Knotweed.

At a glance

Care level

Easy

Plant type

Perennial

Light

Full sun to partial shade

Indoor light

Bright indirect

Water

Every 3 to 7 days

Mature size

1 to 4 feet

Growth rate

Fast

Hardiness

USDA 4 to 9

Bloom time

Mid summer to mid autumn

Care

Keep the soil consistently moist, as these plants really dislike drying out completely. Watch for wilting leaves, which is a clear sign they need a drink. They appreciate a spot with plenty of room to spread out.

Watering

Keep the soil evenly moist but not waterlogged.

Feeding

Apply a balanced liquid fertilizer during the active growing season.

Pruning

Cut back dead foliage in late winter or early spring to encourage fresh growth.

Repotting

Repot when the roots fill the container or the plant becomes top heavy.

Soil

Moist, fertile, well draining soil

Soil pH

6.0 to 7.5

Temperature

-34 to 30°C

Humidity

40 to 70%

Safety

People

The leaves may cause mild skin irritation in sensitive individuals due to the presence of oxalates.

Children

The plant is not considered toxic, but it is best to keep children from eating the foliage due to potential digestive upset.

Contact allergens

The sap can cause contact dermatitis in some people.

Airborne allergens

Pollen may trigger mild allergic reactions in sensitive individuals during peak bloom.

Edibility

Some species are edible, but many contain oxalic acid and should be consumed in moderation or avoided if you have kidney issues.

How to identify it

• Spike like flower clusters
• Lance shaped or ovate leaves
• Swollen nodes on stems
• Ocrea sheathing the stem base

Height

0.3 to 1.5m

Spread

0.45 to 1.2m

Leaves

green, deciduous

Flowers

pink, white, red, 0.5 to 1cm

Problems to watch for

• Japanese beetles · These beetles can skeletonize leaves during the summer months.
• Leaf spot · Fungal spots may appear on foliage during periods of high humidity.

Growing it

Propagation

division, seed, cuttings

When to plant

spring, autumn

Plant them near water features or in rain gardens where their moisture needs are naturally met.

Grows well with

• Hostas · They share a love for moist, shady conditions.
• Ferns · Their textures contrast beautifully with the lance shaped leaves of Persicaria.

Similar species

• Polygonum · They are closely related and share similar growth habits.

Science

Family

Polygonaceae

Genus

Persicaria

Order

Caryophyllales

Class

Magnoliopsida

Native range

Temperate regions of the Northern Hemisphere.

Conservation

Not of conservation concern.

These plants often colonize moist, disturbed areas and provide essential cover for small insects.

Value to wildlife

nectar source

Pollinators

bees, butterflies

Uses

ground cover, pollinator garden, border plant, wetland restoration

Taxonomy resolved against GBIF.

Questions people ask

Are they invasive?

Some species like Persicaria wallichii can be aggressive, so check your local invasive list.

Do they need full sun?

Most prefer partial shade, though some tolerate full sun if the soil stays moist.

Where the name comes from

The name Persicaria comes from the Latin word for peach, referring to the leaf shape, while the common name knotweed refers to the swollen nodes on the stems.

Good to know

• They are commonly called knotweeds.
• Many species thrive in damp soil where other plants struggle.
• The flowers are often described as looking like tiny bottle brushes.