the trial were not rated for their garden performance as they are not recommended for landscape use (see Japanese knotweed sidebar).

Bistorta amplexicaulis 'Summer Dance'

Bistorta amplexicaulis 'Rosea'

Persicaria polymorpha

Three knotweeds earned five-star excellent ratings for their consistently healthy and robust habits, superior floral displays, and winter hardiness. Nine additional taxa earned four-star good ratings.

Bistorta amplexicaulis, mountain knotweed, has a rounded clumping habit and features ovate to lanceolate leaves with acuminate or sharply tapered tips to 6 inches long. The slightly puckered leaves are amplexicaul, which means they clasp the stems at the base.

Dense floral spikes in shades of red, pink, purple, or white top plants from summer to fall. Interestingly, the flowers of mountain knotweed do not open in logical patterns—from top to bottom (basipetally) or base to apex (acropetally)—instead, opening randomly and often at the middle of the inflorescence first. ‘Summer Dance’ embodied all the best attributes of the species and was the standard whereby other cultivars were judged. A profusion of uniquely bright coral-red flowers on slender spikes—their closest color match was ‘Firetail’—bloomed above lime-green leaves for three months. ‘Summer Dance’ was the largest—58 inches tall and 60 inches wide—and most vigorous of the mountain knotweeds in the trial and did not suffer any winter losses.

The pale pink flowers of long-blooming Bistorta amplexicaulis ‘Rosea’ towered generously above dark green leaves from mid- July to late September. While the flower show was exceptional, our evaluators did not find the soft pink as satisfying as the vivid colors of ‘Firetail’ or ‘Summer Dance’. The tiny flowers opened randomly along tapered spikes up to 7 inches long and were busy with bees for many weeks. Stout ‘Rosea’ formed big, bushy mounds reaching 55 inches tall and 58 inches wide by the end of summer. ‘Rosea’ was fully winter-hardy, and like other mountain knotweeds, it did not have an aggressive or weedy habit.

White dragon or giant fleece flower, Persicaria polymorpha, was a boldly robust but mild-mannered knotweed. It had the vigor and stature of Japanese knotweed but with a distinctly clumping growth form. White dragon’s shrubby habit, to 6 feet tall and almost 7 feet wide, remained upright and attractive all summer. Tiny white blossoms clustered in large frothy plumes opened in late May and bloomed for much of the summer. Floral fragrance was described as a light, pleasant scent or a disagreeable odor depending on the evaluator. Flowers aged to pinky red seedheads. Minor cosmetic damage from Japanese beetles was observed on the lance-shaped, dark green leaves in multiple years, but no winter injury was noted.

Bistorta amplexicaulis 'Fire Tail'

Bistorta carnea

Bistorta amplexicaulis 'Rosea'

Flower displays were generally effective, with varied floral habits and production levels among the different knotweeds. The stubby spiked inflorescences of Bistorta affinis and B. officinalis were densely crowded with small flowers. Their cultivars were in shades of pink, which aged to darker tones before finally turning burnt orange. The inflorescences ranged from 3 to 4 inches long and less than an inch wide; B. affinis ‘Dimity’ was the narrowest at ⅝ inch wide and B. officinalis ‘Superba’ the widest at ⅞ inch. Due to the long bloom period—May to October—all floral and color stages were present concurrently. The floral spikes of B. amplexicaulis by contrast were longer and narrower at 4 to 7 inches long and ½ inch wide. The inflorescence sizes noted in Table 2 were measured at the point when the color show—a mix of buds and open flowers—was at its best, approximately three weeks after the first flowers opened. Pollinators were commonly observed during the bloom period and bees were particularly heavy feeders on all the B. amplexicaulis cultivars.

At 14 inches long, the panicles of Persicaria polymorpha were the largest inflorescences in the trial. Beginning in late May, the white flowers were enhanced by light reddish floral stems. Flowers aged tannish and due to their size and quantity, eventually detracted from the overall ornamental quality before the bloom period ended. The spikes of the P. virginiana cultivars were only slightly shorter than P. polymorpha but were thread-like by comparison to its broad plumes. The foot-long inflorescences looked like slender whips with the individual flowers spaced widely apart on the floral axis. The small flower sprays of P. microcephala differed greatly from other knotweeds in the trial. Unlike the generous flower production observed on most taxa, the tiny white baby’s-breath-like flowers were sparsely produced each year. Knotweeds do not display a traditional peak bloom stage like many other perennials, because the flowers open randomly on an inflorescence, and flowering is protracted due to continual production of new inflorescences.

Leaves in shades of green were typical, but several cultivars featured colorful markings or variegation. A few green-leaved taxa took on ruddy tones or became red-spotted in the hotter periods of the summer. The leaf tips of Bistorta affinis including ‘Border Jewel’ and ‘Dimity’ regularly turned red in summer; their foliage was semievergreen in mild winters. Additionally, B. carnea, B. officinalis ‘Superba’, and Koenigia weyrichii also displayed red leaf tips beginning in July.

The cultivars of B. amplexicaulis commonly exhibited red spots during the summer months; spotting was considered a minor cosmetic issue overall.

