Bristly lady's thumb.

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

Common problems

• Aphids — Small insects that cluster on new growth and suck plant sap.

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

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.