Persicaria amplexicaulis 'Firetail', Red knotweed
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Perennials

Persicaria amplexicaulis 'Firetail'

Red knotweed

Polygonaceae

Himalayas (Pakistan, India, Nepal, Bhutan), southwestern China, central Asia

At a Glance

TypePerennial
HabitClumping
FoliageDeciduous
Height36-48 inches (90-120 cm)
Width36-48 inches (90-120 cm)

Growing Zones

USDA Hardiness Zones

4 - 8
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
These zones indicate the coldest temperatures this plant can typically survive.
Frost Tolerancehardy

Key Features

Attracts Pollinators
Attracts Butterflies
Deer Resistant
Maintenancevery low

Overview

A vigorous, clump-forming perennial in the family Polygonaceae, a cultivar of Persicaria amplexicaulis native to the Himalayas, China, and central Asia, growing in moist mountain meadows, woodland margins, and stream banks. 'Firetail' forms bold, spreading mounds of large, lance-shaped to ovate, corrugated leaves 4–8 inches (10–20 cm) long on sturdy, branching stems 36–48 inches (90–120 cm) tall. From July through October — one of the longest bloom seasons of any hardy perennial — the plant produces a continuous succession of slender, tapering flower spikes 2–3 inches (5–8 cm) long densely packed with tiny deep crimson-red flowers. The vivid, nearly pure red color of 'Firetail' is among the most saturated reds available in hardy perennials. Plants spread steadily by short rhizomes but are not invasive; the bold foliage and long bloom season make this one of the most reliable and floriferous perennials for the Pacific Northwest, where the cool, moist summers suit it perfectly. Non-toxic.

Native Range

Persicaria amplexicaulis (the parent species) is native to the Himalayas (Pakistan, India, Nepal, Bhutan), southwestern China, and central Asia, growing in moist mountain meadows, open woodland margins, and stream banks at elevations of 6,000–14,000 feet (1,800–4,300 m).

Suggested Uses

Planted in mixed perennial borders, woodland edges, and rain gardens at 36–48 inch (90–120 cm) spacing. The deep crimson flower spikes combine dramatically with yellow or orange late-season perennials (rudbeckia, helenium, kniphofia) and ornamental grasses. Effective as a bold specimen in moist, partially shaded positions. One of the most reliable and long-blooming perennials for the Pacific Northwest, particularly in the cool, moist conditions west of the Cascades.

How to Identify

Identified by large, lance-shaped to ovate, corrugated leaves with stem-clasping bases on sturdy branching stems, producing a continuous succession of slender, tapering spikes 2–3 inches (5–8 cm) long densely set with tiny deep crimson-red flowers from July through October. The long, vivid crimson tapering spikes over bold corrugated foliage for 3–4 months of bloom is characteristic of this cultivar.

Appearance

Size & Dimensions

Height3' - 4'
Width/Spread3' - 4'

Colors

Flower Colors

red
crimson

Foliage Colors

green

Fall Foliage Colors

yellow
orange

Bloom Information

Bloom Period

~16 weeks
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SummerFall
Blooms July through October in zones 4–8 — approximately 16 weeks of continuous flower production, one of the longest bloom seasons among hardy perennials. New flower spikes emerge continuously from the base throughout the season. No deadheading is required to maintain bloom. Peak flower production is in August and September; the display continues until hard frost. In the Pacific Northwest, bloom persists reliably until mid-October.

Detailed Descriptions

Flower Description

deep crimson-red; slender, tapering bottlebrush spikes

Foliage Description

medium to dark green, lance-shaped to ovate, large, corrugated, stem-clasping

Growing Conditions

Sun Requirements

Full Sun
Partial Shade
Requires 3-6 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 7.0(Neutral)
357912
Soil Types
loamclay
Drainage
moist

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

1-2 years

Care & Maintenance

Care Guide

Plant in full sun to partial shade in consistently moist, fertile, humus-rich soil with a pH of 5.5–7.0. Requires consistent moisture — performance declines significantly in dry conditions. Tolerates clay soils and occasional waterlogging. Best performance in cool, moist Pacific Northwest summers where it blooms for 4 months. Space at 36–48 inches (90–120 cm) — plants spread steadily by short rhizomes. Divide every 3–4 years in early spring to control spread and maintain vigor. No staking required; stems are sturdy.

Pruning

Cut all stems to ground level in fall after frost or in late winter before new growth emerges. No deadheading needed during the growing season — spent spikes can be removed for tidiness, but new spikes emerge continuously without intervention. Divide every 3–4 years in early spring by lifting the clump and separating rhizome sections; replant vigorous outer portions at 36 inch (90 cm) spacing.

Pruning Schedule

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fallearly spring

Maintenance Level

very low

⚠️ Toxicity Warning

Non-toxic