Introduction to Weeds: What are weeds and why do we care?

Prepared by Dwight D. Ligenfelter, Extension Agronomist,
Department of Agronomy, Penn State University

Description of a Weed

There are numerous definitions of a weed, including:

  • a plant out of place and not intentionally sown
  • a plant growing where it is not wanted
  • a plant whose virtues have not yet been discovered. (R.W.Emerson)
  • plants that are competitive, persistent, pernicious, and interfere negatively with human activity (Ross, et. al.)
  • and many others
No matter what definition is used, weeds are plants whose undesirable qualities outweigh their good points, according to man. Our human activities create weed problems since no plant is a "weed" in nature. Though we may try to manipulate nature for our own good, nature is persistent. Through the manipulation process, certain weeds are controlled, while, other more serious weeds may thrive because favorable growing conditions for them also have been meet. Weeds are naturally strong competitors and those weeds that can best compete always tend to dominate. Both humans and nature are involved in plant breeding programs. The main difference between the two programs is that man breeds plants for yield, while nature breeds plants for survival.

Characteristics of weeds

Certain characteristics are associated with and allow the survival of weeds. Weeds posses one or more of the following:

a) abundant seed production;
b) rapid population establishment;
c) seed dormancy;
d) long-term survival of buried seed;
e) adaptation for spread;
f) presence of vegetative reproductive structures; and
g) ability to occupy sites disturbed by human activities.

There are approximately 250,000 species of plants worldwide; of those, about 3% or 8000 species behave as weeds.

Weeds are troublesome in many ways. Primarily, they reduce crop yield by competing for water, light, soil nutrients, and space. Other problems associated with weeds in agriculture include:

a) reduced crop quality by contaminating the commodity;
b) interference with harvest;
c) serve as hosts for crop diseases or provide shelter for insects to
d) limit the choice of crop rotation sequences and cultural
practices; and
e) production of chemical substances which are toxic to crop plants
(allelopathy), animals, or humans.

Costs of weeds

Weeds are common on all 485 million acres of U.S. cropland and almost one billion acres of range and pasture. Since weeds are so common, people generally do not understand their economic impact on crop losses and control costs. In 1991, the estimated average annual monetary loss caused by weeds with current control strategies in the 46 crops grown in the United States was $4.1 billion. If herbicides were not used, this loss was estimated to be $19.6 billion. Losses in field crops accounted for 82% of this total (Bridges; WSSA, 1992).

Another source estimates that U.S. farmers annually spend $3.6 billion on chemical weed control and $2.6 billion for cultural and other methods of control. The total cost of weeds in the United States could approach $15 to $20 billion dollars (Ashton and Monaco, 1991). Also, weed control and other input costs (e.g., seed, fertilizer, other pesticides, fuel) vary with the crop. For example, in the mid-90s, herbicides for soybeans cost $30/acre or about 47% of the $63/acre in total purchased input. For corn, the cost was $32/acre or about 28% of the $114/acre in total purchased input. And for wheat it was $6 or about 6% of the total $96/acre inputs. Several factors help determine the relative costs of herbicides from one crop to another and include the competitive ability of the crop, the weeds present, the contribution of non-chemical control practices, the tillage method, management decisions, and the value of the crop. (Ross and Lembi, 1999)

Benefits of weeds

Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits. Some attributes include:

  • soil stabilization;
  • habitat and feed for wildlife,
  • nectar for bees;
  • aesthetic qualities;
  • add organic matter;
  • provide genetic reservoir;
  • human consumption; and
  • provide employment opportunities.
Weeds have a controversial nature. But to the agriculturist, they are plants that need to be controlled, in an economical and practical way, in order to produce food, feed, and fiber for humans and animals. In this context, the negative impacts of weeds indirectly affect all living beings.