Alien Invasion: Plants on the Move www.weedinvasion.org
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In previous units students learned how invasive weed species came to the United States and the manner in which the growth of weeds allow them to spread when natural limiting factors are not present. Students investigated the impact of weeds on biodiversity and the aesthetic and practical consequences to humans.
It is important to remember that controlling invasive plant species is a response to a problem that will persist unless restoration of a healthy environment also becomes part of the solution. Pulling weeds may slow the spread of weeds, but it does not alter the conditions that first favored the invasion. By understanding the complex web of interactions that comprise an ecosystem, land managers may better prescribe a combination of control methods that eliminate the alien species.
Invasive weed species have the advantage of few natural enemies and an ability to persist in a wide range of habitat and environmental conditions. It is rare that a single method of control is effective. Land managers use the following methods to control invasive weeds:
Chemical control includes applying herbicides using several methods. Concerns about chemical control include polluting the land and water, killing non-target plants and animals, and the ability for chemicals to persist and accumulate in the environment. However, when a land manager thoroughly evaluates a situation and determines that herbicide use is necessary, the benefits of chemical control will outweigh the potential adverse environmental impacts. People who work with herbicides must always apply the herbicides properly and according to instructions on the product label.
Mechanical control includes pulling weeds by hand, removing weeds with weedeaters and chain saws, and mowing with the rotary and flailing method. Like chemical controls, many of these methods are not specific to the invasive weed. For example, machinery typically cuts, chips, and grinds everything in its path, including insects, small mammals, and reptiles. For some invasive species, mechanical control methods can be effective, especially if the method is used in conjunction with proper timing during the plant’s growth cycle.
Fire is one method of mechanical control. Timing and understanding fire’s effect on the plant are important considerations. Many plants are rated for to their resistance to fire. A plant that is highly flammable or has low resistance to fire may respond to control by fire. However, fire is not necessarily the best control method for all plants with low resistance. For example, cheatgrass is highly flammable and has low resistance, but its prolific seeds are resistant to fast, low temperature fires. The seeds may also benefit from the nutrients released to the soil by the fire.
Mechanical control and fire are often expensive methods, because both methods are labor intensive. Both methods may accelerate the invasion rather than eliminate it, and both methods require having specific knowledge of the target plants and timing the work carefully.
Cultural control involves tilling, discing, planting competitive vegetation, fertilizing, and rotating crops. Managed livestock grazing is also in this category.
Biological control is the final method available to control invasive weeds. The most common method of biological control involves introducing a host-specific agent. It is very important to target a specific host with the agent. If the agent impacts only a particular plant, the plant’s population may be reduced to an acceptable level as the agent increases. And, the agent’s population may be kept in check as its specific host decreases. Biological control programs are dynamic, and populations of weed and control agents will fluctuate. Biological control means that populations of undesirable weeds may be reduced to some acceptable level; biological control methods do not eliminate undesirable weeds. A successful biological control program is difficult to accomplish.
The natural herbivores for invasive weed species are not present in the United States These herbivores must be imported too, which introduces yet another alien to the ecosystem. Scientists must conduct extensive research and testing before introducing biological control agents.
Integrated Weed Management
The practice of combining several control methods is called Integrated Weed Management. Land managers often use several control methods, especially when a combination of methods narrows the range of impact to other species. Controlling invasive weed species is a complex undertaking. Teaching students how to evaluate the effectiveness of weed-control methods and to understand why some methods might be inappropriate for a situation is more practical than teaching students how to control invasive weeds.
About the Weed Control Unit
In this unit, students will investigate the human response to the problem of invasive weed species through chemical, mechanical (including fire), cultural, and biological control. Using multiple methods of control is sometimes referred to as Integrated Weed Management (IWM).Lessons focus on helping students understand that the solutions to managing alien weed species involve restoring healthy, native ecosystems and managing agricultural landscapes, parks, and gardens.
Lessons
Elementary School
Once Upon a Town – Reviews how weeds came to the United States ; introduces chemical, mechanical, cultural, and biological control methods; and explains the difference between management and eradication.
Weeds vs. Bugs – Explores the dynamic relationship between a host and control agent and underscores how an invasive species can out-compete native species.
What About Weeds? – Emphasizes increasing public awareness of weeds by developing presentations or products that catch the attention of the public.
Middle School
And Just How DO You Kill a Weed? – Emphasizes experimental design and analysis as students investigate how plants regenerate and methods of controlling plants.
High School
Under/Out of Control – Reinforces seven methods available to control weeds and encourages investigation of and participation in local weed-control efforts.
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