Flower profile

Japanese knotweed

Reynoutria japonica

Introduced in the lower 48

Japanese knotweed is a wildflower that USDA PLANTS lists as introduced in the lower 48 states. It's on our flower lists for 11 states, and across the U.S. it's most often seen in bloom in August–October.

41states

11of our state lists

36,547U.S. sightings*

*iNaturalist research-grade observations, as of October 17, 2026.

On our lists for: Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont

🚫

Regulated invasive plant in Massachusetts, New York, Pennsylvania, Vermont, Connecticut, New Hampshire, Maine and Delaware. It's on those states' official invasive plant lists (Massachusetts: Prohibited plant (sale, import and propagation banned); New York: Prohibited invasive species (6 NYCRR § 575.3); Pennsylvania: Class B noxious weed; Vermont: Quarantine List, Tier 3; Connecticut: Prohibited invasive plant (some listings take effect on later dates; see the statute) (as Polygonum cuspidatum); New Hampshire: Prohibited invasive plant; Maine: Do Not Sell invasive plant (illegal to sell or import) (as Fallopia japonica); Delaware: Invasive Plant List (sale and propagation prohibited) (as Fallopia japonica)). Please don't plant or share it there. ^{MA Prohibited Plant List 6 NYCRR Part 575 PA noxious weed list Vermont Quarantine List CGS § 22a-381d NH Agr 3800 Maine Do Not Sell list 3 DE Admin. Code 806}

Quick look

Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a species of herbaceous perennial plant in the knotweed and buckwheat family Polygonaceae. Common names include Japanese knotweed and Asian knotweed. It is native to East Asia in Japan, China and Korea.
From Wikipedia's article Reynoutria japonica, CC BY-SA 4.0.

It belongs to the Polygonaceae family. ^iNaturalist
USDA PLANTS describes it as an herb (forb) / a subshrub with a perennial life cycle. ^{USDA PLANTS}
USDA PLANTS lists it as introduced in the lower 48 states and records it in 41 states. ^{USDA PLANTS}
In Massachusetts, most iNaturalist sightings of it in flower come in August–October. ^iNaturalist
In New York, most iNaturalist sightings of it in flower come in August–October. ^iNaturalist
iNaturalist has 6,440 research-grade sightings of it in Massachusetts, ranking #13 among the 110 flowers on our Massachusetts list. ^iNaturalist
iNaturalist has 6,145 research-grade sightings of it in New York, ranking #26 among the 109 flowers on our New York list. ^iNaturalist
iNaturalist has 3,998 research-grade sightings of it in Pennsylvania, ranking #46 among the 111 flowers on our Pennsylvania list. ^iNaturalist

State flower gift ideas

This one is a regulated invasive plant, so here are state flower gifts instead.

🌱 Sample spot. Handmade gift picks from independent shops will appear here once our shop partnership is approved. These cards are placeholders, not real products.

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State flower print
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Birth-flower necklace
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Wildflower seed favors
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Botanical tea towel
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Pressed-flower frame
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Flower-garden mug
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When does it bloom?

Flowering records by month across the U.S. (iNaturalist research-grade observations marked “Flowering”) (1,098 records)

Bloom time in each of our 11 states

State	Type	Peak bloom	Sightings*
Massachusetts	Introduced	August–October	6,440
New York	Introduced	August–October	6,145
Pennsylvania	Introduced	August–September	3,998
Vermont	Introduced	August–September	2,753
New Jersey	Introduced	August–September	2,597
Maryland	Introduced	June–October	1,638
Connecticut	Introduced	August–October	1,293
New Hampshire	Introduced	August–September	1,274
Maine	Introduced	August–September	1,032
Rhode Island	Introduced	few records	543
Delaware	Introduced	few records	140

