Pericaria 'J.S. Caliente'Funny how easily gardeners can be scared by 'big' plants. It seems that just one person complaining that a plant has outgrown it's space can send shockwaves through gardening circles, and scare gardeners off a plant with the same speed as food scares frighten people off particular foods. It's a bit sad really, because while food scares can be about specific dangers to human health (albeit probably often exaggerated by the media) there are many gardeners who live in fear of perfectly good garden plants.Take Persicarias; they suffer from two afflictions, their common name of 'Knotweed' brings up fears of 'Japanese Knotweed' (Fallopia japonica, which is related to Persicaria, but more on that later), and also being naturally fairly big plants. Big plants should not be feared- I often use a mantra with gardeners that it is easier to reduce a big plant in size than to stretch out a small plant. If you can grow a plant that needs dividing once a year to keep it looking nice then that 10 minute job must be worthwhile to keep the effect? The fact is that Persicarias don't need dividing annually, in fact they will go for several years without attention. The secret to growing these plants is space, both to allow them to grow healthily and to use them to their maximum effect.

Persicaria amplexicaulis and it's cultivars are the plants that people will most likely come into contact with. These superb and resilient herbaceous plants flower from late summer until the hardest frosts, with spires of red, pink or white flowers held aloft above thick arrowhead-shaped leaves. When grown with grasses and late summer flowering plants the effect can be very impressive, as seen in this picture, taken at Knoll Gardens in Dorset.


Due to increasing popularity new cultivars of Persicaria amplexicaulis are being released, and while many may seem, to the casual observer, to be similar the new cultivars seem to be taking good garden plants and making them better. This does not mean, however, that the older cultivars are in any way bad; I have a P. amplexicaulis 'Rosea' in a container with over 150 flower spikes on it! (Oct. '13). New developments are being made to create 'fatter' flowerheads which can be more colourful from a distance, although tall slender flowerheads still definitely have their place in the garden.


Above are five cultivars of Persicaria amplexicaulis that I currently grow. My biggest plant at the moment is P. a. 'Rosea' (1), which has tall and thin spires of rose pink flowers. Useful although less showy (so far) is the white P. a. 'Alba' (2), which with thinner flower spikes seems to need to be a large clump before you really see any effect. P. a. 'Blackfield' (3) is a fairly new introduction, bred by Belgian plant breeder Chris Ghyselen, and has rich blood red flowers on spikes noticeably fatter than many cultivars. P.a. 'J. S. Caliente' (pictured at the top of this page) is similar but with more orange-red flowers. P. a. 'Pink Elephant' (4) is a fairly dwarf plant, reaching only 18" (45cm) tall in flower, and is notable for it's angled wavy flower spikes (possibly showing P. a. var. pendula in the parentage, but I don't have this at the moment to check against). P. a. 'Orange Field' (5) is a little misleading in that the flowers are definitely salmon pink, possibly with a hint of orange to some people's eyes. 'Pink Elephant' and 'Orange Field' are also Chris Ghyselen introductions, as is the chubby red flowered P. a. 'Fat Domino' below (at RHS Rosemoor).


Persicaria vaccinifolia (number (6) in the above picture) looks in many ways like a miniature P. amplexicaulis in flower, but this species has a different spreading habit. Whereas P. amplexicaulis expands as a clump at the base P. vaccinifolia spreads as a dense mat of growth, rooting as it goes. If grown in a small space with small plants this species will rapidly make a nuisance of itself, smothering small plants as it goes. Don't be seduced by it's small stature in a pot, and instead release this species to make a fine ground covering plant under shrubs, where it will prove resilient and hard wearing. In autumn this species is clothed with short spikes of white/pink flowers before the cold weather turns the leaves a rich burgundy red- yes, this deciduous species even manages some great autumn colour! The thin stems remain all winter and then, just when you're wondering if it's going to come back the whole mat bursts back into life with tiny fresh green leaves. In this photograph P. vaccinifolia is growing happily in a wall at the RHS garden at Rosemoor.

Persicaria affinis and it's cultivars look like larger versions of P. vaccinifolia, and make excellent groundcover plants, especially in difficult positions. I don't grow any P. affinis cultivars myself, but I've seen enough of them in cultivation to be suspicious that some of that cultivars might be mixed up, and that possibly seedling plants have muddied the waters of this species. If someone bred a new and interesting form of this species then I would be tempted to grow that, but as it is I don't really have the inclination to grow this species at this time.

