Noxious weeds are invasive plant species that can cause harm to the environment, agriculture, livestock, and even human health. Identifying noxious weeds is an important first step in managing and controlling them. This comprehensive guide provides photos and descriptions of some of the most common and problematic noxious weeds. Read on to learn more about recognizing these damaging plants.
Common Noxious Weeds and Their Identification
Spotted Knapweed (Centaurea stoebe)
Spotted knapweed is a short-lived perennial that can grow to over 3 feet tall. Its purple flowers have black-tipped bracts giving the ‘spotted’ look that gives this plant its name. The leaves are pinnately divided into lobes.
This invasive weed crowds out native plants and degrades wildlife habitat. It releases chemicals into the soil that hinder the growth of other plants. Spotted knapweed is commonly found along roadsides, pastures, prairies, and woodland areas.
Leafy Spurge (Euphorbia esula)
Leafy spurge is an erect perennial that grows 1-3 feet tall from an extensive root system. The alternate leaves are narrow and linear in shape. Small greenish-yellow flowers grow in umbrella-shaped clusters. When damaged, the plant exudes a milky sap.
This aggressive weed spreads rapidly by roots and seeds. It is toxic to cattle and displaces native grasses and wildflowers. Leafy spurge invades pastures, prairies, roadsides and woodlands throughout much of North America.
Houndstongue (Cynoglossum officinale)
Houndstongue is a biennial that forms a low-growing rosette in its first year. In subsequent years, it grows a flowering stem 1-4 feet tall. The leaves are hairy and grayish-green. Small red-purple flowers develop into prickly bur-like fruits.
This noxious weed contains alkaloids toxic to livestock, especially horses. The sharp fruits can damage fleece and hide. Houndstongue invades disturbed soils such as pastures, roadsides and recently burned areas.
Yellow Starthistle (Centaurea solstitialis)
Yellow starthistle is an annual or winter annual that grows 1-3 feet tall. Its bright yellow flower heads are armed with sharp straw-colored spikes up to 1 inch long. Leaves are blue-green and deeply lobed.
This aggressive invader forms dense infestations that crowd out desirable vegetation and reduce wildlife habitat. The sharp flower bracts can inflict painful wounds and injure livestock. Yellow starthistle often invades overgrazed pastures, rangelands, prairies and roadsides.
Japanese Knotweed (Polygonum cuspidatum)
Japanese knotweed is a large perennial shrub growing up to 10 feet tall. The stout, bamboo-like stems have reddish-brown blotchy markings. Leaves are alternately arranged and heart-shaped. Small greenish-white flowers bloom in late summer.
This extremely invasive plant forms dense thickets that exclude native vegetation and alter natural ecosystems. It is very difficult to control due to its vigorous root system. Japanese knotweed grows in disturbed soils along roadsides, stream banks, yards and woodland edges.
Purple Loosestrife (Lythrum salicaria)
Purple loosestrife is an erect perennial herb growing 3-7 feet tall. Square stems bear lance-shaped leaves in pairs or whorls of three. Showy magenta flower spikes bloom in mid to late summer.
This wetland invader crowds out native plants that provide food and habitat for wildlife. A single mature plant can produce up to 2.7 million seeds per year. Purple loosestrife infests wetlands, stream and river banks, ponds and reservoirs throughout North America.
Canada Thistle (Cirsium arvense)
Canada thistle is an aggressive perennial that spreads by an extensive creeping root system. Stems are slender, grooved and spiny. The lobed leaves are spiny on the edges and underside. Small lavender flowers occur in clusters at the ends of branches.
This noxious weed reduces crop yields and poisons cattle. It forms dense infestations that crowd out desirable native plants. Canada thistle invades disturbed areas such as roadsides, fields, pastures and rangelands. Mowing does not kill this weed, it must be continually controlled and monitored.
Common Tansy (Tanacetum vulgare)
Common tansy is a perennial herb growing 2-5 feet tall. Stems and leaves are covered in fine hairs giving a silvery appearance. Fern-like leaves are deeply divided into numerous narrow segments. Dense clusters of small yellow button-like flowers top each stem.
