Growing degree-days have been dramatically increasing over the last 3 weeks. With this has been the continuous emergence of different weed species. Some of the very first weed species that emerged this spring and many springs in the past belong to the smartweed (Polygonaceae) family. A brief review of the Polygonum species commonly found in Illinois may prove to be beneficial in the identification and subsequent control of these species. |
One key characteristic that all members of the Polygonaceae family have is that of swollen nodes. In fact, the genus name Polygonum means "many knees." These nodes are covered with a clear or whitish membranous sheath called an ocrea.
Ocrea pubescence and size often help distinguish many of the smartweed species from one another.
The first Polygonum species that people generally see emerge in the spring is prostrate knotweed (Polygonum aviculare L.).
Prostrate knotweed is a summer annual that is generally found on hard compacted soils or damaged areas. The growth habit of this species is low to the ground, hence the name prostrate knotweed. This species is often seen emerging through cracks in sidewalks and parking lots and is usually not too much of a problem in agronomic crops. However, some features that make this species tough to control are slender and wiry stems and leaves that are tiny and oblong, providing very little surface area for herbicide interception. Erect knotweed (Polygonum erectum L.) is another Polygonum species that is found in Illinois. It is very similar to prostrate knotweed, except it grows in a more upright fashion and has more of an oval-shaped leaf.
Other early-emerging, annual Polygonum species include Pennsylvania smartweed (Polygonum pensylvanicum L.)
and ladysthumb (Polygonum persicaria L.). These two species are very similar in appearance and are generally distinguishable from one another during early vegetative growth by examining the ocrea. Stems of Pennsylvania smartweed and ladysthumb are branched and can be green or reddish, and as with all species in the Polygonaceae family, they are swollen and jointed at the node. Leaves of these species are alternate and lanceolate to elliptical in shape, and often the center of both leaf surfaces is marked with a purple blotch. Many people feel that this purple blotch separates the two species since it resembles a thumbprint, (i.e., ladysthumb). However, this blotch can be seen on either species, and the distinguishing feature between Pennsylvania smartweed and ladysthumb is the pubescence on the top of the ocrea. Ladysthumb has a fringe of hairs at the top of the ocrea,
whereas Pennsylvania smartweed does not.
Both of these species can be problems in agronomic cropping systems since they can tolerate a range of soil types and conditions. Effective herbicides for controlling these species, especially in burndown situations, are dicamba-containing products, such as Clarity, Banvel, and atrazine. These herbicides are generally more effective on these species compared with 2,4-D and glyphosate. Remember there needs to be at least 1 inch of precipitation, followed by a 14-day (8 fl oz) or 28-day (>8 to 16 fl oz) planting interval if a dicamba product is used as a burndown prior to soybean planting.
The last common annual Polygonum species that we often encounter in Illinois is wild buckwheat (Polygonum convolvulus L.). Wild buckwheat is a weed of landscapes, orchards, and nurseries, and can be a problem in agronomic crops, especially small-grain crops. It usually grows in cultivated areas and is well adapted for a wide range of climates and soil types. Wild buckwheat generally emerges from mid-May through June and is a fast-growing viney species that can grow along the ground or intertwine itself around other plants. If not controlled, wild buckwheat can shade and/or strangle other plants and can often interfere with mechanical harvesting of the crop. Wild buckwheat has heart-shaped leaves
and may be confused with field or hedge bindweed. Two big distinguishing characteristics between wild buckwheat and the two bindweed species are (1) wild buckwheat is an annual and the bindweeds are perennials, and (2) since wild buckwheat is in the Polygonaceae family it has an ocrea that surrounds the stem at the base of each leaf, where the bindweeds do not.
There are four perennial Polygonum species that are often encountered in Illinois. These include swamp smartweed (Polygonum coccineum Muhl.), curly dock (Rumex crispus L.), red sorrel (Rumex acetosella L.), and Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.). Swamp smartweed resembles Pennsylvania smartweed and ladysthumb in appearance; however, it is usually much larger and is a perennial that can reproduce by long, creeping, woody rhizomes.
Because of this extensive root system it can be a strong competitor with other plants and is often difficult to control. Also because of this extensive root system, it is known to many farmers as devil's shoestring.
Another perennial in the Polygonaceae family that can be a problem in agronomic crops, especially in no-till situations, is curly dock. This is a taprooted perennial that develops a basal rosette of wavy-margined leaves.
Curly dock reproduces by seed, and the young leaves of the seedlings are generally egg shaped.
Perennial plants emerge from the taproot in mid-spring and can be controlled by tillage. Remember that since it is a member the Polygonaceae family, it will have an ocrea at the base of the leaves.
Red sorrel is a rhizomatous perennial that can be a problem in pastures that are not maintained. Identification is usually quite easy with this species due to its leaf shape: arrowhead shaped with two narrow and spreading basal lobes.
Red sorrel plants accumulate a high concentration of soluble oxalates, which give them a sour taste and can occasionally cause fatalities in livestock, particularly sheep.
The final Polygonum species that we will describe is Japanese knotweed. This species is also a rhizomatous perennial. It is very aggressive and fast growing and looks like a woody shrub at maturity.
Japanese knotweed was actually introduced as an ornamental in the late 1800s and has become an ecological threat in many parts of the United States because of its aggressive growth habit. It spreads primarily from rhizomes and is difficult to control by both mechanical and chemical means. Key characteristics to identifying Japanese knotweed are the hollow, bamboo-like stem, the thick rhizomes, and the joints on the stem surrounded by the ocrea.
This has been a brief overview of many of the Polygonum species that we encounter in Illinois. Remember that the key to controlling any of these species is proper identification. For further recommendations for control of these species refer to chapters 2 and 3 in the Illinois Agricultural Pest Management Handbook.--Christy Sprague and Aaron Hager