Persicaria (also known as Bistort or Knotweed) 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.
Where to grow Persicarias
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.
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.
Looking after Persicarias
After a few hearty frosts the leaves and stems 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.
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.