Do not let the common name –knotweed put you off, there are many worthy species available from garden centres, nurseries and me, see My Services.
They are hardy and relatively fast growing without being aggressively invasive. They can easily be kept within their desired location and are perfect for reducing weeds. In winter they die down to ground level. This has the benefit of allowing us to associate them with early flowering bulbs increasing year round interest.
Persicarias do not root too deeply so any existing species with no bulbs beneath are relatively easy to lift, divide and re-plant the healthiest divisions. These and newly acquired plants can then be inter-planted with early flowering bulbs when they become available. It is always handy to note in your diary where, when and what bulbs you intend to plant. Snowdrops, winter aconites, daffodils and other early bulbs will happily grow in association with them adding a welcome early layer of colour in before the persicarias green up again.
Spring flowering bulbs are in the shops now and are best planted in the coming weeks. This will necessitate the cutting back of perhaps other existing herbaceous, where an early addition of colour is desired, such as Euphorbia griffithiii ‘Fireglow’. The milky sap of euphorbias can be skin irritating and I have read persicarias can also cause irritation, so it is always advisable to wear gloves when handling plants.
You may think cutting back a plant still looking good a bit cruel. Maybe so but I always think back to the words of Fred Beechcroft Parker. He was a 79 year old ex-clergyman and owner of a private arboretum in Te Puke , New Zealand: I met him on my overseas experience nearly 25 years ago. His home was furnished with furniture he had made himself over many years, from trees he had planted on his land. The soil and climate of this part of the North Island allows rapid growth. In exchange for food and accommodation I did some work for him. On one occasion we had to decide which one of three beautiful trees growing too closely together had to be cut down. In my youthful, naive opinion he chose the nicest tree, but it was the right one to make space for the other two. As he said “Sometimes we have to make decisions that are cruel but take heart when nature decides to destroy it is the cruellest of them all”. So like a hurricane I happily cut away, perhaps a little more orderly.
Persicaria affinis ’Superba‛ and P.affinis ‛Donald Lowndes’ make excellent front of border or slope stabilisation plantings. Both are similar, growing to about 20- 30cm, the latter being more compact with smaller tighter leaves.Taller P.campanulata with Pyrus salicifolia ‘Pendula‛ makes a dense weed supressing clump.
Other tall growing species I have are P.amplexicaulis ’ Fat Domino’ (red flowers) and P. amplexicaulis ’High Society‛ (pink flowers). I grow them towards the centre of my mixed border and admittedly they are prone to keeling over in strong persistent winds .On a recent visit to Cambo gardens In Fife I saw P. amplexicaulis varieties associated with clump forming grasses (calamagrostis and miscanthus species ).Together with other taller clump forming herbaceous perennials they have been allowed to mature, forming a more wind resistant, weed supressing, self -supporting mass planting. A magnificent display from which I have learnt ideas for next year on how I will help support my persicarias and other prone to flop plants.