Topics on this page:
- About knotweed
- Identification
- How to locate
- Phenology overview
- Phenophases - "show me what to look for"
About Japanese knotweed
Japanese knotweed is an invasive plant in North America. It grows quickly and reproduces vegetatively, meaning that escaped plant fragments (e.g., a piece of root) can grow into clone plants that colonize new areas and form dense leafy thickets. Its stems and rhizomes take advantage of cracks in concrete and other hard materials. This aggressive growth can compromise the integrity of bridges, drains, retaining walls, building foundations, and more. Knotweeds can form dense stands that degrade natural habitats and displace native plants.
Identification
Japanese knotweed is a perennial herbaceous plant that can appear shrub-like and grow up to 10 feet tall. It's hollow, bamboo-like stems are green with reddish nodes, become tough with age, and have multiple branches. The leaves are simple, alternate, and measure up to 6 inches long by 4 inches wide. The leaf shape is broadly ovate with a pointed tip and square base. Plants produce flowers in creamy whitish clusters at the upper leaf axils in late August and September and can produce small 3-angled black-brown papery fruit.
Be aware of look-alike plants:
- In Minnesota, common knotweeds include Bohemian, Japanese, and Japanese var . compacta (also called dwarf knotweed”). Giant knotweed is rare in this region. All four plants are Noxious Weeds on Minnesota’s Prohibited-Control list. Pesky Plants staff can advise you on how to collect data, no matter what kind of knotweed you observe.
- Giant knotweed plants are much taller than Japanese knotweed and have significantly larger, thinner leaves with heart-shaped bases.
- Bohemian knotweed is a hybrid between Japanese knotweed and giant knotweed. It is generally taller and has larger leaves with more heart-shaped bases.
- Var. compacta (i.e., "dwarf") Japanese knotweed is shorter in height (up to 3 feet) and has smaller, rounder, thicker leaves. See examples of var. compacta on iNaturalist by observers dintherius, mary_g, matthew6416, spins, and ecokyle.
Identification resources:
- Kinds of knotweeds and Pesky Plant Trackers (PDF)
- Japanese knotweed profile page by Minnesota Department of Agriculture
- Japanese knotweed profile page by Minnesota Department of Natural Resources
- Japanese knotweed identification training module by Midwest Invasive Species Information Network
- Minnesota Noxious Weeds Book (PDF, pages 24 and 25, Minnesota Department of Transportation)
- Biology, Ecology and Management of Knotweeds (PDF by Montana State University)
Locate a knotweed plant to observe
Proximity: As a volunteer, you will want to locate and track Japanese knotweed close to where you live, work, or regularly visit. Planning ahead will make future observations easy. Below are tips and tools to help you search your local environment for pesky plants to track.
Access to sites: Japanese knotweed often grow on public domain lands, such as edges of sidewalks, bike paths, and roads. Exercise caution and sound judgement when observing plants on public domain lands. You can also locate these plants growing on public lands, such as parks and nature reserves. Follow these guidelines for getting permission to make observations on public lands. Do not enter others' private property without permission.
Habitat:
- Japanese knotweed is tolerant of partial shade.
- Look for it in landscaped areas, where Japanese knotweed is sometimes intentionally planted as an ornamental hedge.
- Also, check roadsides, streambanks, and riverbanks because fragments can be transported by people moving soil and by waterways.
Kinds of knotweed: Some observers may not be able to locate Japanese knotweed but will instead locate a "dwarf knotweed" (var. compacta) or a hybrid (Bohemian) knotweed. Observations of these plants are also valuable. To learn how to observe knotweeds other than Japanese knotweed, see the "About knotweeds" section of the FAQ page.
Maps: Special maps (such as EDDMapS, below) let you see where Japanese knotweed has already been reported.
- Tip: Zoom into your neighborhood to see if Japanese knotweed has already been reported.
- Tip: Filter the map to view only “positive” reports.
- Tip: Check the date of reports and prioritize recent reports.
- To view the map in a new window, use this link: https://www.eddmaps.org/distribution/viewmap.cfm?sub=19655
EDDMapS: Japanese knotweed reports
EDDMapS: Bohemian knotweed reports
Phenology
Phenophases for Japanese knotweed are available on the species profile page by USA-NPN.
- Leaves appear as early as mid-March. Leaves persist through the fall and turn a rusty color as conditions become cold. It is common to see dead leaves persist on the plant through winter.
- Flowers appear in about August and can be active through September.
- Fruits appear in about late-September and persist on the plant through fall, sometimes into winter.
Volunteers collect phenophase data using Nature's Notebook (mobile app, website, or paper datasheets).
Knotweed phenophases
The videos and text below show what to look for when assessing phenophases in knotweed.
Initial growth: New growth of the plant is visible after a period of no growth (winter or drought), either from above-ground buds with green tips, or new green or white shoots breaking through the soil surface. Growth is considered "initial" on each bud or shoot until the first leaf has fully unfolded. For seedlings, "initial" growth includes the presence of the one or two small, round or elongated leaves (cotyledons) before the first true leaf has unfolded. (Source: USA-NPN)
Leaves: One or more live, fully unfolded leaves are visible on the plant. For seedlings, consider only true leaves and do not count the one or two small, round or elongated leaves (cotyledons) that are found on the stem almost immediately after the seedling germinates. Do not include fully dried or dead leaves. (Source: USA-NPN)
Flowers or flower buds: One or more fresh open or unopened flowers or flower buds are visible on the plant. Include flower buds or inflorescences that are swelling or expanding, but do not include those that are tightly closed and not actively growing (dormant). Also do not include wilted or dried flowers. (Source: USA-NPN)
Open flowers: One or more open, fresh flowers are visible on the plant. Flowers are considered "open" when the reproductive parts (male stamens or female pistils) are visible between or within unfolded or open flower parts (petals, floral tubes or sepals). Do not include wilted or dried flowers. (USA-NPN)
Fruits: One or more fruits are visible on the plant. For Fallopia japonica, the fruit is tiny and capsule-like, maturing to shiny black-brown, and is enclosed within remnant flower parts that become tan, papery "wings". (Source: USA-NPN)
Ripe fruits: One or more ripe fruits are visible on the plant. For Fallopia japonica, a fruit is considered ripe when its outer covering has turned tan, dry and papery. (Source: USA-NPN)
Recent fruit or seed drop: One or more mature fruits or seeds have dropped or been removed from the plant since your last visit. Do not include obviously immature fruits that have dropped before ripening, such as in a heavy rain or wind, or empty fruits that had long ago dropped all of their seeds but remained on the plant. (Source: USA-NPN)