Germination temperature
Cool to mild with a day-night swing; 24 °C (75 °F) and above pushes seed back into dormancy.
Herb growing guide
Persicaria longiseta is an easy annual smartweed with slender pink flower spikes, native from Pakistan and the Himalaya to East Asia. Its seed is dormant when shed and needs a short cold, moist spell before sowing into cool, fluctuating spring conditions.
Cool to mild with a day-night swing; 24 °C (75 °F) and above pushes seed back into dormancy.
Shallow: on the surface or just covered.
No exact day count is published; expect a few weeks in cool weather.
No strict light requirement; full sun to part shade for plants.
Short cold-moist stratification, up to 6 weeks.
Direct sow, or sow in pots and transplant young.
Fresh smartweed seed is dormant. In a Japanese study of seven Persicaria species, all needed moist chilling to break dormancy, and Persicaria longiseta needed a shorter chilling period than most. Do not leave stratified seed to dry out before sowing: in field studies in Akita, air-drying the seed reduced seedling emergence.
Not needed. Dormancy is broken by cold and moisture, not by damaging the seed coat.
Recommended. Mix the seed with damp sand or vermiculite, seal it in a labelled bag and keep it in the refrigerator for 2-6 weeks. The study used chilling periods of up to 6 weeks, and this species was among the quickest to respond. In cold climates you can instead sow outdoors in autumn and let winter do it.
Not needed. Cold-moist stratification already wets the seed fully.
Sow into cool, mild weather. Germination of this species was speeded up by fluctuating temperatures, while holding seed at 24 °C (75 °F) or 30 °C (86 °F) sent all seven species back into secondary dormancy. On the Indian plains, sow at the end of the cool season; in the hills, sow in spring.
Sow on the surface or cover with a thin layer of fine soil. In the Japanese study, seedlings emerged from surface soil, and buried seed tends to remain dormant.
Any ordinary, moisture-retentive garden soil or seed mix. Smartweeds commonly grow on damp ground and lake shores.
Keep the surface moist until seedlings are established.
The study found no strict light requirement, so a light covering of soil is fine.
No exact day count is published. Expect seedlings within a few weeks of sowing stratified seed into cool weather; if the weather turns hot before they appear, resow next cool season.
Thin direct-sown seedlings, or move pot-raised seedlings while they are still small. Plant in sun or part shade in moist soil.
As an annual it grows, flowers and sets seed in one season. Water in dry spells; little feeding is needed.
Cause: unbroken dormancy, or seed held in warm conditions. Fix: stratify for up to 6 weeks in the refrigerator and sow into cool weather.
Cause: buried seed cycles in and out of dormancy. In the Akita study, germinability fell sharply each October and outdoor emergence dropped from September. Fix: sow in late winter or spring, not late summer.
Cause: the seed dried out after chilling. Fix: sow straight from the refrigerator while still damp.
Cause: abundant seed that can stay alive in soil for years. Buried seed of this species kept over 40% viability after 35 months in the Akita trials. Fix: cut off flower spikes before the seed ripens.
This species is listed as invasive in Maryland, USA, where it is non-native. Outside its native Asian range, grow it in containers and never discard seed heads in the wild.
In cold areas, an autumn outdoor sowing gives natural stratification and seedlings in spring.
Grow it at a pond margin, in a damp border or in a pot that is kept moist.
It looks very similar to water pepper, Persicaria hydropiper; check the long bristles on the leaf sheaths that give longiseta its name.
Give the seed a short cold spell and sow in spring for pink flower spikes all season.
Yes. Fresh seed is dormant, and research showed it needs moist chilling, though less than other smartweeds; up to 6 weeks in the refrigerator in damp sand is enough.
Warm conditions of 24 °C and above push the seed back into dormancy. Sow in the cool season or spring instead.
It can. The seed stays alive in soil for years and it is invasive in parts of North America. Remove flower spikes before they set seed.