Knotweed, Giant

Fallopia sachalinensis

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Common look-alikes:

Identification

Appearance

Giant knotweed is an herbaceous perennial that is woody in appearance and can grow over 10 feet tall. Root extend deeply into the soil creating a dense impenetrable mat.

Foliage

Alternate, simple, dark green. Leaves are 6-14 inches long and have a heart-shaped base coming narrow to a point.

Flowers

Numerous small, greenish-white flowers appear in the leaf axils of the upper stems. Giant knotweed blooms have both male and female parts in the same flower.

Fruits

Fruits are papery and broadly winged. Each fruit contains a 3-sided achene that is small, shiny and brown. Small amounts of seed are viable and have no dormancy requirement.

Biology

Ecological Threat

Knotweeds are capable of quickly forming dense stands where they can crowd out native vegetation. Thickets can clog small waterways and displace streamside vegetation, increasing bank erosion and lowering the quality of riparian habitat for fish and wildlife. Once established, these stands are very difficult to eradicate.

Origin

Introduced to North America as an ornamental plant in the late 1800s.

Habitat

Meadows, fields, river banks, lake shore.

Life Cycle

Early in the season, new shoots can grow three to four inches per day. Knotweed grows three to 12 feet tall. The plant’s greenish white flowers are functionally unisexual...Blooms are up to 4” long and occur during August-October.

Management Options

This species is considered a watch list species.

Mechanical Control

The key to successful knotweed management is controlling the rhizomes. Manual and Mechanical Mechanical methods alone are largely ineffective. It may be possible to grub or pull single plants if they are not well established and soil conditions allow for complete rhizome removal. Small portions of the rhizome system not removed have the potential to resprout. The herbaceous stems of knotweed can be cut or mowed quite easily. Cutting alone will not control the plant but when performed after June 1 will significantly reduce the height of the regrowth.

Chemical Control

Be careful not to damage or kill nearby native plants when conducting management work.
Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.

Chemical Several herbicides, such as glyphosate, are effective in controlling this species. If the plants grow in a wetland, be sure to use an aquatic approved herbicide. Check label directions and state requirements. Foliar herbicide applications made after July 1 and before the first killing frost are most effective at injuring the rhizomes. During this time of year carbohydrates produced in the leaves are moved to the rhizomes for growth and storage. Foliar-applied herbicides move through the plant with the carbohydrates.