Lauren Lazaro, Donnie Miller, Christopher Mudge, Albert Orgeron, Daniel Stephenson and Ronald Strahan

Weeds are the largest economic threat to agriculture as a whole, whether in row crops, aquatics, rangelands or pastures. In Louisiana, the same conditions that help crops flourish also bring weeds. The long growing season, warm weather and high levels of precipitation allow weeds to emerge and set seeds over longer periods of time. A wide variety of soil types and agricultural systems also means a wide variety of weeds.

Following are some of the weeds that grow in Louisiana that are a threat not only to agricultural production but to landscapes and waterways.

Row Crop Production Systems

Divine nightshade (Solanum nigrescens) is a perennial broadleaf weed belonging to the black nightshade family. Divine nightshade has been identified in sugarcane fields in 19 of the 24 sugarcane-producing parishes. Divine nightshade develops into highly branched bushlike plants that measure up to 6 feet in height and width. The flowers are composed of five white petals arranged around yellow stamens and are found in clusters of four to 12. Upon fertilization, berry development begins. They are green but become blackish at maturation.

Italian ryegrass (Lolium mulitflorum), also known as annual ryegrass, is a winter annual grass that is primarily found in small grains, such as wheat. The leaves are rolled in the bud with hooked auricles in the collar region with a membranous ligule. Its leaves often have a glossy appearance on the lower surface. Italian ryegrass leaf sheaths are often red tinged at the base. This can distinguish Italian ryegrass from perennial ryegrass, which is similar in growth habit and appearance. In Louisiana, Italian ryegrass has resistance to glyphosate in 13 parishes and suspected resistance to paraquat in three parishes.

Itchgrass (Rottboellia chochinchinensis) is a major weed issue in corn, sugarcane and along roadsides but also in pastures and rights-of-way. It has been mainly a problem weed from Rapides Parish southward. This summer annual grass has an abundance of short, stiff hairs on the leaf sheaths, especially near the collar region, which can irritate human skin. The leaves are flat and taper to a point, which also have short hairs on the leaf surface. Itchgrass tillers profusely, causing it to be competitive with crops. One of the two spikelets is stalked and sterile, while the other is stalkless and fertile.

Nealley’s sprangletop (Leptochloa nealleyi) is an emerging summer annual grass weed in rice found along roadsides and in ditches or pastures in Louisiana. Nealley’s sprangletop has a fringed membranous ligule like that on Amazon sprangletop, another common rice weed. This erect and robust weed has flat culms and closely resembles vaseygrass.

Palmer amaranth (Amaranthus palmeri) is an erect summer annual weed commonly found in several cropping systems. This aggressive weed can be commonly confused with similar pigweed species, such as waterhemp or redroot pigweed. The leaves are alternate and are lance-shaped or egg-shaped. Leaf petioles are long in comparison to other pigweed species. Palmer amaranth has one central stem with several lateral branches. Small, green flowers are produced in dense, compact panicles. Male and female flowers are found on separate plants. In Louisiana, Palmer amaranth has resistance to glyphosate in 23 parishes.

Ragweed parthenium (Parthenium hysterophorous), or false ragweed, is a summer annual broadleaf weed that forms a basal rosette with finely lobed leaves. Plants then form a branched, hairy stem with lengthwise grooves. Leaves have hairs on the upper and lower leaf sides. Flowers are small with five white ray florets. Ragweed parthenium has been known to be toxic to mammals. The weed proliferates along roadsides and in both disturbed and undisturbed habitats. The plant has shown tolerance to glyphosate and other herbicides.

Aquatic Systems

Crested floating heart (Nymphoides cristata), or white snowflake, is a floating-leaved aquatic plant that has invaded the state during the past decade. It can be identified by slender bundles of tuberous roots on the underside of its floating leaves and showy white flowers that bear an erect fold of tissue that runs down the length of the upper side of the petal. Established rooted plants produce many heart-shaped leaves, and the plant will grow in water deeper than 10 feet.

Cuban bullrush (Oxycaryum cubense) is a perennial aquatic plant that is spreading across Louisiana. Its plants produce submersed and emergent triangular stems with long, narrow emergent leaves and has small, reddish rhizomes below the surface of the water. Cuban bullrush is capable of sexual and asexual reproduction. During initial colonization the plants use other floating or emergent aquatic plants for habitat before outcompeting other plants and forming large free-floating islands.

Turfgrass, Roadside and Pasture Systems

Cogongrass (Imperata cylindrica) is a summer perennial grass that can displace entire communities of native plants and wildlife. In Louisiana, cogongrass is most often found in St. Tammany and Washington parishes along roadsides and hay fields. Congongrass has bright green leaves with serrated margins and a distinctly off-center white midrib. Cogongrass leaves emerge directly from underground horizontal rhizomes. Rhizomes are white, segmented and covered with reddish-brown scales. Flowering occurs immediately after winter dormancy.

Chinese tallow (Triadica sebifera) is one of the most invasive weeds infesting pastures and pine forests in Louisiana. It’s a deciduous tree that can reach up to 60 feet in height. Its leaves are heart-shaped, and its trunk can grow to 3 feet in diameter with bark that is light grey and furrowed. Flower petals are not present, but the sepals are yellowish green in color. The seed capsules or fruit are three-lobed and release three white, waxy seeds. Mature seeds remain attached into the winter and resemble popped popcorn. Birds are the most effective method of seed dispersal.

Torpedograss (Panicum repens) is a perennial grass weed that infests turfgrass and landscape beds and spreads primarily through creeping rhizomes. Seeds produced by torpedograss are not viable. Torpedograss prefers wet soils but has good drought tolerance once established. It can tolerate frequent mowing but if left unmanaged can grow up to 3 feet in height. It has waxy silver-green leaves that are pointed at the tip. Torpedograss produces stolons and rhizomes, which are sharply pointed at the tip, resembling a torpedo.

Lauren Lazaro is an assistant professor in the School of Plant, Environmental, and Soil Sciences; Donnie Miller is a professor, at the Northeast Research Station, St. Joseph; Christopher Mudge is an adjunct professor in the School of Plant, Environmental, and Soil Sciences; Albert Orgeron is an associate professor at the Sugar Research Station, St. Gabriel; Daniel Stephenson is a professor at the Dean Lee Research Station, Alexandria; and Ronald Strahan is a professor in the School of Plant, Environmental, and Soil Sciences.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

Divine nightshade has been identified in sugarcane fields in 19 of Louisiana’s 24 sugarcane-producing parishes. Photo by Al Orgeron

Italian ryegrass, also known as annual ryegrass, is a winter annual grass that is primarily found in small grains, such as wheat. Italian ryegrass has resistance to glyphosate in 13 parishes and suspected resistance to paraquat in three parishes. Photo by Donnie Miller

Crested floating heart, or white snowflake, is a floating-leaved aquatic plant that has invaded the state during the past decade. Photo by Chris Mudge

Cuban bullrush. Photo by Chris Mudge

Chinese tallow and a torpedograss infestation. Photo by Ron Strahan