Japanese knotweed

Japanese knotweed (Reynoutria japonica) is in the knotweed family family (Polygonaceae). Across the U.S., iNaturalist users have logged 1,069 research-grade flowering observations of it in our sample, peaking in September.

Nationally it blooms mainly from August to September, but timing varies a lot by state and elevation — see the table below.

When to photograph Japanese knotweed: observations by month (U.S.)

About Japanese knotweed

Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a large species of herbaceous perennial plant of the knotweed and buckwheat family Polygonaceae. It is commonly known as Asian knotweed or Japanese knotweed. It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats and is classified as an invasive species in several countries.From Wikipedia via iNaturalist — article, CC BY-SA.

Japanese knotweed by state

Main season = months with at least 30% of the state's peak-month count.

Related wildflowers (knotweed family)

California BuckwheatEriogonum fasciculatum

Low SmartweedPersicaria longiseta

Desert TrumpetEriogonum inflatum

Sulfur BuckwheatEriogonum umbellatum

American jumpseedPersicaria virginiana

Naked BuckwheatEriogonum nudum

American bistortBistorta bistortoides

Arrow-Leaved TearthumbPersicaria sagittata

Photography tips

• Photograph flowers on calm, overcast mornings to avoid wind blur and harsh shadows.
• Get low: a shallow depth of field (f/2.8–f/5.6) isolates a bloom against soft color.
• Stay on trails — trampling meadows kills plants and ruins next year's display.
• Try a kaleidoscope abstract of a single bloom with our online kaleidoscope tool.

FAQ