This aggressive plant spreads rapidly and produces chemicals that inhibit the growth of other plants. Common tansy is toxic to humans and livestock. It invades pastures, roadsides, streambanks and disturbed areas.
Oxeye Daisy (Leucanthemum vulgare)
Oxeye daisy is a perennial herb with flowering stems reaching 2-3 feet tall. Basal leaves are oval-shaped. Upper stem leaves are smaller and more pointed. The flower heads have white ray flowers surrounding yellow centers.
This invasive weed crowds out desirable forage plants and reduces crop yields. Oxeye daisy contains substances toxic to livestock. It invades pastures, meadows, prairies, roadsides and other disturbed sites throughout North America.
Field Bindweed (Convolvulus arvensis)
Field bindweed is a persistent perennial vine with spreading, twining stems. Arrowhead-shaped leaves occur alternately along the stems. Funnel-shaped white or pink flowers bloom in summer. Underground rhizomes spread aggressively.
This fast-growing vine climbs over and smothers crops and ornamental plants. It competes aggressively with other plants for moisture and nutrients. Field bindweed is very difficult to control due to its extensive root system. It grows in gardens, landscaped areas, crop fields and roadsides.
Burdock (Arctium minus)
Burdock is a biennial plant with large rhubarb-like leaves. In its second year, flower stalks 3-7 feet tall arise from a basal rosette. The spherical flower heads are covered in hooked bristles that cling to clothing and animal fur.
Burdock crowds out forage plants and the burs reduce livestock wool and hide values. The taproot is deep making this weed difficult to control. Burdock grows in pastures, fence rows, roadsides and disturbed areas.
Diffuse Knapweed (Centaurea diffusa)
Diffuse knapweed is a short-lived perennial or biennial reaching 1-3 feet tall. Solitary flower heads have white to lavender flowers with spinose bract tips. Leaves are finely divided into narrow segments covered in short hairs.
This aggressive invader infests rangelands, pastures and roadsides, crowding out native vegetation. It spreads through vigorous seed production and lateral roots. The foliage contains compounds toxic to livestock if ingested in large quantities.
Russian Knapweed (Rhaponticum repens)
Russian knapweed is a long-lived perennial that grows 2-4 feet tall. Pink to purple thistle-like flowers occur at the tips of upright branching stems. The stems and leaves are covered in gray hairs. Deep and spreading roots form large colonies.
This invasive weed is toxic to horses. It crowds out native species and reduces wildlife habitat and forage production. Russian knapweed is difficult to control and spreads aggressively by lateral roots. It invades pastures, rangeland, roadsides and ditch banks.
Wild Parsnip (Pastinaca sativa)
Wild parsnip is a biennial or short-lived perennial reaching 4-5 feet at maturity. First year plants form a basal rosette. Second year flowering stems have broad, coarsely toothed alternate leaves. Flat-topped yellow umbel flowers bloom in summer.
This invasive plant can cause severe blistering rashes and burns on contact. It displaces native vegetation and spreads aggressively by seed. Wild parsnip grows along roadsides, fields, pastures and disturbed areas.
Chinese Lespedeza (Lespedeza cuneata)
Chinese lespedeza is a perennial, woody-stemmed legume shrub reaching 6 feet tall. Alternate leaves have three leaflets. Clusters of small white to purple pea-like flowers bloom in late summer and fall.
This invader crowds out native plants and forms dense monocultures. It has little value to wildlife. The shrubby growth can overtake prairies, fields, roadsides and open woodlands. Seeds remain viable in the soil for many years.
Dalmatian Toadflax (Linaria dalmatica)
Dalmatian toadflax is an erect perennial up to 3 feet tall. Waxy blue-green leaves clasp the stem. Snapdragons-like yellow flowers with orange centers and a long spur bloom May to September. An extensive creeping root system allows rapid vegetative spread.
This aggressive invader displaces desirable native species in rangelands, forest openings and along trails. Livestock and wildlife rarely browse toadflax due to its unpalatable waxy foliage. Prescribed fire increases infestations.
