With a common name like "knotweed" you might think you need to steer clear of Persicaria. But while certain species are considered invasive many remain excellent ornamental plants, providing good ground cover, interesting foliage forms and a range of decorative flower spikes in shades of white, pink and red.
In recent times they were classified under their now synonym Polygonum. They are in the Polygonaceae family, alongside the dreaded Japanese knotweed, Fallopia japonica. The common name "knotweed" refers to swollen nodes on the stems that look like joints. There are around 150 species, mostly annual or perennial herbs, but there are some aquatics, scramblers or woody subshrubs.
There are several excellent species for the garden, which are long-flowering, vigorous and form dense ground cover at differing heights. The lowest are P. affinis and P. vacciniifolia Award of Garden Merit (AGM), making them suited to the front of an herbaceous border, under shrubs or in a rock garden, positioned where they will not smother other plants such as delicate alpines. A nice feature of these two species is their late flowers that turn to a rusty red or brown and persist into winter.
At medium height is P. bistorta, a species that copes well with dry soils. The popular variety ‘Superba’ AGM produces soft-pink flower spikes in late spring or early summer — earlier than other species.
P. amplexicaulis and its numerous cultivars are tall, robust, clump-forming plants that provide late-summer colour. Established varieties include the dwarf, dark-red flowered ‘Inverleith’ and the rose-pink flowered ‘Rosea’.
Breeding is now aiming to create fatter, more colourful flower heads, such as the blood-red flowered ‘Blackfield’ PBR and appropriately named ‘Fat Domino’ PBR, but the tall, slender flower heads of the older varieties remain a nice design feature.
There are also some highly popular forms with striking foliage. P. microcephala ‘Red Dragon’ PBR is perhaps the best-known. It has lance-shaped purple leaves with silver chevrons. The more recently introduced ‘Silver Dragon’ PBR has similar chevron markings and silvery-red leaves. P. ‘Purple Fantasy’ also comes highly recommended.
They prefer a moisture-retentive, not too fertile soil in sun or partial shade. They do not thrive in dry soils, though P. bistorta is said to cope tolerably well in such conditions. Plant them at a distance from thirsty trees and shrubs if you can and give them a mulch to lock in moisture.
Many of the species are spreaders, but choose a spot in your garden wisely and site them where they can be allowed to create a mass of planting, or select a clump-forming type. Dig up the edges of spreading types and divide clump formers every three years, in early spring, if you want to control their spread and to keep them healthy. For self-seeding types such as P. affinis and P. bistorta, be vigilant about deadheading after flowering.
What the specialists say
Claire Austin, owner, Claire Austin Hardy Plants, Powys
"Persicaria are excellent for ground cover, especially the P. affinis types, which sit neatly at the front of the border. They will attract insects and are good for cutting.
"All P. amplexicaulis types are worth considering for late-summer colour, particularly P. amplexicaulis ‘Fat Domino’ as it is less vigorous than others. P. bistorta ‘Superba’ is great for late-spring colour.
"They are very easy to grow in a soil that does not dry out, but they can get very big so need to be divided every three years to keep them well-behaved. They are also liable to self-seeding."
Ben Probert, horticultural consultant, Pen & Trowel Gardening, Cornwall
"Persicaria are very useful plants for summer and autumn and I’m always happy to recommend them. They’re tough, flower well and stand up impressively to bad weather. P. amplexicaulis varieties are chunky — there’s no denying that — but if they can be given plenty of space they’ll provide excellent ground cover.
"New varieties like ‘JS Caliente’, ‘Blackfield’ and ‘Orangefield’ lead the way, but older cultivars like ‘Rosea’ are still worth having. Varieties of P. affinis make superb dense mats on walls and around the base of large shrubs, and are invaluable to anyone wanting good hard-working ground cover plants.
"Two Persicaria excite me very much at the moment. P. ‘Purple Fantasy’ is a fantastic foliage plant, with green arrowhead leaves boldly marked with chocolate brown. Mine has survived two winters now and seems as happy in a container as it is in the ground. With an outward, slightly flopping habit this has a great deal of potential for hanging baskets. Pink flowers appear in June and last through until the frosts but, as I say, this is more of a foliage plant.
