Which Plants Are Considered Weeds? Here's a Complete Guide

Weeds are plants that grow where they are not wanted. They can be invasive, aggressive and compete with desired vegetation in gardens, lawns, fields and natural areas. Determining which plants are considered weeds depends on the location and purpose of the site. Here is a comprehensive guide to identifying common weed species.

What Makes a Plant a Weed?

Plants are considered weeds when they possess some key characteristics:

They are unwanted in a specific location and compete with desired plants.
They spread aggressively and are difficult to control.
They produce large amounts of seeds that easily disperse.
They grow and reproduce quickly.
They are tolerant of a wide range of conditions.
They are resilient and difficult to fully eradicate.

Some plants are only weeds in certain situations. A wildflower growing in a lawn may be considered a weed, while the same species is welcome in a meadow or prairie restoration area.

Major Categories of Weeds

Weeds can be divided into several major categories:

Annual Weeds

These weeds complete their lifecycle in one growing season. They germinate from seeds, grow, flower, set new seed and die in a single year. Examples include crabgrass, pigweed and foxtails.

Biennial Weeds

Biennial weeds take two years to complete their lifecycle. The first season they form roots, stems and leaves. The next season they flower, set seed and die. Common biennial weeds are burdock, wild carrot and wild parsnip.

Perennial Weeds

Perennial weeds persist for multiple years. Some spread aggressively by underground roots or rhizomes. Examples include dandelion, bindweed, thistle, milkweed and goldenrod.

Noxious Weeds

Noxious weeds are non-native plants that are considered harmful, toxic or economically destructive. State and federal agencies regulate them. Some examples are poison hemlock, spotted knapweed and leafy spurge.

Common Garden and Landscape Weeds

Several weeds frequently plague gardens, lawns and landscaped areas:

Crabgrass – This annual grassy weed spreads rapidly. It takes over thin lawns and competes with desired grasses.
Dandelion – Thebright yellow flowers and distinctive seed heads make dandelion a noticeable perennial weed. It has a long taproot and grows low to the ground.
Creeping Charlie – Also called ground ivy, this fragrant, creeping perennial spreads via thin stems that root at nodes. It carpets areas of moist shade.
Lamb’s Quarters – The pale green leaves have a dusty coating on their surface. Lamb’s quarters is an annual weed that grows rapidly and produces abundant seeds.
Plantain – With broad leaves and small green spikes that turn brown, plantain is a perennial weed that invades lawns and landscape beds.
Purslane – Succulent leaves and red stems identify this annual weed. Purslane spreads along the ground, rooting as it goes and produces tiny yellow flowers.

Agricultural Weeds

Weeds reduce agricultural productivity by competing with crops for water, sunlight and nutrients. Some common agricultural weeds include:

Wild Oat – This grassy annual weed invades wheat, oat and barley fields. The seeds often contaminate grain harvests.
Canada Thistle – An aggressive perennial that forms large, spreading patches in crops and rangeland. Difficult to control due to its deep, branching roots.
Field Bindweed – With arrow-shaped leaves and white trumpet flowers, this vining perennial has deep, extensive roots. It wraps around crop plants and pulls them down.
Johnsongrass – A tall annual grass that reproduces by seeds and fast-growing rhizomes. Johnsongrass competes strongly with corn, soybeans and other crops.
Pigweed – Several pigweed species, like redroot and palmer pigweed, are major weed problems in agricultural fields due to their rapid growth and prolific seed production.

Aquatic and Wetland Weeds

Wet areas, lakes, ponds and waterways also have common weeds, including:

Cattail – This prolific perennial has strap-like leaves and forms dense stands in marshes, ditches and shorelines. It spreads by underground rhizomes and seeds.
Purple Loosestrife – An invasive perennial with showy magenta flowers. It overtakes wetlands, crowding out native plants. Reproduces by prolific seeding and spreading roots.
Eurasian Watermilfoil – Feathery underwater leaves make this submerged perennial easily identifiable. It forms thick mats that impede water recreation and outcompete native aquatic plants.
Common Reed (Phragmites) – Tall plumes identify this aggressive perennial grass. It forms virtual monocultures in wetlands, shorelines and ditches, displacing other vegetation.

Control Methods

Managing and controlling weeds requires an integrated approach combining multiple methods:

Manual removal – Hand pulling, hoeing and digging can eliminate weeds, especially with small infestations. The entire root system must be removed to prevent regrowth.
Mowing and cutting – Regular mowing and clipping weakens perennial weeds by preventing flowering and seed production. However, it must be continued long-term.
Smothering – Materials like mulch, cardboard and landscape fabric placed over weeds block sunlight and prevent growth and regrowth.
Burning – Flames from controlled burns kill existing weeds and seeds. Best for large acreages and brushy, woody weeds.
Herbicides – Chemical weed killers effectively control many weeds but must be applied properly to avoid environmental impacts.
Biological control – Natural enemies like insects, diseases and grazing animals selectively control certain noxious weeds in some cases.

An integrated weed management plan combines several methods customized for the location, weed species and infestation extent. Persistence and multiple control tactics are needed to keep weeds in check over the long-term.

Conclusion

Weeds are adaptable, tenacious plants that spread prolifically in locations where they are unwelcome. They compete aggressively with crops, forage, gardens and desired vegetation. Many factors like growth habit, lifespan and reproduction define a plant as a weed. A variety of methods can be combined to manage infestations for the long term. Knowledge of weed identification, biology and control options leads to effective management and sustained prevention of these undesirable plants. With vigilance and persistence, weeds can be overcome.

Which Plants Are Considered Weeds? Here’s a Complete Guide

Weeds are unwanted plants that grow and spread prolifically. Many factors determine if a plant is considered a weed, including its location, growth habit, competitiveness and origins. Weeds can be categorized as annuals, biennials, perennials and noxious weeds. Some of the most common garden, landscape, agricultural and aquatic weeds have identifiable features and growth characteristics. Controlling weeds requires an integrated approach with manual removal, mowing, smothering, herbicides and other methods used in combination. Knowledge of weed identification traits and control techniques allows effective management of these unwelcome plants.