(Bistort or Knotweed) Persicaria are handsome, carefree plants that can be divided into two - ones that are big and ones that are small. The biggest will grow to 180cm (6ft) high, the shortest to just 30cm (1ft). The tiny flowers form lovely spikes, some as thin as a thread of beads, some fat and poker-like. All are carried on stems that emerge from relatively large, oval leaves.
Although the preferred position for Persicarias is in a soil that stays moist, they are easy to grow any soil that stays cool, whether it is clay or sand. They don’t like hot and dry soil and they are equally as happy in a sunny or lightly shaded (the sort you find along the edges of trees) situation.
Above: Persicaria affinis 'Darjeeling Red', Persicaria amplexicaulis 'Rosea', Perscaria amplexicaulis 'Fat Domino'
Persicarias In The Garden
I grow the larger Persicarias in borders at least 120cm (4ft) wide. This allows other perennials to be included and gives enough ‘run’ room for them to sit comfortably amongst the companions. Anything shallower makes them look squashed. Phloxes, heleniums, echinaceas, and anything bushy look great with Persicarias, and as the slender flowers are dispersed evenly across the clump other plants sitting behind the clump will be easy to see. Shorter varieties are perfect for the front of a border. The flowers are also loved by bees, and there is one additional bonus, when cut and put in a vase, the flowers last for ages in water.
Above: Persicaria bistorta 'Superba', Persicaria amplexicaulis 'Alba', Persicaria polymorpha
Persicarias Are Great For
Persicarias will grow in any soil that retains moisture and does not dry out in summer. If it does dry out the plant will fail to reach its full glory. All will grow in a situation where the amount of sunlight is limited. This might be a location that only gets sun for a part of the day, or the sunlight is dappled by over head trees. Be careful in this location because, due to tree or shrub roots, the soil can dry out.
Bees as well as other flying insects, absolutely adore Persicaria, especially in the latter part of the summer. I have discovered that the flowers are great for cutting. The only draw back is that the tiny individual flowers will drop quickly, and although untidy, they make very little mess.
How To Care For Persicaria
After a few hearty frosts the leaves and stems of types of Persicaria amplexicaulis collapse into a heaps of brown that will provide over-wintering insects (such as ladybirds) with cover. However tidier, less rushed gardener might like to remove the foliage once the leaves turn brown. Any earlier and you might miss the lovely autumn colours of the leaves.The heap of old, brown foliage is perfect for protecting over-wintering insects. Remove this before new leaves start to shoot in spring. Because they are large plants, with long rhizomes, I think it is a good idea to dig them up and divide them every few years. This can be done any time after the foliage has died down. If they are left to get too large this is can be more difficult task, whereas if it is done every few years the while the clumps are not so big a small portion can be returned to the ground and it won’t take long for the clump to recover its original vigour.
Over winter Persicaria affinis types tend to leave a woven matt of stems that bear a few leaves. In spring new leaves will sprout from the stems, but not always on all the stems, which can make them look untidy. I tend to cut all the stems (which will root along the ground) back to the main plant. The new shoots which appear will then be neater and fresher.
Persicarias Are Good with
Bigger Persicarias look good with larger perennials that can cope with the robust nature of these plants. Smaller ones are ideal with Stachys byzantina. All are great for covering the ground to prevent weeds.