Weeds are plants that grow where they are not wanted. They compete with desired plants for water, nutrients, sunlight and space. Some weeds are worse than others, making control difficult and reducing crop yields. Knowing the worst weed species allows farmers and gardeners to focus control efforts on the most problematic plants. This article explores the very worst weeds and methods for identification and management.

The Worst Agricultural Weeds

Several weeds stand out as particularly problematic in agricultural settings:

Palmer Amaranth

Palmer amaranth (Amaranthus palmeri) has rapidly become one of the worst weed pests across the U.S. This aggressive plant prolifically produces seeds and can tolerate drought. It’s resistant to several herbicides and grows quickly, reducing crop yields.

Identification: Palmer amaranth grows as a reddish-colored stem with long, narrow oval leaves. It can reach over 7 feet tall with upright seed heads. Tiny, smooth seeds readily fall to the ground.

Control: Integrate cultural, mechanical and chemical control. Plant crop rows close together to shade out weeds. Hoe regularly when plants are small. Use herbicide rotation and mix multiple modes of action.

Canada Thistle

Canada thistle (Cirsium arvense) infests fields across North America. Its creeping roots allow rapid spread and make control difficult. Each plant can produce over 5,000 seeds that persist in soil. This aggressive perennial weed chokes out crops.

Identification: Canada thistle has spiny, lobed leaves with crinkled edges and round flower heads. It emerges in spring with a basal rosette before sending up flower stalks.

Control: Prevent spread by cleaning equipment. Persistently mow or till to stop seed production. Use systemic herbicides for long-term control.

Johnsongrass

A major pest across the southern U.S., Johnsongrass (Sorghum halepense) reproduces from seeds and rapidly spreading rhizomes. It’s prolific, drought tolerant and outcompetes crops, reducing yields. The vigorous root system makes control difficult.

Identification: Johnsongrass is a tall, coarse, perennial grass reaching over 6 feet. Flat, broadly-bladed leaves have a prominent white midrib. Reddish seed heads emerge from the top. Rhizomes sprout new plants.

Control: Use systemic herbicides, repeated mowing/tilling, or heavy mulches for several years. Prevent rhizome spread by cleaning equipment. Rotate herbicide modes of action.

Kochia

Kochia (Bassia scoparia) has spread across agricultural fields throughout North America. This tumbleweed readily develops herbicide resistance. In drought, the deep taproot and ability to abort seed allows survival. Heavy infestations smother crops.

Identification: Kochia is an annual, upright plant reaching 2-6 feet with many hairy branches. Leaves are green or gray-green, narrow and pointed. Inconspicuous green flowers form in leaf axils.

Control: Use cultivation and varied herbicide rotation when plants are small. Hand rogue before seed set. Prevent spread by cleaning equipment.

Horseweed

Also called marestail, horseweed (Conyza canadensis) is a rapidly-spreading annual weed across the U.S. Prolific seed production and herbicide resistance contributes to its invasion of crop fields, reducing yields.

Identification: Horseweed is an erect winter annual or biennial with multiple hairy stems. Leaves are alternate, hairy and pointed. Flower heads are small and white. Seeds are wind dispersed.

Control: Till before planting, use varied herbicide rotation, hand rogue, and prevent seed production.

Worst Agricultural Grass Weeds

Grassy weeds present unique control challenges in farm fields and orchards. Problematic grass weeds include:

Barnyardgrass

Barnyardgrass (Echinochloa crus-galli) is a summer annual grass weed widespread across North America. Prolific seed production allows rapid spread. Plants compete strongly with crops and reduce yields.

Identification: Emerging plants have a reddish hue. Mature plants reach 3-5 feet with open seed heads. Leaves are wide and flat with a rounded tip.

Control: Use pre-emergence herbicides in spring. Cultivate shallowly to reduce new seedlings. Hand rogue before seed set. Rotate herbicide modes of action.