Bistorta amplexicaulis 'Golden Arrow'

Chevron markings on Persicaria virginiana

Japanese beetles on Bistorta amplexicaulis 'Summer Dance'

The vibrant yellow leaves of Bistorta amplexicaulis ‘Golden Arrow’ lightened to yellow-green as summer went on. Sunscald, noted periodically during hot and/or dry periods, caused burnt or bleached out leaves that eventually turned brown. Chevron markings were prominent on the leaves of Persicaria microcephala, P. virginiana, and P. filiformis. The tricolored leaves of P. microcephala ‘Red Dragon’ started out purple-brown with a silvery chevron and aged to green with purple and silvery markings. Persicaria ‘Silver Dragon’ had a similar look, but its multi colored leaves were silvery green with ruddy edges and a pale chevron bordering a reddish blotch. The green leaves of P. runcinata ‘Purple Fantasy’ were highlighted by dark purple chevrons with a pale nimbus. Reddish brown to maroon chevrons were prominent on P. filiformis and P. virginiana. Persicaria filiformis ‘Compton’s Form’ featured a silvery overlay with a large chocolate chevron; ‘Lance Corporal’ had light green leaves with a pronounced deep maroon chevron; and the cream-and-yellow-splashed green leaves of ‘Painter’s Palette’ had red-brown chevrons.

Plant habits were generally clump-forming with variable sizes and vigorousness observed. Weakened habits attributed to competition from adjacent plants were reported on several taxa including Bistorta amplexicaulis ‘Alba’, Persicaria ‘Pink Elephant’, P. virginiana ‘Painter’s Palette’, P. filiformis ‘Compton’s Form’, P. filiformis ‘Lance Corporal’, and Koenigia weyrichii. Stoloniferous or rhizomatous knotweeds included B. affinis, B. carnea, B. officinalis and their related cultivars. None of the spreading knotweeds caused problems with neighboring plants, other than intermingling along their common edges. Reseeding was observed on P. virginiana and P. filiformis ‘Lance Corporal’, with the latter noted as reseeding excessively. Persicaria microcephala ‘Red Dragon’, P. runcinata ‘Purple Fantasy’, and P. ‘Silver Dragon’ were occasionally presumed dead because they emerged in June, much later than other species. The foliar mounds of taxa with longstalked inflorescences ranged from 6 to 8 inches tall (B. affinis), 15 – 30 inches tall (P. virginiana and P. filiformis), and 24 – 42 inches tall (B. amplexicaulis). Plant heights with inflorescences are shown in Table 2.

No diseases were observed during the trial period, but Japanese beetles were occasional pests—foliar damage was mainly a minor cosmetic nuisance. Japanese beetle damage was observed in one or more years on Bistorta amplexicaulis cultivars—‘Alba’, ‘Atrosanguineum’, ‘Firetail’, ‘Golden Arrow’, ‘Rosea’, and ‘Summer Dance’; B. carnea; B. officinalis ‘Superba’; Persicaria ‘Pink Elephant’; P. virginiana; and P. filiformis cultivars—‘Compton’s Form’ and ‘Lance Corporal’. Minor rabbit and deer browsing was randomly observed; repeated browsing in the same season was not noted on any taxon.

While most knotweeds in the trial are listed as cold-hardy to USDA Hardiness Zones 4 or 5, Persicaria microcephala is rated for Zone 6 and warmer. The lack of success with P. microcephala ‘Red Dragon’ was likely a cold temperature issue; it was replanted once, and plants did not survive the winters of 2012 – 13 and 2014 – 15. Of the Bistorta amplexicaulis group, ‘Inverleith’ was an anomaly because it did not survive a winter whereas other cultivars did. ‘Inverleith’ was replanted twice in different locations within the trial garden; subzero temperatures were recorded in each winter when losses occurred: -16°F in 2013 – 14, -4°F in 2015 – 16, and -5°F in 2017 – 18. Despite an expectation of cold-hardiness, several taxa suffered crown injury and/or plant losses in multiple years including Persicaria ‘Pink Elephant’, P. ‘Silver Dragon’, P. virginiana, P. virginiana ‘Painter’s Palette’, and P. filiformis ‘Compton’s Form’ and ‘Lance Corporal’. Crown loss was noted in most winters on the affinis-group, but ‘Border Jewel’ and ‘Dimity’ rejuvenated more quickly from crown injury than the species.

Bistorta amplexicaulis 'Taurus' 

Bistorta amplexicaulis 'Rosea' 

Bistorta officinalis 'Superba'

Competition from neighboring plants likely played a part in crown injury and plant losses observed on Persicaria virginiana, P. filiformis, and their associated cultivars. Although injury and losses were noted following winter, the affected plants had been significantly encroached upon in the preceding year by Reynoutria japonica ‘Devon Cream’ and R. japonica var. compacta, both of which were vigorously rhizomatous. With rigorous attention to removing errant shoots of the Japanese knotweeds after the initial trial years, the Virginia knotweeds and Asian jumpseeds recovered their vigor, and crown injury was not observed after the winter of 2014 – 15. The exception was P. filiformis ‘Compton’s Form’, which died out completely in winter 2014 – 15. Late frost damage to leaves was only ever observed in mid-May 2016 on Bistorta amplexicaulis ‘Alba’ (40%), B. amplexicaulis ‘Rosea’ (10%), P. polymorpha (10%), and P. virginiana (30%).