HPIPM:Common knotweed

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Caryophyllanae
Order	Caryophyllales
Family	Polygonaceae
Genus	Polygonum

Scientific Name

Polygonum aviculare

Common Name

prostrate knotweed

Common knotweed (Polygonum aviculare L.)

Author: Joe Julian, compiled from the following sources:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7484.html

http://www.cdfa.ca.gov/phpps/ipc/weedinfo/polygonum-knotweeds.htm

http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artaug07/bj-small.html

http://www.illinoiswildflowers.info/weeds/plants/pros_knotweed.htm

Identification and Life Cycle

Common or prostrate knotweed, Polygonum aviculare, is also known as wiregrass, wireweed, matweed or doorweed, is a prostrate annual or short-lived perennial plant with numerous slender, wiry stems that are highly branched to form prostrate mats. However, in cultivated conditions it may grow slightly erect to 4 to 8 inches. Seedlings are initially upright with strap-shaped, embryonic or cotyledon first leaves that are 1/4 to 3/4 inch long. There is a single taproot that can penetrate to more than 18 inches. Leaves are bluish green in color with blades narrowly ovate in shape (about 1/5 to 4/5 inch in length). The leaf stalk is short and stem nodes are encircled by papery leaf stipules. These stem nodes are slightly swollen giving the typical “knot”-like appearance from which the common name is derived. Flowers are small and inconspicuous; they are borne in the upper leaf axils. The colors of the flowers range from white to green, and they may have a pinkish tinge. The seed is part of an achene or simple fruit that is three-sided, dark brown, not shiny, and about 1/8 inch long.

Common knotweed can be confused with spotted spurge in gardens and mowed areas. An easy way to distinguish them is by the white milky sap that is exuded from broken stems of spotted spurge.

The flowers of common knotweed have five sepals, 2 to 3 mm long, which are green at the center and white or pink-tinged in their outer area. Flowers contain variable numbers of stamens (the male pollen producing organs) and two or three pistils (the female pollen accepting organs). The brilliant red coloration of the sepals seen in closed blooms happens after fertilization. The red sepals then enclose the developing fruit.

Habitats

It is found in field crops, row crops, orchards, yards, gardens and turf. It is tolerant of soils compacted by trampling with foot traffic and is therefore frequently found along paths and walkways. This weed gets a good start with early moisture and establishes a taproot, which allows it to survive the summer drought.

Impacts

Common knotweed readily invades areas where other weeds may have difficulty surviving such as trampled and compacted areas. Examples include turf, roadsides, sports fields, vacant lots, gravel parking areas, gardens, agricultural crops, foot paths, and dirt roadways. It is particularly troublesome in alfalfa fields, where soil is compacted from wheel traffic. In turf it invades open areas caused by heavy wear. It is a host for the parasitic weed dodder and some disease organisms such as powdery mildew fungi. Predatory (beneficial) insects are attracted to the flowers which they use as a food source.

Biology and Ecology

Common knotweed germinates in late winter or early spring, when sufficient moisture is available. It often germinates in soil cracks in compacted soil. Though it germinates in early spring, it grows slowly and upright before becoming prostrate. If mowed, it remains prostrate and spreads. It can form mats 3 to 4 feet in diameter. It reproduces primarily by seed and by vegetative means with the help of long, stout rhizomes. Seeds develop on the plant low to the ground and seedlings readily survive mowing. Like other species in the genus Polygonum, seed in the soil are probably long-lived. Flowering may occur from March through October. Seeds can be transported to new sites by water, wind, as a contaminant in fill-dirt, or on the soles of shoes.

Biological Control

There are currently no registered biological control agents for use on any of the Polygonum species.

Mechanical and Cultural Control

One of the most important management methods is to prevent soil compaction, which provides the conditions under which this weed grows best. Arrange landscapes so that soil is less likely to become compacted. Spread out foot and vehicle traffic over a broader area. Use fences or hedges to reduce traffic and install rock or pavement pathways where traffic cannot be avoided. Do not trample areas soon after irrigation or rainfall. Arrange soccer fields and athletic areas so that heavily used areas such as goals, midfields, and sidelines can be rotated.

Aeration: Loosening the soil in lawns to provide better drainage and a better environment for more desirable species can be beneficial. If areas are compacted, loosen the soil and overseed with a locally adapted grass seed.