Conservation status

Federal (U.S. ESA): Not listed No U.S. Endangered Species Act listing in the USFWS ECOS database. ^{USFWS ECOS}
NatureServe global: GNR Not yet ranked worldwide. ^NatureServe
Massachusetts: Not listed Not on Massachusetts Endangered Species Act list (321 CMR 10.90). NatureServe: SNA, not applicable (usually because it isn't native there) in Massachusetts. ^{MESA list}
New York: Not listed Not on New York protected native plants (6 NYCRR § 193.3). NatureServe: SNA, not applicable (usually because it isn't native there) in New York. ^{6 NYCRR § 193.3}
Pennsylvania: Not listed Not on Pennsylvania classified native plants (17 Pa. Code Ch. 45, Subchapter B, as amended 2023). NatureServe: SNA, not applicable (usually because it isn't native there) in Pennsylvania. ^{17 Pa. Code Ch. 45}
Vermont: Not listed Not on Endangered and Threatened Plants of Vermont (10 V.S.A. chap. 123; list of Feb 10, 2022). NatureServe: SNA, not applicable (usually because it isn't native there) in Vermont. ^{Vermont E&T list}
New Jersey: Not listed Not on New Jersey Endangered Plant Species List (N.J.A.C. 7:5C-5.1). NatureServe: SNA, not applicable (usually because it isn't native there) in New Jersey. ^{N.J.A.C. 7:5C-5.1}
Maryland: Not listed Not on Maryland threatened and endangered plants (COMAR 08.03.08.05, .06 and .08). NatureServe: SNA, not applicable (usually because it isn't native there) in Maryland. ^{COMAR 08.03.08}
Connecticut: Not listed Not on Connecticut Endangered, Threatened and Special Concern Plants (RCSA 26-306; 2015 list). NatureServe: SNA, not applicable (usually because it isn't native there) in Connecticut. ^{CT E/T/SC list}
New Hampshire: Not listed Not on New Hampshire Official Rare Plants List (Ncr 312, eff. Jan 1, 2020). NatureServe: SNA, not applicable (usually because it isn't native there) in New Hampshire. ^{NH Ncr 312}
Maine: Not listed Not on Maine Rare Plant List (Maine Natural Areas Program, 2021). E and T are Maine's official endangered and threatened plants (12 M.R.S.); SC and PE are tracking categories. NatureServe: SNA, not applicable (usually because it isn't native there) in Maine. ^{Maine rare plant list}
Rhode Island: Not listed Not on Rhode Island Rare Plants 2016 (RI Natural History Survey, from the RI Wildlife Action Plan; informational, Rhode Island has no state plant-protection listing law). NatureServe: SNA, not applicable (usually because it isn't native there) in Rhode Island. ^{RI rare plants 2016}
Delaware: Not listed Not on Rare and Uncommon Plants of Delaware (DNREC, 2024; informational, Delaware has no state plant-protection list). NatureServe: SNA, not applicable (usually because it isn't native there) in Delaware. ^{DE rare plants 2024}
Rare elsewhere: No other U.S. state ranks it S1, S2, SH or SX.

Checked October 17, 2026. Rankings and lists can change, so follow the links for the latest.

Can you eat it?

For peopleEdible (per NC State Extension)

NC State Extension notes: “Young stems are edible with a sour rhubarb flavor.” ^{NC State Extension}

For petsNot well documented

It isn't on the ASPCA plant list, and NC State Extension doesn't flag it for pets.

🌿 Never eat a wild plant unless you're certain of its ID; this isn't medical advice. Worried about a person? Call Poison Control at 1-800-222-1222. Worried about a pet? Call your vet.

Where does it grow?

USDA PLANTS records Japanese knotweed in 41 states plus Washington, D.C.

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recorded by USDA PLANTS on our state lists not recorded

See the full list of states

Alaska, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Hampshire, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin; District of Columbia.

Massachusetts counties with USDA records: Barnstable, Berkshire, Dukes, Essex, Franklin, Hampden, Hampshire, Middlesex, Nantucket, Norfolk, Plymouth, Suffolk, Worcester.

New York counties with USDA records: Albany, Bronx, Broome, Cattaraugus, Cayuga, Chautauqua, Chemung, Delaware, Dutchess, Erie, Greene, Kings, Monroe, Nassau, New York, Niagara, Oneida, Onondaga, Orange, Putnam, Queens, Rensselaer, Richmond, Rockland, Schenectady, Seneca, Steuben, Suffolk, Ulster, Warren, Washington, Westchester.