Foxtail Species

Several foxtail (Setaria spp.) species aggressively invade agricultural fields. These annual grasses produce abundant seed on bristly spikes that can injure crop harvests. Deep root systems make control difficult.

Identification: Most foxtail species are clump grasses under 3 feet tall with bristly, vertical seed heads. Leaves are broad with hairy ligules. Seeds have a pointed tip.

Control: Rotate pre- and post-emergence herbicides. Cultivate to reduce new seedlings. Mow/hand rogue before seed set.

Crabgrass

Crabgrass (Digitaria spp.) is a summer annual grassy weed of agricultural fields and orchards worldwide. Fast growth and prolific seed heads quickly overwhelm crops. The low growth habit avoids shading by crops.

Identification: Crabgrass has spreading stems with flat leaves reaching 6-8 inches long. Characteristic crab-like seed heads sprout digitate spikes.

Control: Use pre-emergence herbicides in spring before seeds germinate. Shallow cultivation and mowing also help prevent establishment.

Quackgrass

Quackgrass (Elymus repens) is an aggressive, cool-season perennial grass. The extensive rhizome system readily invades fields, reducing crop yields. It’s difficult to control and spreads rapidly.

Identification: Quackgrass produces jointed, hollow stems with elongated seed heads. Leaves are bright green and hairy on the upper surface with a clasping auricle. Rhizomes are yellowish-white.

Control: Repeated tillage is needed to chop up rhizomes before planting crops. Use systemic herbicides in fall. Prevent spread by cleaning equipment.

Worst Aquatic Weeds

Several invasive aquatic plants can quickly take over ponds, lakes and slow-moving waterways, making boating, fishing and swimming difficult. Watch for these weeds:

Eurasian Watermilfoil

Eurasian watermilfoil (Myriophyllum spicatum) is one of the most problematic aquatic weeds across North America. Dense stands disrupt recreation, displace native plants, reduce fish habitat, and alter water quality.

Identification: Feathery, submerged leaves form dense stands extending to the water surface. Leaves have 12-21 slim leaflet pairs. Reddish stems branch profusely.

Control: Monitor for and remove new infestations. Use approved aquatic herbicides. Reduce nutrients and mechanically harvest. Grass carp eat this weed.

Hydrilla

Extremely invasive hydrilla (Hydrilla verticillata) has spread across the southern U.S. Rapid growth allows it to form thick mats on water surfaces, altering aquatic ecosystems. Fragments spread infestations.

Identification: Slender stems branch profusely underwater and grow up to 25 feet long. Pointed leaves are arranged in whorls of 4-8. Small white flowers appear on long stalks.

Control: Detect and treat new infestations. Use EPA-approved aquatic herbicides. Sterile grass carp provide long-term control. Dredge or mechanically remove plants.

Waterhyacinth

Waterhyacinth (Eichhornia crassipes) forms dense floating mats that clog waterways. Infestations severely limit boating, fishing and swimming access. It spreads rapidly by stolons and seeds.

Identification: Thick, waxy round leaves with inflated petioles form dense mats. Showy purple flowers rise on upright stalks. Plants interlace with black, spongy roots hanging subsurface.

Control: Hand pull small infestations. Use approved herbicides. Introduce weed-eating insects. Harvest physically with specialized boats.

Worst Invasive Terrestrial Weeds

Invasive weeds disrupt native ecosystems, increase fire risk and reduce biodiversity. Watch for these aggressive invaders:

Spotted Knapweed

Highly invasive spotted knapweed (Centaurea stoebe) has spread across the western U.S., forming dense stands on rangelands. It displaces native vegetation, reduces forage, and degrades wildlife habitat.

Identification: A biennial or short-lived perennial, knapweed sprouts rosettes before sending up 1-3 foot branched stems. Pink to purple flowers have dark spots on bract tips. Leaves are pale green and deeply lobed.

Control: Hand pull small infestations. Use biocontrol insects. For large infestations apply systemic herbicides; revegetate treated areas.