Scotch Broom (Cytisus scoparius)
Scotch broom is an upright, leggy, perennial shrub growing 3-10 feet tall. Green stems bear small, alternate leaves. Bright yellow pea-like flowers occur in spring and early summer. Pods burst open when ripe, flinging seeds up to 20 feet away.
This aggressive invader takes over large areas, crowding out native species and altering wildfire regimes. It is toxic to livestock and forms impenetrable thickets. Scotch broom invades meadows, pastures, forestlands and roadsides, especially in coastal areas.
Musk Thistle (Carduus nutans)
Musk thistle is a biennial or winter annual reaching 2-6 feet tall. Large, grayish-green lobed leaves have a light green midrib and spiny margins. Nodding flower heads are large, usually solitary, with deep pink to purple disk flowers surrounded by numerous spiny bracts.
This noxious weed crowds out desirable forage plants and reduces wildlife habitat. The spiny foliage and bracts deter livestock grazing. Musk thistle infests pastures, rangeland, roadsides and logged areas.
Perennial Pepperweed (Lepidium latifolium)
Perennial pepperweed is an erect perennial growing up to 5 feet tall. Stems are smooth, reddish and highly branched. Waxy lance-shaped leaves occur alternately along the stems. Dense clusters of small white flowers produce peppery-tasting seeds.
This invasive weed displaces native wetland vegetation and reduces wildlife habitat. Toxic compounds in pepperweed can harm livestock. It spreads aggressively by extensive lateral roots and is very difficult to control. Perennial pepperweed invades riparian areas, floodplains, marshes and irrigation ditches.
Impacts of Noxious Weeds
Noxious weeds pose a serious threat to agriculture, the environment, and even human health. Here are some of the major negative impacts caused by uncontrolled noxious weed infestations:
- Reduce crop yields due to competition for resources
- Lower the quality and quantity of forage for livestock
- Increase costs of agricultural production
- Contaminate agricultural products like hay and seed crops
- Cause injuries to livestock and humans via thorns, toxins or burns
- Increase soil erosion along streambanks and hillsides
- Create fuel for wildfires and alter natural fire regimes
- Reduce wildlife habitat and biodiversity
- Crowd out and displace native plant communities
- Alter hydrology and water availability in natural areas
- Clog waterways and irrigation canals
Controlling and preventing the spread of noxious weeds is critical for protecting agricultural production, ecosystems, and public health. Their impacts underscore the importance of early detection, education, and integrated management efforts.
Control and Management of Noxious Weeds
Effectively managing noxious weeds requires an integrated approach utilizing a variety of control methods. Here are some of the most common techniques and strategies:
Manual and Mechanical Controls
- Hand pulling, digging and using hand tools for small infestations
- Mowing and string trimming to prevent seed production
- Cutting taproots below ground to kill certain weeds
- Tillage to disrupt roots and bury weed seed banks
- Tarping or mulching to block light and smother plants
- Controlled burns to damage aboveground growth
Cultural Controls
- Re-vegetating treated areas with desirable plants
- Using weed-free crop seed and plant stock
- Cleaning equipment, clothing and animals when leaving infested areas
- Using weed barrier fabrics and mulch in landscaped areas
Biological Controls
- Grazing with goats, sheep or cattle to selectively graze weeds
- Introducing host-specific insects that damage certain weeds
- Fostering native weed predators like mites and beetles
Chemical Controls
- Applying selective herbicides to weedy areas
- Spot spraying individual weeds while avoiding desirable plants
- Using pre-emergent herbicides to prevent germination
Prevention and Monitoring
- Cleaning equipment, boots, and clothing after working in infested areas
- Avoiding invasive ornamental plants in landscaping
- Regularly surveying for new weed arrivals
- Installing weed wash stations at entry access points
An integrated plan that combines multiple methods and frequent monitoring provides the best control. Priority should be given to detecting and treating new weed invasions early before they become established. Preventing weed seed production and dispersal is key to long-term management.
Common Questions about Noxious Weeds
What makes a plant considered a noxious weed?
A weed species is designated as