Mulches: A variety of mulches can be applied to planting beds and other landscaped areas to prevent establishment of common knotweed. Mulching with landscape fabrics can be effective if the fabric is overlapped and no light is allowed to penetrate to the soil. Use a polypropylene or polyester fabric or black polyethylene (plastic tarp) to block all plant growth. Rock or organic mulches such as bark or compost can be used over the top of synthetic fabrics. If used alone, organic mulches should be 3 to 4 inches thick. Finer mulch material is not desirable since weeds seed may easily grow in it. Coarser material will drain readily and reduce seedling establishment of common knotweed. Mulch needs to be replenished each year to maintain cover thickness and eliminate light penetration to the soil. Common knotweed is easy to remove with common weeding tools, such as a swivel hoe. For the home gardener, frequent manual removal along with mulching should be sufficient to manage this weed in most situations. Carefully digging out small clumps when discovered can prevent new colonies from establishing. However, rhizomes and fragments left in the ground or nearby can regenerate and spread infestations. Repeated cutting of stems (4 or more times per season), especially in conjunction with shading by black plastic or heavy shade cloth, depletes rhizome energy reserves and can help control infestations.

Prevent knotweed from producing seed by controlling young plants. This will reduce the amount of seed present in the soil in succeeding years.

Large colonies of this species are extremely difficult to dig up due to their high rhizome densities. Digging of large colonies is not recommended as it is very labor intensive and unlikely that all below ground material can be removed. Small patches may be dug, however care should be used in removing plant material as improper disposal can spread the species further. Repetitive cutting or mowing within a single growing season to deplete stored reserves and remove photosynthetic tissue has been effective. Eradication of the rhizome system is necessary for control of this aggressive invasive species.

Chemical Control

There are many herbicides that will control common knotweed, but they are not generally required in home garden situations. The selection of the herbicide is governed by the site and, should there be one, the crop. Remember that many of these herbicides can have negative effects on desirable plants as well and should be used with care, especially in a landscape situation.

For herbicide recommendations for specific grain rotations and weeds in Montana, please see the MSU Herbicide Chooser Tool.

Examples of herbicides that can be used to manage common knotweed

Always consult herbicide labels for additional rate, crop rotation restrictions, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Herbicide Active Ingredient, Trade Name	Mode of Action	Product per Acre	Application Time or Growth Stage
Rangeland and Pastures
Diglycolamine salt of 3,6-dichloro-o-anisic acid	Group 4 (synthetic auxins)
*Clarity		8-16 oz/A, 16-24 oz/A	Apply the lower rate to small, actively growing weeds and the higher rate for established weed growth.
Alfalfa
pendimethalin	Group 3 (Microtubule inhibition)
*Prowl H20		1-4 qt./A	Applications can be made in the fall after the last mowing/cutting, during winter dormancy, in the spring, or between cuttings. Applications should be made prior to the alfalfa reaching 6 inches in regrowth and prior to weed emergence.
Potatoes
pendimethalin	Group 3 (Microtubule inhibition)
*Prowl H20		1.5 - 3 pts./A depending on soil texture	For preemergence: apply after planting, but before potatoes and weeds emerge,or after dragoff. Early Postemergence: apply from crop emergence to the 6-inch stage of growth. DO NOT apply Prowl H2O postemergence if potatoes are under stress from cold/wet or hot/dry conditions or crop injury may occur.
Wheat and Barley
Diglycolamine salt of 3,6-dichloro-o-anisic acid''	Group 4 (synthetic auxins)
*Clarity		2-4 oz/A	Early season applications to fall-seeded grains must be made prior to the jointing stage.Early season applications to spring-seeded wheat must be made before wheat exceeds the 6-leaf stage. Apply 2 - 3 oz/A before spring-seeded barley exceeds the 4-leaf stage.
Spring and Winter Wheat
pendimethalin	Group 3 (Microtubule inhibition)
*Prowl H20		1.5-3 pts./A	As a postemergence treatment from the 1st-leaf stage of wheat until before the flag leaf is visible or emerged.Prowl H2O should be applied prior to weed emergence as emerged weeds will not be controlled by this treatment.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.Updated Nov 2008

References

For more information and images please visit IPM Bugwood: http://www.ipmimages.org/search/action.cfm?q=prostrate%20knotweed