Himalayan knotweed

Himalayan knotweed: how do you recognise it?

Habit

Low, creeping perennial about 20 cm high and 60 cm wide, forming mats by creeping rootstocks.

Foliage

Narrow, lance-shaped, 5–15 cm, dark green and glossy; bronze-red in autumn, partly evergreen in mild winters.

Flower

Thick, upright spikes of 5–8 cm made up of small flowers, from July to October.

Colour

First bright pink, then deeper pink to red as the flower ages.

Roots

Creeping, branching rootstocks with which the plant gradually forms a carpet.

Flowering: July–October · Plants: March–May and September–October · Pruning: March

Why grow it?

• A dense, low-maintenance ground cover that suppresses weeds.
• Long flowering from July to October with pink spikes that turn red.
• Bees visit the flowers when little else is in bloom.
• Lovely autumn colour: the foliage turns bronze-red.
• Suits sun or partial shade, along paths, over low walls and in rock gardens.

Himalayan knotweed: where does it grow best?

• Sun or partial shade; in full sun it needs more water.
• Ordinary garden soil: sand, loam or clay, acid to chalky. Humus-rich soil is ideal.
• Normally moist; in dry weather the leaf edges turn brown.
• Allow 30–40 cm spacing for a quickly dense carpet, or 50–60 cm for slower growth.

Light

Sun and partial shade

Soil type

Sand, loam or clay

Acidity

Not fussy

Moisture

Normal

Size

0.2 m tall, 0.6 m wide

Hardiness

Hardy (to -25 °C)

Goes well with

• Lady’s mantle: Soft yellow-green flowers and leaves contrast nicely with the pink spikes.
• Cranesbill: Blue or pink flowers in the same sunny to partly shaded spot.
• Ornamental grasses: Airy stems above the low, dense carpet.
• Japanese anemone: Flowers at the same time, in white and pink shades.

Himalayan knotweed: when and how to plant it?

Plant in March–May and September–October.

1. Plant in March–May or September–October, when the soil is neither frozen nor too wet.
2. Dig a hole big enough for the root ball and work some compost into the bottom.
3. Set the plant at the same depth as in the pot and firm the soil.
4. Keep 30–40 cm between plants; the carpet closes in two to three years.
5. Water in at once and keep the soil moist during the first season.
6. Lay a thin layer of compost or bark between the plants to suppress weeds until they meet.

Care

Water

In the first year weekly in dry weather. After that, only in long dry spells, especially in sunny spots.

Feeding

In spring a spadeful of compost or a little organic fertiliser.

Mulching

A thin layer of compost in spring keeps the soil cool and moist.

Deadheading

Not needed: the spikes stay attractive. Cut them off if they look untidy.

Dividing

Divide the plant every 3–4 years when the centre goes bare, and replant the healthy pieces.

Containing

Keep the carpet in check by cutting the edges with a spade in spring.

Tip: Plant it in partial shade or where the soil does not dry out: in full sun the leaf edges scorch. Cut the edges every spring to keep the carpet within bounds.

Himalayan knotweed: when to prune it?

Pruning months: March.

When: In March, before the new shoots start to grow.

How:

• Cut off dead foliage and dried spikes, and trim the carpet down to a few centimetres above the ground.
• After flowering you can cut off faded spikes if you want a tidy mat.
• Cut the edges with a spade to limit spreading.
• Remove bare or dried-out patches and replant healthy pieces there.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April or September–October: lift a clump, divide it with a spade or knife into pieces with roots and replant at once, 30–40 cm apart.

Cuttings

In June–July: take 8–10 cm tip cuttings or pieces of rooted stem. Pot them up in compost and keep them moist until rooted.

Himalayan knotweed: how does it get through the winter?

Hardy to −25 °C.

• Hardy down to about −25 °C: no protection is needed in Belgium.
• The foliage turns bronze-red and partly stays in place in mild winters; in hard winters it dies back.
• In a pot or container: stand it against a wall and do not let the roots dry out.

Diseases and pests

• Brown leaf edges: Mostly in dry weather or full sun. Water, mulch the soil and plant it in partial shade instead.
• Bare centre: Old plants go bare in the middle. Divide them and replant the healthy pieces.
• Spreading too far: It spreads by rootstocks. Cut the edges and pull out unwanted runners.
• Aphids: On young shoots in spring. Rinse off with water or let the ladybirds do the work.

Through the year

• March: Cut back to a few centimetres. Add compost. Plant or divide.
• April: Plant or divide. Cut the edges.
• May: Plant. Mulch.
• June: Take cuttings. Water in dry weather.
• July: Flowering begins. Water in dry weather.
• August: Full flower. Bees on the flowers.
• September: Flowering. Plant or move.
• October: Bronze-red autumn colour. Plant.
• November: Rest. Leave the foliage.
• December: Rest.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Persicaria comes from the Latin ‘persica’ (peach), after the peach-like leaves of some species.
• It was long called Polygonum affine and is also sold under the name Bistorta affinis.
• Knotweeds are related to buckwheat and rhubarb: they belong to the same family, the Polygonaceae.