Pennsylvania counties with USDA records: Allegheny, Armstrong, Beaver, Bedford, Berks, Blair, Bradford, Bucks, Butler, Cambria, Cameron, Carbon, Centre, Chester, Clarion, Clearfield, Clinton, Crawford, Cumberland, Dauphin, Delaware, Erie, Jefferson, Lackawanna, Lancaster, Lehigh, Luzerne, McKean, Mercer, Mifflin, Monroe, Montgomery, Northampton, Philadelphia, Schuylkill, Somerset, Tioga, Venango, Warren, Washington, Wayne, Westmoreland, York.

More Massachusetts flowers

American jumpseed

Persicaria virginiana

Native

🌼 Blooms: July–September

Low smartweed

Persicaria longiseta

Introduced

🌼 Blooms in MA: June–October

Mile-a-minute weed

Persicaria perfoliata

Introduced

🌼 Blooms: June–October

Sources

USDA, NRCS. PLANTS Database, profile POCU6.
iNaturalist, Reynoutria japonica: taxonomy, research-grade observation counts and “Flowering” annotations, retrieved October 17, 2026.
Wikipedia, Reynoutria japonica (lead quoted under CC BY-SA 4.0).
Massachusetts: Massachusetts Endangered Species Act list (321 CMR 10.90).
Massachusetts: Massachusetts Prohibited Plant List (MDAR) (Text from MDAR's 2023 scientific-name PDF (mass.gov blocks automated downloads); 2024-2026 additions listed on the mass.gov page: Alnus glutinosa, Pyrus calleryana, Miscanthus sinensis, Aralia elata, Rhodotypos scandens, Symplocos paniculata.).
New York: New York protected native plants (6 NYCRR § 193.3).
New York: New York prohibited and regulated invasive species (6 NYCRR Part 575).
Pennsylvania: Pennsylvania classified native plants (17 Pa. Code Ch. 45, Subchapter B, as amended 2023).
Pennsylvania: Pennsylvania Noxious Weed List (3 Pa.C.S. § 1519, PA Dept. of Agriculture) (PDA's page spells garlic mustard “Allaria petiolata”; we match it as Alliaria petiolata).
Vermont: Endangered and Threatened Plants of Vermont (10 V.S.A. chap. 123; list of Feb 10, 2022).
Vermont: Vermont Quarantine List, Regulated Introduced Plant Species Rule (eff. June 16, 2026).
New Jersey: New Jersey Endangered Plant Species List (N.J.A.C. 7:5C-5.1).
New Jersey: New Jersey prohibited invasive plant species (Invasive Species Management Act; propagation banned from Jan 1, 2027, sale from Jan 1, 2030).
Maryland: Maryland threatened and endangered plants (COMAR 08.03.08.05, .06 and .08).
Maryland: Maryland Prohibited Invasive Plant List (COMAR 15.06.04.06; MDA, updated July 2026).
Connecticut: Connecticut Endangered, Threatened and Special Concern Plants (RCSA 26-306; 2015 list).
Connecticut: Connecticut prohibited invasive plants (Conn. Gen. Stat. § 22a-381d).
New Hampshire: New Hampshire Official Rare Plants List (Ncr 312, eff. Jan 1, 2020).
New Hampshire: NH Prohibited Invasive Species List (Agr 3800, Table 3800.1).
Maine: Maine Rare Plant List (Maine Natural Areas Program, 2021). E and T are Maine's official endangered and threatened plants (12 M.R.S.); SC and PE are tracking categories.
Maine: Maine Do Not Sell invasive plant list (01-001 C.M.R. ch. 273).
Rhode Island: Rhode Island Rare Plants 2016 (RI Natural History Survey, from the RI Wildlife Action Plan; informational, Rhode Island has no state plant-protection listing law).
Delaware: Rare and Uncommon Plants of Delaware (DNREC, 2024; informational, Delaware has no state plant-protection list).
Delaware: Delaware Invasive Plant List (3 Del. Admin. Code § 806-3.0).
USFWS, ECOS species database (ESA listing check), October 17, 2026.
NatureServe Explorer, Reynoutria japonica (global and state ranks).
Edibility and toxicity: NC State Extension.
Photo: Steven Lamonde via iNaturalist · CC BY 4.0 · resized.