Taming Persicaria

Sunday, 21 October 2012

Persicaria odorata - Vietnamese mint

I have always hesitated about writing in this blog about the genus Persicaria, for fear that readers would think me crazy for growing a plant with such weed potential, especially one whose common name is 'knotweed'!. However, the emerging foliage this spring of a couple of the different ones that I grow has made me want to eulogise them just a bit and talk about how to minimise their negative qualities.

Many Persicaria plants used to be called Polygonum, and the taxonomy of the genus is still rather confusing at times. In English gardens, species and cultivars with showy flower spikes are often grown in borders - such as P. affinis, P. amplexicaulis and P. campanulata - but as with so many perennials, these ones don't perform terribly well in Sydney's climate.

Persicaria capitata

However, there are others that do, and they tend to be those with interesting and often alluring foliage. One of the first plants I was ever cognisant of as a child was the low groundcover Persicaria capitata (ht 15 cm), which has bright green foliage with distinctive purplish V-shaped markings, and seemed to be always sporting its strange, pink, bobbled flowers. I agree this can spread quite widely - as it did in my childhood garden, growing along paths and cascading over walls - and its width is given in one of my books as being 'indefinite', but it is a useful plant for any spot where you need the soil covered, whether in sun or shade. At times, the leaves take on an overall purple tinge, which is a useful contrast.

Persicaria virginiana

The V-shaped foliage markings are one of the most characteristic features of the species that I grow. One that is catching my eye at the moment is Persicaria virginiana (sometimes called P. filiformis, which may indeed be the more correct name; it was previously known as Tovara virginiana). In my garden, it is herbaceous, dying right back to the ground in winter and re-emerging in spring with rich green leaves with distinctive purplish-brown bands. It can apparently grow taller than 1 metre, but mine it gets to about 80 cm in height. It grows best in reasonable soil, in a position sheltered from hot afternoon sun and strong winds. I usually support my plant with cradle stakes during summer. If a plant with leaves or flowers of a similar colour to the leaf bands can be grown nearby as an echo, it can be a thrilling combination. Some ideas are dark-leaved Begonia, Iresine herbstii 'Wallisii' or Alternanthera dentata. The down side of this Persicaria is its self-seeding habit. I foolishly left the flower-spikes on the plant last year, thinking that the slim wands of tiny red blooms were rather pretty - this spring, I have had literally thousands of baby plants come up through the lawn and garden bed. This year I will not be letting it flower!

Persicaria virginiana Painters Palette

This species has a few cultivars, one being called 'Painter's Palette'. It is a rather gaudy plant with variegated leaves - a central V-shaped brown mark, yellow patches and reddish tints. It sounds rather horrid but placed in a tropical-style border with hot-coloured Dahlia, Salvia or Alstroemeria and given the contrast of some nearby dark foliage, it can look wonderfully flamboyant. This one has self-seeded in my garden too, so again I need to be more vigilant about cutting off the flower-stalks when they first appear.

Persicaria microcephala Red Dragon

In contrast, I have not (yet) had any problems with Persicaria microcephala 'Red Dragon'. In spring, the fresh burgundy leaves on reddish stems are marked with a dark centre surrounded by a silvery chevron. As summer progresses, the leaves may become greener but they still have some dark hues and markings. The flowers are insignificant. It needs to be pruned back hard in late winter to allow the new leaves to develop. It may need support from cradle stakes when in full growth and can be trimmed back a bit at any time when it seems to have outgrown its spot. It can grow in sun or part-shaded sites and tolerates most soils. The colour of the leaves complements nearby hot-coloured or blue flowers, and it combines well with silver foliage (such as Plectranthus argentatus) to echo its markings. It also makes a dramatic contrast to gold-leaved plants, such as Duranta 'Sheena's Gold'. It grows very readily from cuttings taken in spring or autumn and the foliage is useful in flower arrangements.

Another member of the genus that I grow is Persicaria odorata (pictured at the start of the blog) - better known as Vietnamese mint, a useful culinary herb. It has the same V-shaped leaf markings as the others mentioned. Unlike most of them, however, this is a frost-tender plant hailing from South-East Asia, so needs to be grown as an annual in cooler areas. I confine it to a pot, as it tends to take root wherever the tip of the stems touch the ground. It likes moisture and regular fertiliser. It has many uses in Asian cooking and it also makes an attractive garnish.