Knotweed’s not ALL bad
Japanese thug has a brutal reputation... but some species are stunning – and totally safe to grow
WHEN I told my neighbour that I was going to plant knotweed in a bed next to her property, she gave me a look that suggested I should brace myself for legal action. In order to preserve good relations, I thought it prudent to reassure her that I didn’t mean invasive Japanese knotweed but a well-behaved, garden species.
The plant in question was Persicaria virginiana ‘Painter’s Palette’, a cracking perennial that is grown primarily for its foliage. Forming a 3ft-wide clump, its 20in-tall stems are clothed with large, oval green leaves that are marked with a red chevron pattern, along with irregular patches of cream, yellow and light green.
It’s a bit of a pussycat compared to its ferocious relative, the aforementioned weed from Japan, whose stems can grow up to 4in a day over summer, eventually reaching heights of 10ft.
Below the soil, its underground stems can extend more than 20ft horizontally and are infamous for breaking through tarmac and concrete.
Introduced to Britain as an ornamental in the 19th Century, Japanese knotweed quickly went rogue. These days, it strikes fear into the hearts of homeowners; if the weed is spotted within 23ft of a property, mortgage applications can be refused unless there’s a plan in place to eradicate it.
Both Japanese knotweed (Fallopia japonica) and persicarias belong to the polygonaceae plant family, whose members can be found in a variety of habitats worldwide. Russian vine (Fallopia baldschuanica), rhubarb and French sorrel (Rumux scutatus) are among the clan cultivated in our gardens.
Persicarias gets their common name of knotweed from their swollen leaf joints, or nodes, that are arranged at alternate intervals on stems. In fact, until the 1990s, many persicarias were known botanically as polygonum, a name that derives from ancient Greek meaning numerous-jointed.
Ornamental knotweeds are a diverse bunch, ranging in height from a few inches to 4ft. The majority are grown for their summer flowers that last well into autumn. Available in shades of white, pink and red, they are carried on heads that vary from slender spikes to bulbous clusters resembling miniature bottlebrushes. Some persicarias are held in high regard for their foliage, producing handsome, upright clumps of stems decked out with brightly coloured, variegated leaves. These types do produce flowers, but they tend to be fairly insignificant and can even be removed to prevent them spoiling the foliage effect. Suitable for sun or partial shade, persicarias will flourish in soil that remains slightly moist. Some will tolerate drier conditions for a while, but tend to look a bit rundown by the end of summer. As plants usually hit their stride at this time, prevent an untimely end by avoiding dry soil or at least improving ground prior to planting. Low-growing ones are ideal as weed-suppressing ground cover, while taller varieties will add late interest to beds and borders. They have been an integral cottage garden plant since the late 19th Century, with the likes of Gertrude Jekyll and William Robinson being among the first to extol their virtues. Foliage forms are perfect in jungle schemes alongside other leafy things, such as bananas, canna lilies and Euphorbia mellifera. In my old London garden I teamed Persicaria microcephala ‘Red Dragon’ with Amicia zygomeris and astelia below the branches of a Cercis canadensis ‘Forest Pansy’ tree.
The key to success with persicarias is to give them enough room to spread, rather than shoehorn them into a gap. Every three to four years, restore the vigour of congested colonies by prising up clumps with a fork, dividing into smaller pieces and replanting.
This work is best carried out in early spring or autumn.