Weed Ecology and Biology

Only about 40 percent of the weeds found in the United States are native, while the remaining 60 percent are considered exotic or imported.

Origin of Weeds

Weeds are found throughout the world. However, all are not common in every region. Since weeds can be easily spread, more and more are being disseminated to places where they were not originally found. Only about 40 percent of the weeds found in the United States are native, while the remaining 60 percent are considered exotic or imported. The following are some examples of weeds and their origins:

  • United States—common and giant ragweed, common milkweed, fall panicum, common cocklebur, poison ivy, marestail (horseweed), nightshade, wild or common sunflower, and wild onion
  • South America—pigweed species and prickly sida
  • Europe—quackgrass, chickweed, Canada thistle, common lambsquarters, common purslane, wild garlic, and yellow foxtail
  • Asia or Africa—Johnsongrass, wild carrot, giant foxtail, velvetleaf, kudzu, and witchweed

Questions and Answers Regarding Concerns about Nonnative, Invasive Plants

Adapted from Swearingen, J., K. Reshetiloff, B. Slattery, and S. Zwicker. 2002. Plant Invaders of Mid-Atlatic Natural Areas. Washington, D.C.: National Park Service and U.S. Fish & Wildlife Service.

What Are Native Species?

A native species occurs naturally in a particular place without human intervention. Species native to North America are generally recognized as those occurring on the continent prior to European settlement. Nonnative plants are species that have been introduced to an area by people from other continents, states, ecosystems, and habitats. Many non native plants have great economic value for agriculture, forestry, horticulture, and other industries and pose little to no threat to our natural ecosystems. Others have become invasive and pose a serious ecological threat.

Invasive Plants

Invasive plants, such as exotic honeysuckles, are aggressive, displace native species, reduce land value, and can be difficult and expensive to control.

What Are Invasive Plants?

Invasive plants reproduce rapidly, spread over large areas of the landscape, and have few, if any, natural controls, such as herbivores and diseases, to keep them in check. Many invasive plants share some important characteristics that allow them to grow out of control: (1) spreading aggressively by runners or rhizomes; (2) producing large numbers of seeds that survive to germinate; and (3) dispersing seeds away from the parent plant through various means such as wind, water, wildlife, and people.

How Are Invasive Plants Introduced?

People introduce exotic plants to new areas, on purpose and by accident, through a variety of means. Some species (e.g., kudzu, kochia, multiflora rose, Japanese knotweed, and Johnsongrass) are introduced for use in gardening and landscaping or for erosion control, forage, and other purposes. Others come in unknowingly on various imported products or in soil, water, and other materials used for ship ballast. Many invasive aquatic plants are introduced by dumping unwanted aquarium plants into waterways. Once established in a new environment, some exotic species proliferate and expand over large areas, becoming invasive pests.

How Do Invasive Plants Spread?

Invasive plants spread by seed, vegetative growth (producing new plants from rhizomes, shoots, tubers, etc.), or both. Seeds, roots, and other plant fragments are often dispersed by wind, water, and wildlife. Animals spread invasive plants by consuming fruits and depositing seeds, as well as by transporting seeds on their feet and fur. People also help spread invasive plants by carrying seeds and other plant parts on shoes, clothing, and equipment and by using contaminated fill dirt and mulch. Invasive aquatic plants are often spread when plant parts attach to boat anchors and propellers.

Why Are Invasive Plants a Problem in Natural Areas?

Like an invading army, invasive plants are taking over and degrading natural ecosystems. Invasive plants disrupt the intricate web of life for plants, animals, and microorganisms and compete for limited natural resources. Invasive plants impact nature in many ways, including growing and spreading rapidly over large areas, displacing native plants (including some very rare species), reducing food and shelter for native wildlife, eliminating host plants of native insects, and competing for native plant pollinators. Some invasives spread so rapidly that they displace most other plants, changing a forest, meadow, or wetland into a landscape dominated by one species. Such “monocultures” (stands of a single plant species) have little ecological value and greatly reduce the natural biological diversity of an area.

Invasive plants also affect the type of recreational activities that we can enjoy in natural areas, such as boating, bird watching, fishing, and exploring. Some invasives become so thick that accessing waterways, forests, and other areas is impossible. Once established, invasive plants require enormous amounts of time, labor, and money to control or eliminate. Invasive species cost the United States an estimated $34.7 billion each year in control efforts and agricultural losses.