"The other Persicaria that I’m really excited about is P. ‘Silver Dragon’, a new more silvery leaved version of the now very popular P. ‘Red Dragon’. I’m looking forward to growing this among ferns and hostas in a partially shaded bed.
"There are many brilliant garden plants in the genus, but there are some horrors. P. runcinata can make itself unpopular in gardens with a more favourable climate, while P. wallichii is an elegant but rampant thug that would become a nuisance in even the biggest gardens after a couple of seasons. Even the straight green P. microcephala — from which we get the sublime ‘Red Dragon’ — is a bit of a sod once it gets established.
"Thankfully these thugs are seldom seen in nurseries, but there is an attitude that Persicaria are invasive and this is just not true. Many are big growers but few cause any problems. I still see ‘knotweed’ used as a common name for Persicaria and I really doubt this helps their cause."
Species and varieties
P. affinis ‘Darjeeling Red’ AGM (H5) is a semi-evergreen perennial that forms a mat of lance-shaped leaves, often becoming red in autumn. In summer, dense spikes of small pink flowers are produced, becoming dark red when mature. Height: 25cm.
P. affinis ‘Donald Lowndes’ AGM (H5) is a partially evergreen perennial that forms low mats of dark-green, narrowly elliptic leaves. It produces erect spikes of small pink flowers that darken with age, finally orange-brown. Height: 20cm.
P. affinis ‘Superba’ AGM (H5) is a semi-evergreen perennial that forms a low mat of neat green leaves that turn russet brown in autumn. Spikes of pale-pink flowers are produced between midsummer and autumn, deepening to crimson as they age. Height: 23 cm.
P. amplexicaulis is a tall, robust, clump-forming, semi-evergreen perennial. It has pointed, slightly puckered, oval, mid-green leaves. Produces narrow spikes of bright-red to purple or white flowers on tall stems from midsummer to early autumn. Height and spread: 1.2m.
P. amplexicaulis ‘Alba’ is a tall, upright, clump-forming type, producing large, oval, pointed leaves topped with narrow spires of small, bell-shaped white flowers from midsummer to early autumn. Height: 1m.
P. amplexicaulis ‘Firetail’ is a vigorous, clump-forming perennial with semi-evergreen, lance-shaped, mid-green leaves that spread quickly to form ground cover. Crimson-red flower spikes are borne on long slender stems in midsummer to early autumn. Height: 120cm.
P. amplexicaulis ‘Inverleith’ is a shorter form than many other Persicaria, forming a low mound of large, mid-green leaves and spikes of pinky-red flowers between June and September. Height: 60cm.
P. amplexicaulis ‘Rosea’ is a tall, upright, clump-forming type with large, oval, pointed leaves and narrow spires of small, bell-shaped, soft-pink flowers in summer and autumn. Height: 120cm.
P. bistorta ‘Hohe Tatra’ is a clump-forming, semi-evergreen perennial with strongly veined, finely-pointed ovate leaves and dense spikes of small pale-pink flowers from late spring to midsummer. Height: 50cm.
P. bistorta ‘Superba’ AGM (H7) is a vigorous clump-forming herbaceous or semi-evergreen perennial. It has strongly veined, finely pointed ovate leaves and dense spikes of small light-pink flowers. Height: 90cm.
P. microcephala ‘Red Dragon’ (PBR) is a vigorous spreading perennial with lance-shaped, purplish-green leaves with a bold central splash of purple bordered by a silver chevron marking. Rounded clusters of white flowers are produced in late summer and autumn. Height: 50cm.
P. microcephala ‘Silver Dragon’ PBR is a striking variety with silver foliage that has red undertones and an apple-green flush when mature. Spikes of white flowers are produced between August and October. Height: 60cm.
P. ‘Purple Fantasy’ is a another striking variety with interesting foliage — lyrate bright-green leaves with chevrons of purple and pale green. Produces best colour and patterning in light shade. Small heads of pale-pink to white flowers are produced on very thin stems in the upper leaf axils. Forms large clumps. Height: 60-75cm.
P. vacciniifolia AGM (H5) is a low-growing, creeping perennial that forms a wide mat of small, glossy ovate leaves that turn a rich burgundy red in the autumn. It produces pale-pink flower spikes in late summer and early autumn, on stems that persist through the winter. Height: 15-20cm.
Thank you to Floramedia, which supplied the images for this article from its photo library