A surprisingly diminutive species is Persicaria tenuicaulis, a miniature species from wooded areas of Japan. This species is unusual in many ways; the pewter leaves, the gently clumping nature, the honey scented flowers in early spring, and the fact that this is the only Persicaria species I have found difficult to grow- as with many small leaved herbaceous plants it needs plenty of moisture BUT also excellent drainage. I got this balance wrong and will in due course have to replace this species.

Another early species is Persicaria bistorta, the best known cultivar of which is 'Superba'. This is the easily recognised 'Bistort', a fairly vigorously spreading species with low mounds of waxy leaves and chunky pink flowerheads to about 18" (45cm) tall. It can spread far if allowed to, although it's easy enough to dig out anything you don't want. The cultivar P. a. 'Hohe Tatra' differs in that it has longer and narrower leaves, slightly darker pink flowers and has less of a tendency to wander off! Hohe Tatra is a mountain in Eastern Europe, so it's probably a wild collected form.

Larger Persicaria species are worth growing too! Possibly the best known of the larger Persicarias is Persicaria microcephala 'Red Dragon'. This form makes a large mass of dark red leaves which by autumn are often found poking out of neighbouring plants. As the plant grows up the stems happily scramble through nearby shrubs and perennials, seldom being a nuisance, to add extra interest. Around October Persicaria microcephala and it's cultivars produce small white flowers, but these are fairly dull, and while not detracting from the foliage they don't really enhance it either. Make no mistake, this species is a fantastic foliage plant for any garden, although the straight species can sometimes set seed, so maybe stick to one of the cultivars with the better markings/leaf colour!

I was quite taken with a variety at RHS Wisley called P. m. 'Purple Fantasy'. Although the name seems tacky the variety has bold purple markings on bright green leaves- very eye catching. One day I might find one of these and grow it myself.

Another popular Persicaria is P. virginiana from North America (from Virginia oddly...!). There are several cultivars in the UK, and broadly speaking they follow a similar pattern; upright almost shrubby plants with long thin 'filaments' of flowers (particularly in P. v. var. filiformis). The selections are grown for the markings on their leaves, and these are mostly variations on the V-shaped marking that is present in the species (except for the white flowered P. virginiana 'Alba'). The exception to the rule is P. virginiana 'Painter's Palette', a vile looking plant beloved of some people that always looks either diseased or as though it's been planted under a tree that's home to a roosting flock of birds. I tolerate it but don't grow it myself. Far more upmarket is P. virginiana var. filiformis 'Compton's Red' (also doing the rounds under it's old name 'Compton's Form'); this is superb, with a distinctive metallic sheen to the leaves of this low and compact growing form. It is especially happy in a partly shaded spot, has no discernible 'bad habits' and associates well with larger ferns, Rodgersias and other shade loving plants

For those with space Persicaria campanulata is a great plant! Although the species itself is fairly dull there are some very nice forms around. For many years I have grown P. campanulata 'Southcombe White', a long flowering selection with pure white flowers that turn to a deep pink as they age. Oddly although I grow it and I love it I don't seem to have a photograph of it! P. campanulata 'Rosenrot' is another nice selection, this one having charming pink flowers and red/pink stems too. I saw this plant 'in the flesh' for the first time at Knoll Gardens in Dorset, where it grows enthusiastically in a shaded area, it's spread curbed by shrubs on one side, a low growing bamboo to the other and a large tree at the back. P. campanulata is a spreader, and will take up as much as it's allowed to, but one redeeming feature of this species is that it never really dies back, so during the winter (when the top growth has gone) it's easy to see how big the clump has grown and do some careful 'editing' with a spade by digging up any fresh leaves at ground level and allowing only the parts of the plant that you want to grow in spring to remain. This process is very easy and well worthwhile.

Persicaria polymorpha is very much a collector's plant. It is stout, with thick stems reaching to about adult chest heigh (the height varies according to how wet the soil is in summer), long green leaves and sprays of tiny white flowers. If you take the time to sniff the flowers you will find that they smell a bit like stale urine- don't sniff them! This species is probably more interesting than attractive, but is perfectly hardy and amenable to cultivation in most soils. I've seen it growing in the boggy soil at the edge of a pond where it grew as a large lax mass of foliage and flower, but I've also seen it growing in the fairly dry display beds at Cottage Garden Flowers in Worcestershire (as pictured below) where it made a tight mass of foliage and looked almost like a clipped shrub. Under the large oak trees at Castle Drogo (NT) in Devon it also grows fairly compact, so the availability of water really does affect it's habit.

