Yes, it spreads aggressively by seed and is considered invasive in parts of North America.

Pull the plants before they set seed to prevent further spread.

Plant guide

Bristly lady's thumb.

Persicaria longiseta

Pronounced per-si-KAIR-ee-uh lon-ji-SEE-tuh

Also known as Asiatic smartweed, Asiatic waterpepper, Bristly Lady's Thumb, Oriental Lady's Thumb, Oriental Smartweed, bristled knotweed, bristly lady's-thumb, bunchy knotweed

This hardy plant is a common sight in damp, disturbed areas. It produces lovely spikes of pink flowers that add a nice touch to your garden. You will find it is quite adaptable to different soil conditions.

At a glance

Care level: Easy
Plant type: Annual herb
Light: Full sun to partial shade
Indoor light: Bright indirect
Water: Every 2 to 4 days
Mature size: 1 to 2 feet
Growth rate: Fast
Hardiness: USDA 5 to 9
Bloom time: Mid summer to late autumn

Care

Keep the soil consistently moist to mimic its natural damp habitat. Watch for wilting if the soil dries out too much. It is a vigorous grower so keep an eye on it to make sure it does not crowd out other plants.

Watering: Keep the soil damp at all times.
Feeding: Use a balanced liquid fertilizer during the active growing season.
Pruning: Trim back spent flower spikes to encourage more blooms.
Repotting: Since it is an annual, you will typically replace it each season.
Soil: Moist, fertile soil
Soil pH: Neutral to slightly acidic
Temperature: -10 to 35°C
Humidity: 40 to 80%

Safety

People: There are no reports of significant toxicity to humans.
Children: The small seeds and stems pose a minor choking hazard if ingested by small children.
Contact allergens: The stems can cause mild skin irritation in sensitive individuals.
Airborne allergens: Pollen may cause mild reactions in those prone to seasonal allergies.
Edibility: This plant is not considered edible and should not be consumed.

How to identify it

Pink flower spikes
Bristly leaf sheaths
Lance shaped green leaves
Reddish stems
Small dark seeds

Height: 0.2 to 0.6m
Spread: 0.3 to 0.9m
Leaves: green, deciduous
Flowers: pink, magenta, 0.5 to 1cm
Fruit: black, Autumn

Problems to watch for

AphidsCommon
Leaf SpotUncommon
Powdery MildewUncommon
Slugs and SnailsUncommon

By symptom

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Growing it

Propagation: seed
When to plant: spring

Pair this plant with other moisture loving species to create a lush, low maintenance border.

Grows well with

Joe Pye Weed · They share similar moisture needs and provide a nice height contrast.
Cardinal Flower · Both thrive in the damp conditions this plant prefers.

Similar species

Lady's Thumb · It has a similar growth habit but features a dark spot on its leaves.

Science

Family: Polygonaceae
Genus: Persicaria
Order: Caryophyllales
Class: Magnoliopsida
Native range: Eastern Asia
Conservation: It is considered an invasive weed in many regions outside its native range.

It colonizes open, moist, and disturbed habitats by producing vast quantities of easily dispersed seeds.

Value to wildlife: nectar source
Pollinators: bees, butterflies
Uses: ground cover, pollinator garden, naturalized areas

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 2/9
Soil reaction: 5/9
Light: 6/9
Temperature: 5/9
Moisture: 7/9
Nutrients: 5/9

Taxonomy resolved against GBIF.

Questions people ask

Is it invasive?: Yes, it spreads aggressively by seed and is considered invasive in parts of North America.
How do I control it?: Pull the plants before they set seed to prevent further spread.

Where the name comes from

The name Persicaria comes from the Latin word for peach, referring to the leaf shape, while longiseta refers to the long bristles on the stem sheaths.

Good to know

It is often called bristly knotweed due to the stiff hairs on its stem sheaths.
The plant thrives in damp, disturbed soil like riverbanks or garden beds.
It can produce thousands of seeds in a single season.

Keep your bristly lady's thumb growing

This page is a general guide. Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked.

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This guide was generated with AI on September 26, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.