Polygonum arenastrum
Encyclopedia
Polygonum arenastrum is a summer annual
weed
that is native to Europe and can be found on most continents as an introduced species
and a common noxious weed. Known by many other names, such as mat grass, oval-leaf knotweed, stone grass, wiregrass, and door weed, as well as many others, knotweed was first seen in North America
in 1809 and is now seen throughout all of the United States
and Canada
.
, and survives drought
conditions. The numerous wiry stems grow out from the center of the plant giving it a mat-like appearance. The stems are very long and branching and grow in a zigzag form. The leaves are alternate, oblong, hairless, can be ½ to 2 ½ inches long, and are bluish-green in color. The leaf stalk is short and closely surrounded by papery stipule
s on each node. The stem nodes are slightly swollen and look somewhat like “knots,” thus its common name, knotweed. Flowers, with colors ranging from white to green, are inconspicuous, have no petals, and grow all along the stems. The sepal
s, however, are pinkish with white edges. Seeds are egg-shaped, dark reddish brown in color, and have a smooth shiny surface.
Some people mistake knotweed for Spotted spurge
. An easy way to tell the difference is to break off a stem. Spurges contain milky sap; knotweed does not.
Since common knotweed does well in compacted soils, it is most commonly found in driveways, along roadsides and vacant lots, and in walkways, as well as in turf grass (where there is a lot of footpath wear) and in gardens.
Knotweed is a weed of “waste areas” but has been showing up in grain fields, where is becomes problematic because of its prolific seed production and long viability. It is also a host for other weeds such as beet curly top, and for viruses such as beet ring spot and powdery mildew
.
To prevent this weed, the soil should be aerated and well-drained. Young weeds should be pulled before they set seed.
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...
weed
Weed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
that is native to Europe and can be found on most continents as an introduced species
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
and a common noxious weed. Known by many other names, such as mat grass, oval-leaf knotweed, stone grass, wiregrass, and door weed, as well as many others, knotweed was first seen in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
in 1809 and is now seen throughout all of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
Appearance
Knotweed develops a deep tap root (it can be as deep as 18”), does well in compacted soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
, and survives drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
conditions. The numerous wiry stems grow out from the center of the plant giving it a mat-like appearance. The stems are very long and branching and grow in a zigzag form. The leaves are alternate, oblong, hairless, can be ½ to 2 ½ inches long, and are bluish-green in color. The leaf stalk is short and closely surrounded by papery stipule
Stipule
In botany, stipule is a term coined by Linnaeus which refers to outgrowths borne on either side of the base of a leafstalk...
s on each node. The stem nodes are slightly swollen and look somewhat like “knots,” thus its common name, knotweed. Flowers, with colors ranging from white to green, are inconspicuous, have no petals, and grow all along the stems. The sepal
Sepal
A sepal is a part of the flower of angiosperms . Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud...
s, however, are pinkish with white edges. Seeds are egg-shaped, dark reddish brown in color, and have a smooth shiny surface.
Some people mistake knotweed for Spotted spurge
Spurge
Euphorbia is a genus of plants belonging to the family Euphorbiaceae. Consisting of 2008 species, Euphorbia is one of the most diverse genera in the plant kingdom, exceeded possibly only by Senecio. Members of the family and genus are sometimes referred to as Spurges...
. An easy way to tell the difference is to break off a stem. Spurges contain milky sap; knotweed does not.
Since common knotweed does well in compacted soils, it is most commonly found in driveways, along roadsides and vacant lots, and in walkways, as well as in turf grass (where there is a lot of footpath wear) and in gardens.
Germination and spread
Common knotweed germinates at or near the soil surface in early spring, as soon there is enough moisture, and grows in an upright position before it spreads out like a mat. If mowed, it will still spread and can form a mat as big as 3 to 4 feet in diameter. The seeds, which grow low to the ground, survive mowing and are not disturbed when stepped on by people and animals.Knotweed is a weed of “waste areas” but has been showing up in grain fields, where is becomes problematic because of its prolific seed production and long viability. It is also a host for other weeds such as beet curly top, and for viruses such as beet ring spot and powdery mildew
Powdery mildew
Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the...
.
Eradication
In the home garden, Common knotweed is easy to get rid of by hand pulling. Covering garden beds with landscaping fabric and then mulch is a good way to keep knotweed from taking over. If the seeds do germinate in the mulch cover, they are easy to remove. Herbicides are also an option. Premergent herbicides, those that prevent the seed from germinating, are effective, but the area must first be free of weeds for this type of herbicide to work. Post emergent herbicides, those that kill the growing plant by spraying the green leaves of the plant, may be effective if applied when the plant is very young.To prevent this weed, the soil should be aerated and well-drained. Young weeds should be pulled before they set seed.