Persicaria weyrichii (not pictured here) is very similar to P. polymorpha, and seems to differ mainly in being of smaller stature, having noticeably rounded leaves and setting seed after flowering. I don't know if the seed is viable (I didn't wait to find out!) but if you find yourself growing P. weyrichii, a perfectly good species to grow, you might want to deadhead your plant after it's flowered. The leaves and habit are sufficiently attractive to be worth growing after the flowers. Again a collector's plant.

I've also tried an annual Persicaria, P. orientale. Depending on your own personal taste you'll either find it's common name of 'Kiss Me Over The Garden Gate' endearing and romantic or thoroughly revolting! Either way this is a fun addition to a garden, being suitable for modern 'natural' style gardens or cottage gardens... anywhere where it's 8ft (2.4m) or so of height can be accommodated. Oh yes folks, it's a big-un!

The flowers are strangely dark pink but also very bright and 'in your face', especially on a dull autumn day. For much of the summer the plant concentrates on growing up on a thick almost woody stem, on top of which the leaves almost look like a strange species of 'Dock' (Rumex sp.). In September the fun starts; the plant rapidly grows upwards and branches out, with each branch dividing outwards to hold a fairly thick head of the pink flowers. Each head is about the same length and thickness of an adult's thumb but is held on thin stem, so the flowers dance around in even the lightest breeze. The effect is captivating. My plant has a lean on it after a particularly windy day a few weeks ago but has now grown upright again. You might want to provide a bit of support for this species, but don't let that put you off growing it. Scarcity of seed can be an issue, but searching online usually brings up someone selling seed.

Some Persicarias are weeds, and not everything in cultivation is necessarily worth growing (but that can apply to other plants too...), but Persicarias have so much to offer gardeners and it's such a shame when fear stops people growing these worthwhile and resilient garden plants. As with many plants it is worth doing your research; P. wallichii for example is grown well at RHS Rosemoor, but would grow out of hand if it was allowed to.


Similarly some close relatives of Persicaria occupy a status as horticultural weeds- Fallopia baldschuanica is the exceptionally fast growing climber known as 'Russian Vine', which will grow to 35ft (10m) or so tall almost before your very eyes, so needs to be sited very carefully. Fallopia japonica is the 'Japanese Knotweed' that is running rampant in ditches around the UK, and Fallopia sachalinensis is the 'Giant Knotweed'- even bigger than the 'Japanese Knotweed'. Basically don't trust Fallopia!

Cultivation

The secret of Persicarias comes down to two key things, water and space. Persicarias dislike drying out completely and will often grow to less than their full potential in permanently dry soils. In areas such as the South East of the UK they will grow happily enough but might look a little bedraggled by the end of the summer if they've not had quite enough water, and this is a shame given that they flower from late summer onwards. Site carefully, away from thirsty trees and shrubs in dry areas, and there can be no doubt that a mulch around the base will be beneficial.

These are mostly big plants; P. amplexicaulis varieties will usually spread to about twice the width of their base, so if the base of the plant is 2ft (60cm) across then you should expect the top to be up to 4ft (120cm)! This is useful for making bold statements in the garden, so put them in with other bold plants. Some species, notably P. campanulatus springs to mind, are enthusiastic colonisers, so give them plenty of space to run and you will get a great display from them. The more Persicaria there is the more showy they become. They are tough plants, to if they misbehave then dig chunks out of the running ones and divide the clump formers and they will grow away happily. The best time to divide is in early spring as the new growth gets moving. The scrambling Persicarias, especially P. vaccinifolium, grow very well with big shrubs and will potter away happily covering the ground and keeping all but the toughest weeds at bay. The one rule for Persicarias is that you should never grow a Persicaria with anything smaller than it. This rule applies to all species except for the very well behaved P. polymorpha which is a big but gentle plant.

As far as I know that vast majority of Persicarias are hardy in most sensible gardening regions. Some of the small exotic species can look hardy but aren't, P. runcinata being an example that I've lost to winter cold even in the relatively mild South West of the UK.

 The tiny individual flowers of P. virginiana 'Alba'

Pests and diseases... I'm loathed to say 'none' because that invites someone to prove me wrong, but I've not come across any particular problems. If you're worried that your Persicaria might become a pest (by self seeding, probably P. affinis and P. bistorta) then dead head them after flowering.