Purple Loosestrife

Wetland invader purple loosestrife (Lythrum salicaria) displaces native plants and reduces wildlife habitat across North America. Each plant can produce over 2 million seeds annually.

Identification: Showy flower spikes rise above clumps of downy, lance-shaped leaves. Flowers have five to seven petals emerging from tube-shaped bases. Stems are four-sided.

Control: Remove by hand in moist soil. Apply aquatic-approved herbicides at early growth stage. Release leaf-eating beetles as biocontrols.

Saltcedar

Saltcedar or tamarisk (Tamarix spp.) trees and shrubs have invaded riparian habitats across the U.S. Displacing native vegetation, they reduce wildlife habitat and increase fire risk and soil salinity.

Identification: Feathery, blue-green foliage and pink flowers appear on shrubby trees growing over 15 feet tall. Bark is reddish-brown and smooth when young becoming furrowed with age.

Control: Cut large stems and apply systemic herbicide to stumps. Use foliar sprays on resprouting plants. Remove debris and revegetate sites.

Worst Vine Weeds

Vining weeds spread quickly and climb over native vegetation and man-made structures. Watch for these problematic vines:

Kudzu

Extremely invasive kudzu (Pueraria montana) smothers native vegetation across the southeastern U.S. Rapid growth allows it to engulf trees, buildings and powerlines.

Identification: Large lobed leaves emerge from hairy stems. Clusters of purple pea-like flowers bloom mid to late summer. Aggressive tuberous roots allow rapid spread.

Control: Frequent mowing and tillage weaken established plants. Apply systemic herbicides to foliage repeatedly over several years.

Mile-a-minute

Also called devil’s tearthumb, mile-a-minute weed (Persicaria perfoliata) grows 6-20 feet annually. Barbed stems smother native plants and block access. Prolific seeds spread infestations.

Identification: Triangular leaves have a distinctive silvery midrib. Barbs and downward curving hooks emerge on leaf joints. Small white flowers become metallic blue berries. Rhizomatous roots promote clonal growth.

Control: Remove and destroy root crowns and stems. Release beneficial weevils that eat foliage and prevent seed production. Use labeled herbicides.

Japanese Hops

Invasive Japanese hops (Humulus japonicus) grow densely along forest edges, smothering understory plants. Rapid growth allows young plants to reach 20 feet in one season.

Identification: Opposite, toothed leaves emerge on branched, prickly vines. Male and female flowers grow on separate plants. Female flowers form papery cones containing seeds.

Control: Remove roots and stems manually. Apply labeled systematic herbicides repeatedly over several years to control regrowth. Prevent seed production.

Worst Weed Trees and Shrubs

Several invasive trees and shrubs become problematic weeds outside their native ranges. Identify and remove young plants to prevent spread. Watch for:

Russian Olive

Brought to North America as an ornamental plant, Russian olive (Elaeagnus angustifolia) now aggressively invades riparian areas in western states. It displaces native vegetation and reduces wildlife habitat.

Identification: A small deciduous tree reaching 20 feet tall. Silvery, scaly leaves have a distinctively silver underside. Fragrant yellow flowers produce olive-like fruit. Spines emerge on older branches.

Control: Cut large trees and immediately apply herbicide to stumps. Use foliar herbicides on resprouting plants. Seedlings can be hand pulled.

Autumn Olive

Prolific in eastern states, autumn olive (Elaeagnus umbellata) grows aggressively along field edges and forests. This nitrogen-fixing shrub changes soil composition and crowds out native plants.

Identification: A deciduous shrub growing to 20 feet tall. Oval leaves have distinctive silvery scales underneath. Fragrant yellow flowers produce speckled red berry-like fruit. Stems have spur-like thorns.

Control: Remove above and below ground parts. Monitor and retreat resprouting plants with systemic herbicides.

Mimosa

Mimosa or silk tree (Albizia julibrissin) invades open and disturbed areas, spreading prolifically by seed. This small leguminous tree shades out native plants and is not utilized by wildlife.

