"Taming Persicaria"

Persicaria are vigorous and attractive foliage plants - that need control!
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.

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.

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!

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.

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.

 Reader Comments

1/7  Lyn - 2565 (Zone:10 - Warm Temperate) Monday, 22 October 2012

Oh yes Vietnamese Mint, I had put it into the ground before I discovered how rampant it becomes. It tooks us years to rid our garden of it and even after we had the house demolished (and moved to new house next door) it was still coming up on the old site! Next batch we confined to a pot in the chook run. Even they wouldn"t eat it... Ah, how we learn as we grow! Thanks, Lyn. Interesting that the chooks wouldn"t eat the plant! I don"t think I quite realised how vigorous it can be. Deirdre

2/7  Robin - 2121 (Zone:10 - Warm Temperate) Monday, 22 October 2012

Welcome back, Deirdre. Another interesting and timely blog. I had been wondering whether to throw Persicaria capitata as some people say it is a weed and spreads everywhere but I agree that it can have its place in a difficult spot, if controlled. Spring is sprung - I wonder where the rain is? Hope you got a bit of rain today, Robin - though it was pretty sparse here. That Persicaria is good for those difficult spots and I guess I am sentimentally attached to it as well! Deirdre

3/7  Richard - 2112 (Zone:10 - Warm Temperate) Monday, 22 October 2012

I love the lime green hue that P virginiana has as it really lifts a shaded spot, and though it does seed rampantly, is easy to pull out. Vietnamese mint is a water marginal plant in its native environment and so can be grown in shallow water on the edge of a pond - contained in a pot, of course!! I grow mine in a large waterwell pot to keep up the moisture. Thanks for that tip on the mint. I also love that lime-green colour of P. virginiana, especially when it first emerges in spring. I am still pulling out seedlings! Deirdre

4/7  Anne - 2518 (Zone:10 - Warm Temperate) Monday, 22 October 2012

yes welcome back Deirdre. I really enjoyed that blog - I remember Persicaria capitata from when I was a child too and my mother always called in Bachelors Buttons. I confined my Vietnamese mint to a pot as well but somehow it doesn"t grow as well. It is a spicy addition to meals. Thanks, Anne. Glad that someone else has a childhood memory of that plant. Re the mint, see Richard"s comment above about it being a water-loving plant. I think mine has suffered previously due to insufficient water. Deirdre

5/7  Clare - 3123 (Zone:10 - Mediteranean) Monday, 22 October 2012

It must be the difference in climate? I grow Vietnamese mint in my Melbourne garden in a dampish spot and it has never spread far at all! However, I do now prefer to grow a fresh clump every year by putting a cutting in water until roots form as the old clumps get a bit "woody" and ungainly. The leaves of fresh clumps are much nicer and tastier. I too love P.capitata for its usefulness and colour and it"s not hard to control. I didn"t know its name until your blog Deirdre. Thanks! Yes, climate would explain that difference in vigour, I think. The cold winters in Melbourne perhaps slow it down. I think all the species I mentioned can survive Melbourmne winters, however. Deirdre

6/7  Stephanie - 2075 (Zone:10 - Warm Temperate) Monday, 22 October 2012

Hmmm, I find that Persicaria capitata grows very well for some.Is it a capricious plant? I see it thriving on total neglect in some gardens,yet where I would like it to grow as a rampart ground cover, it lazes around and defiantly refuses to dominate the verge area beneath the liquidambers. Stephanie It does sound annoying that it won"t grow there when it seems to grow in most rugged spots. Maybe give it more water just to get it going? Or maybe tuck some compost around it to give it a boost. Hope you can get it going there. Deirdre

7/7  Margaret - 2122 (Zone:10 - Warm Temperate) Saturday, 24 November 2012

I rather like the Persicaria and have some you mentioned. Once had the "Vietnamese mint", but it absolutely ran riot and I eventually discarded it altogether. Persicaria microcephala "Red Dragon" behaves well, although it forms a large clump, but it is very hardy. I have it growing in some parts of the garden as a companion to begonia "Irene Nuss", and they make a handsome couple.

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