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Submodule 1: What are invasive weeds?

General Characteristics of Invasive Plants

All organisms require a niche…a particular place for them to grow, flourish, and reproduce. Plants invading western rangelands have these same requirements, and they seem especially good at aggressively filling this niche once they are introduced to a new environment. It is well known that many introduced plants do not become invasive and do not destroy native habitats. Unfortunately, it is difficult to determine which plants will be invasive and which will grow peacefully in a new environment. Most of what is known about the “invasiveness” of certain plants is learned after an infestation has occurred.

 

Before learning about the characteristics of invasive plants, let’s look at some general characteristics of native and non-invasive plants.

• Native plants are those that have evolved in a particular geographical area over thousands and perhaps millions of years; they have an ecological equilibrium established in their environment. During this time they have adapted to this geological location by developing characteristics that make them uniquely adapted to their environment. Some of these characteristics are:
• Ability to successfully reproduce by creating sufficient numbers of progeny that are able to survive, grow to maturity, and replace parent plants without impinging on the success of neighboring plants and other species.
• Ability to accommodate local environmental conditions including temperature, humidity, rainfall amounts, light intensity, and soil type and fertility while not out-competing other indigenous species.
• Ability to live in a balance with local pests such as viruses and insects, and herbivory by native animals.
  
  
• Invasive plants are able to upset the ecological equilibrium that is a hallmark of native and non-invasive plants. Once invasives become established, meaning once they begin to reproduce in their new environment, they are able to proceed to invade this new environment. Let’s look at some common characteristics of invasive plants that allow them to out-compete native plants for resources:
• Rapid growth and short life cycle compared to native plants that allows invasive plants to overtake disturbed land more quickly.
• Increased photosynthetic rate that contributes to rapid growth and early maturity.
• Early maturity allows invasives to get seeds out and growing before native plants. Some annual invasive plants go from seed to seed in a few short weeks!
• Production of large quantities of seeds allows invasive plants to establish many offspring in a larger area.
• Effective seed dispersal enables some invasive plants  to rapidly spread and invade large areas.
• Vegetative reproduction, or reproduction without seeds using underground stems and cuttings, gives invasive plants another option for reproduction.
• Altered growth habits such as early leafing out and remaining green longer allow invasive plants to photosynthesize more efficiently and for extended periods of time. This in turn allows them to grow faster and produce more vegetative and reproductive growth.
• Production of dense shade increases the ability of invasive plants to grow, reproduce, and outcompete native plants for sunlight.
• Long seed dormancy and staggered germination allows invasives to have seeds ready to grow over longer periods of time and during different growing seasons.
• Prolific flowering increases the number of seeds produced.
• Resistance to grazing can allow invasive plants to grow and spread more rapidly than neighboring, grazed native plants.
• Allelopathy is the production and release of chemicals into the surrounding soil environment that stunts the growth other plants. This helps invasives create a monoculture by preventing the growth of natives in the near-environment.
• Dense root mass production by invasives prevents the establishment of roots by native plants and provides for massive carbohydrate storage, which makes control efforts very difficult.
• Deep root system compared to native plants that is usually accompanied by early, rapid root growth. This increases the invasives ability to sequester valuable water on dryland ranges.
• The advantage of living in an environment free from their native predators allows invasives to thrive in the face of pests that would naturally control the growth and numbers of native plants.

Recall that only some non-native plants are actually invasive. Most introduced plants never even become established in their new habitat. Several non-native, non-invasive plants are the primary food crops grown in the U.S.; these include corn, wheat, soybeans, and potatoes. In addition, many others are grown as ornamentals (iris, willow trees and lilac bushes) and for other uses.

 

 

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