Identification: Feathery, fern-like leaves with numerous leaflets fold up at night. Showy pink puffy flowers. Flat seed pods release hard seeds. Multiple trunks arise from a single root crown.

Control: Cut trees and immediately treat stumps with labeled herbicide. Monitor for seedlings and resprouting. Foliar herbicides also effective.

Key Tips for Weed Identification

Examine leaf shape, flowers, seed heads and growth habit for identifiable features. Compare to photo guides or online weed databases.
Dig up roots or rhizomes to diagnose problem perennials. Annuals will not have this extensive underground growth.
Time weed emergence and flowering to aid identification. Weed development follows patterns based on species.
Crush a leaf or stem and smell. Some weeds like Palmer amaranth have characteristic odors when crushed.
Check for hairs, prickles, waxy coatings or milky sap by running your fingers along stems and leaves.
Use a hand lens to look closely at leaf structures, thorns, and hairs for tiny identifying features.
Consider where the weed is growing. Many species prefer specific conditions like moist ground or disturbed soils.
Compare unknowns to known problem weeds. Notice differences that indicate a new invader.
Take several clear pictures of the unknown weed including leaves, flowers and other structures.
Bring weed samples to your local agricultural extension office for expert identification.

Frequently Asked Questions About Identifying Weeds

How can you tell the difference between grassy weeds and crop grasses?

Grassy weeds can often be distinguished from crop grasses by examining the ligule (where leaf blade attaches to stem), auricle (clasping structure) and seed head structures using close-up photos or a hand lens. Weed grasses also often have faster, more aggressive early growth than crops.

What are the small flowering weeds in lawns called?

Several low growing lawn weeds produce small flowers. Common examples include clover, lawn burweed, corn speedwell, parsley piert and hop clover. Compare foliage and flowers to photo identification guides. Digging up plants can also help diagnose root types.

Why do some weeds get woody stems but others don’t?

Woody or stiff stems emerge on older weed plants that live for multiple years (perennials). Annuals that sprout from seed, flower, and die each year will not develop woodiness. Examples of weeds with woody stems are thistles, knapweeds, woody vines and some brush species.

What is the bushy blue flowering weed called?

Many weeds produce small blue or purple flowers. Examples with bushy growth habits include purple loosestrife, chicory, vervain, henbit, bugloss and viper’s bugloss. Compare foliage and other identifying traits to pictures for exact identification.

How can you tell if an unfamiliar plant is a weed or a regular wildflower?

Traits that indicate a weed versus a desirable wildflower are aggressive spread through seeds or roots, forming dense pure stands that crowd out other plants, and growing where they are not wanted, such as in a lawn or garden. Wildflowers will usually have lower seed production and growth, mixing in well with other native plants.

Why do some weeds get really big and tall but others stay low to the ground?

Weed species have differing growth habits based on the ecosystem and strategy that allows them to best compete and spread in that area. Tall weeds with significant biomass such as burdock and horseweed tower over crops. Low weeds like oxalis and spurge avoid shading and utilize available space beneath crop canopies.

Conclusion

Identifying weeds by appearance, growth habit and ecosystem behavior allows agricultural producers, land managers and gardeners target control measures towards the worst offenders. Careful weed identification also prevents unnecessary use of time, labor and herbicides controlling benign plants. When in doubt, use available resources like local weed experts, photo databases and guides to positively identify plants before embarking on control measures. Focusing efforts on the very worst weed species provides the most efficient means of maintaining productive, healthy lands.

Identifying the Worst Weeds and the Best

Weeds are unwanted plants that grow and spread aggressively, competing with crops or disrupting ecosystems. Some weeds stand out as particularly problematic based on traits like rapid growth, profuse seed production, ability to spread vegetatively, and resistance to control measures. Knowing the most destructive weeds infesting an area allows for targeted management to prevent major losses in crop yield and environmental quality. This article explores identification of the very worst agricultural and invasive weeds along with