Carpet knotweed or snail knotweed is an undemanding groundcover that delights with its changing flower color from June to September.
From afar, you could mistake the carpet knotweed for a heather or a slightly undersized cultivar. Because this groundcover also fascinates with its purple-pink inflorescences, which shine through the garden green like colorful ears of grain. Especially with regard to heather, the supposed similarities quickly change if you take a closer look at the carpet knotweed.
The similarities between carpet knotweed (Bistorta affinis) and heather are mainly in the appearance of the flowers. The ground cover, which can be up to 25 cm high, forms a similarly dense carpet of plants, which is reminiscent of the dense pink cushions of the common heather. However, a closer look reveals some significant differences.
For example, the leaves of the carpet knotweed are more oval to lanceolate and have white hairs on the underside. In contrast, much smaller, scaly leaf needles are found in heather. Also, heather herbs such as common heather mostly grow as evergreen shrubs, whereas snail knotweed is a deciduous perennial. The differences between the two plants become even clearer with regard to their site conditions. In addition, the assignment of the carpet knotweed to the correct plant genus often creates a risk of confusion.
Carpet knotweed in the garden
The exact genus of the carpet knotweed has changed repeatedly over time. Initially counted among the knotweeds (Persicaria) by the first descriptor David Don, the ground cover was later temporarily attributed to the bird knotweeds (Polygonum). Only Edward Lee Greene divided the carpet knotweed into the currently valid genus of the meadow knotweed (Bistorta). For this reason, the flowering perennial is also known as Carpet Knotweed.
| Snail knotweed / carpet knotweed (Bistorta affinis) | |
|---|---|
| Growth: | creeping, carpet-like, ground-covering |
| Growth: | 5 - 25 cm |
| Growth: | 20 - 30 cm |
| Planting distance: | 20 - 30 cm |
| Location: | Sun to partial shade |
| Floor: | fresh, well-drained, loamy, sandy |
➔ Special Blossoms
All of the knotweed genera mentioned, including the carpet knotweed, belong to the knotweed family (Polygonaceae). Another difference to the heathers, which belong to the heather family of the same name. Apart from that, both plants are characterized by a spiked inflorescence and a dense, creeping growth. With the carpet knotweed, the flower spikes even conjure up multicolored accents in the garden. The closed flower buds initially color the bed in a strong purple. The small bell-shaped flowers of the plant later sprout from them, replacing the strong red-violet with a delicate pink to white-pink. This results in a unique play of colors that lasts from June to September thanks to a relatively long blooming period.
➔ Suitable plant concepts
Knotweed is suitable for numerous planting concepts. Since the flowering perennial is a wild plant, natural garden styles such as the cottage garden, the natural garden or the wild flower garden are ideal. Carpet knotweed is suitable here, among other things, as a bed border, upholstery perennial or ground cover for borders. Due to its resemblance to heather, it also looks wonderful in the heath garden. However, in contrast to the drought-loving heather herbs, Bistorta affinis prefers moist locations. A circumstance that makes carpet knotweed interesting as a bank or pond planting in the water garden.
Snail knotweed / carpet knotweed - location and planting
Carpet knotweed originally comes from the Himalaya mountains. In addition to numerous other nicknames, it is therefore also known as Himalayan knotweed. Due to its origin, the plant is perfectly accustomed to cold climates. A impressive winter hardiness down to -40 °C is also evidence of this. Ideally, however, the location should have a sunny to semi-shady location. The following applies: the sunnier the location, the more lush the blossom.
As far as the site substrate is concerned, the ground cover prefers freshly moist soil. As with heather, sandy-loamy soils are best suited to the needs of the carpet knotweed. Calcareous substrates are also well tolerated. In contrast to heather, however, the pH of the soil should be less acidic andinstead be in the neutral range with 6.5 to 7.5 points.
» Tip: Good plant partners for the snail knotweed and similar moisture-loving plants such as water daffodil, Chinese reed or sedge. They are particularly good as neighboring plants when planting in a pond. In the bed, the ground cover also harmonises well with bluebells or meadowsweet.
➔ Planting carpet knotweed in 3 steps
1. Step: The best planting time for the carpet or snail knotweed is spring. Choose a planting date after the ice saints to ensure that there are no more late frosts.
2. Step: Deeply loosen the ground and remove all ground obstacles. A light basic fertilization of humus and compost also gives the plant a good start.
3. Step: Carpet knotweed can grow up to 60 cm wide. Therefore, make sure that the planting distance is at least 30 to 40 cm. There should not be more than 5 to 7 plants per square meter.
Watering and fertilizing snail knotweed / carpet knotweed
For proper care, the moisture-loving carpet knotweed should be watered regularly. A lack of water quickly leads to wilted leaves on this plant. Regular waterings are therefore necessary, especially during the dry summer months. You can easily use calcareous tap water for this.
When it comes to fertilization, the ground cover is not particularly hungry for nutrients. A handful of compost spread around the root area during the flowering period will do the trick. Alternatively, other organic fertilizers such as horn shavings can be used.
Pruning and propagating snail knotweed / carpet knotweed
In contrast to heather, pruning the carpet knotweed is actually not necessary. Nevertheless, it can be carried out if necessary. To do this, cut back all withered flower stalks close to the ground in autumn after flowering. Only the foliage should remain on the plant for improved winter protection. In addition, the reddish colored autumn leaves of Bistorta affinis are also quite decorative.
» Speaking of winter protection: straightly cut carpet knotweed needs some covering with brushwood in winter to protect against frost.
➔ Snail knotweed / carpet knotweed - multiply by root division
For the rejuvenation of the plant, as well as for propagation, a root division is recommended for carpet knotweed. To do this, dig up the perennial in the spring and divide itCut rootstock into two equal parts with a spade or sharp knife. You can then plant the sections back directly to the desired spot in the garden bed.
Carpet knotweed - interesting varieties
Carpet Knotweed is available in a few exclusive cultivars. Some of them have been awarded the so-called Award of Garden Merit (AGM). The British award honors plants that are particularly suitable for garden culture in Great Britain due to their good resistance. And these varieties also thrive here with us. The AGM varieties are
- Carpet Knotweed 'Darjeeling Red'
- Carpet Knotweed 'Donald Lowndes'
- Carpet Knotweed 'Superba'
There are also a few other cultivars that are also predestined for cultivation in the garden. Basically, the varieties differ in their individual flower color, leaf color and growth habit. In some cases, the growth height also differs. Below is a brief overview.
| Types | Special Features |
|---|---|
| Carpet Knotweed 'Darjeeling' (Bistorta affinis 'Darjeeling') | • pink to dark purple flower spikes • lanceolate leaves • clump-forming habit • growth height of 15 to 20 cm |
| Carpet Knotweed 'Dimity' (Bistorta affinis 'Dimity') | • pink to dark purple flower spikes • dark green leaves, red autumn foliage • bushy, clump-forming habit • growth height of 15 to 25 cm |
| Carpet Knotweed 'Donald Lowndes' (Bistorta affinis 'Donald Lowndes') | • pink flower spikes • strong green leaves • flat growing • growth height from 10 to 25 cm |
| Carpet knotweed 'Kabouter' (Bistorta affinis 'Kabouter') | • red-white flower spikes • dark green leaves, red autumn foliage • low growing • growth height from 5 to 20 cm |
| Carpet Knotweed 'Superba' (Bistorta affinis 'Superba') | • strong pink flower spikes • dark green, lanceolate leaves • bushy to clump-forming habit • growth height of 15 to 30 cm |
Diseases and Pests
No particular signs of damage to the carpet knotweed are known.