How to Prevent Spread of Invasive Plants

Become familiar with invasive plant species in your area (Table 1). When selecting plants for landscaping, avoid using known invasive species and those exotic species exhibiting invasive qualities. Ask for native plant alternatives at your nursery. Obtain a list of plants native to your state from your native plant society, state natural resources agency, or the U.S. Fish and Wildlife Service. If you have already planted invasives on your property, consider removing them and replacing them with native species.

Table 1. List of selected invasive plant species common to the Northeast. For additional information about these and other invasive plants refer to the Center for Invasive Species and Ecosystem Health website.

Aquatic Plants

  • Eurasian watermilfoil (Myriophyllum spicatum)
  • Hydrilla (Hydrilla verticillata)
  • Water chestnut (Trapa natans)

Herbaceous Plants

  • Garlic mustard (Alliaria petiolata)
  • Japanese knotweed (Polygonum cuspidatum)
  • Japanese stiltgrass (Microstegium vimineum)
  • Purple loosestrife (Lythrum salicaria)
  • Giant hogweed (Heracleum mantegazzianum)
  • Bamboos, exotic (Bambusa, Phyllostachys, and Pseudosassa species)
  • Spotted knapweed (Centaurea biebersteinii)


  • Autumn olive (Elaeagnus umbellata)
  • Bush honeysuckles, exotic (Lonicera species)
  • Japanese barberry (Berberis thunbergii)
  • Multiflora rose (Rosa multiflora)
  • Privets (Ligustrum species)
  • Winged burning bush (Euonymus alata)
  • Butterfly bush (Buddleja species)


  • Bradford pear (Pyrus calleryana ‘Bradford’)
  • Norway maple (Acer platanoides)
  • Tree of Heaven (Ailanthus altissima)


  • Kudzu (Pueraria montana v. lobata)
  • Mile-a-minute (Polygonum perfoliatum)
  • Oriental bittersweet (Celastrus orbiculatus)
  • Porcelainberry (Ampelopsis brevipedunculata)
  • Japanese honeysuckle (Lonicera japonica)

Classification of Weeds

Almost all plants are categorized by some sort of plant classification system and given a scientific name to identify them anywhere in the world. Weeds are also classified by various means. In general, they can be classified by their structure and appearance (for example, dicots [broadleaves] and monocots [grasses and sedges]), habitat, or physiology. A common categorization system groups them according to their life cycle (how long they live). The three major life cycle groups are annuals, biennials, and perennials.


Annuals are generally divided further into summer annual and winter annual weeds. Summer annuals germinate in the spring, mature, produce seed, and die in one growing season. Large crabgrass, giant foxtail, smooth pigweed, common lambsquarters, common ragweed, velvetleaf, hairy galinsoga, and common purslane are examples of troublesome summer annuals.

Winter annuals germinate in late summer or fall, mature, produce seed, and then die the following spring or summer. Examples of winter annuals include common chickweed, henbit, shepherdspurse, downy brome, and annual bluegrass. (Some annual bluegrass subspecies can occasionally function as a perennial.)


Biennial weeds grow from seed anytime during the growing season. They normally produce a rosette of leaves close to the soil surface the first year, then flower, mature, and die during the second year. A true biennial never produces flowers or seeds the first year. There are relatively few biennial weeds. Some examples include wild carrot, common burdock, bull and musk thistle, and poison hemlock.


Perennial weeds live for more than two years and can be divided into two groups: simple and creeping. Simple perennials form a deep taproot and spread primarily by seed dispersal. Some examples of simple perennials include dandelion, broadleaf plantain, curly/broadleaf dock, and common pokeweed. Creeping perennials may be either herbaceous or woody and can spread by both vegetative structures as well as by seed. Some common herbaceous perennials include Canada thistle, common milkweed, hemp dogbane, creeping buttercup, slender speedwell, ground ivy, quackgrass, and yellow nutsedge. Some examples of woody perennials include poison ivy, multiflora rose, Japanese knotweed, bamboo, brambles, wild grape, and Virginia creeper. Creeping perennials become established by seed or by vegetative parts. Since perennial weeds live indefinitely, their persistence and spread is not as dependent upon seed as the other two weed groups.

Annuals, Biennials and Perennials

A. Common chickweed can be a problem in field crops, gardens, lawns, and many other areas. B. Biennials, such as wild carrot, are controlled more easily during their first year of growth. C. For perennial weed control, the best time to either mow or apply an effective herbicide is during the bud to bloom growth stage and/or in the fall.