Flowers, Fruits, and Leaves
Japanese knotweed is a USA-NPN regional plant species. Regional species are ecologically or economically important but are distributed more locally than calibration species. The USA-NPN integrates these observations to understand better plant responses within the different geographic regions of the nation. In addition, this species is potentially invasive. Observations on its phenology will provide valuable information toward understanding its potential for spread and for its control. The NPN does not promote planting or cultivation of this or any invasive plant.
Japanese knotweed is an erect, multi-stemmed, perennial, herbaceous forb to subshrub plant growing 3 to 10 feet tall, and can form large patches with creeping underground stems. Its small, greenish-white to white flowers are densely clustered along showy, branched spikes, and have either male or female parts that occur on separate plants. Male flower stalks are mostly erect, and female flower stalks are drooping.
Japanese knotweed is found on disturbed sites in preferably moist habitats. It grows on a wide range of soil types and colonizes bare volcanic soils, including those high in sulfur in its native range. It is tolerant of shade, high temperatures, salinity, and drought. It is found near water sources, along stream and river corridors, low-lying areas, waste places, utility rights-of-way, and travel corridors, fence rows, and old homesites.
You should observe...
Here are the phenophases you should observe about this plant.
New growth of the plant is visible, 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.
In at least one location on the plant, a fully unfolded leaf is visible. For seedlings, consider only true leaves and do not count the cotyledons (one or two small, round leaves) that are found on the stem almost immediately after the seedling emerges.
One or more fresh flowers or flower heads (inflorescences) are visible on the plant. Flower heads include many small flowers that usually do not open all at once. Do not include wilted or dried flowers that remain on the plant, or heads whose flowers have all wilted or dried.
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 unfolded or open flower parts. Do not include wilted or dried flowers that remain on the plant.
One or more fresh fruits are visible on the plant.
One or more ripe fruits are visible on the plant.
|Recent fruit drop
One or more fresh 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.
If drought seems to be the cause of leaf senescence for a plant, please make a comment about it for that observation.
This species has separate male and female flowers. If you know whether the flowers you are observing are male or female (or both), please make a comment about it for that observation.
Note that individuals of this species with only male flowers will not produce fruit.
The shoots of Japanese knotweed are strong enough to grow up through concrete sidewalks and parking lots.
Gardens with this plant