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Managing Invasive Plants

About Plants

Photo of elk grazing in habitat provided by native plant communities.
Native plant communities provide habitat for plants and animals. Photo credit: USFWS

Understanding plants and their environment, and how plants grow and reproduce, is important to effectively manage invasive and native plants. Managers consider this information when developing their IPM plans and selecting control methods.

PLANTS AND THEIR ENVIRONMENT

Diagram showing the an ecosystem, habitat, and plant community.

The groupings of plants that coexist and interact with one another in an area are called plant communities. A plant community is a component of habitat, which is a place that provides living organisms with food, water, shelter, and space. Understanding plant communities within the context of their environment is important for managing nonnative plants invading native plant and wildlife habitat.

Although the term plant community generally embraces only plants, plant communities are influenced by the surrounding ecosystem. An ecosystem contains all the interactions among living (plants, animals, soil organisms) and nonliving (nutrients, minerals, moisture, disturbances) components. Examples of interactions include nutrient, mineral, and water cycles, and natural disturbances like fire.

Photo of a dense stand of saltcedar.
Dense stands of saltcedar can alter ecosystem processes such as the water cycle. Photo credit: S Dewey/Utah State Univ., www.forestryimages.org

The presence of invasive plants, and their management, can affect the interactions of both living and nonliving components of an environment at several levels: plant communities, habitats, and ecosystems. For example, the invasive plant saltcedar (Tamarix spp.), which dominates many riparian habitats in the southwestern United States, can greatly affect the water cycle. Dense stands of saltcedar trees can dry up pools, springs, and streams. As saltcedar stands replace native vegetation along waterways, their roots trap sediment, causing the stream channel to narrow. As a result, the flow rate and depth of the water may increase, as well as flooding.

Because of the interconnectedness of the environment, it is important to consider how managing invasive plants will potentially affect other parts of an ecosystem. In some areas of the southwest, saltcedar has replaced native vegetation used for habitat by an endangered bird, the southwestern willow flycatcher (Empidonax traillii). Because the bird now uses the invasive trees for habitat, managers are concerned that completely removing saltcedar will further threaten the willow flycatcher. Reestablishing desired plant communities when invasive plants are removed is an important aspect of management.

PLANT SYSTEMS AND GROWTH FORMS

All plants—native and invasive—share some of the same fundamental characteristics. Understanding how plants function and recognizing their growth forms can help focus control efforts when and where invasive plants are most vulnerable.

Plant Systems and Their Functions

Vascular plants, which include just about all large land and aquatic plants such as trees and shrubs, flowering plants, and ferns, have three systems: vascular system, shoot system, and root system. These systems serve important functions in the processes required for plants to survive and reproduce.

 Diagram of plant systems and their functions.

Invasive plant management methods often disrupt processes or inhibit the function of various plant systems to injure or kill the plant. For example, cutting off the shoot system of a plant can prevent it from making food through photosynthesis, or producing flowers and seeds. Biological control agents such as insects or diseases may be released to control invasive plants. They may feed upon, bore through, or infect plant roots, thus reducing the plant’s ability to absorb water and nutrients from the soil. Though the mechanisms are often complex, some herbicides work by disrupting vital plant processes such as photosynthesis.

Plant Growth Forms

Plant stems, leaves, roots, flowers, cones, and seeds help distinguish one plant from another and may provide information about how a particular plant grows and reproduces. Based on the characteristics of their roots and shoots, plants can be grouped into six general growth forms.

Diagram of plant growth forms.

PLANT REPRODUCTION AND DISPERSAL

In general, plants increase their numbers and distribution by reproducing from vegetative structures or by producing seeds or spores. A number of plants are able to reproduce both by seed and by vegetative means. Understanding how plants reproduce and spread aids managers in developing more effective prevention strategies and control methods.

Vegetative structures

Photo of yellow toadflax with rhizomes.
Yellow toadflax is an invasive plant that spreads vegetatively by rhizomes. Photo credit: S Dewey/Utah State Univ., www.forestryimages.org

Vegetative reproduction is a process by which new plants are produced from plant parts other than seeds or spores. It is both a natural process in many plant species and one used by horticulturists to start new plants from existing plants. Examples of vegetative reproductive structures are rhizomes (underground stems) and stolons (stems running along the surface of the ground) from which new plants develop.

Plants that spread vegetatively by rhizomes and stolons may produce dense, localized infestations. They may also disperse greater distances if their vegetative structures break off and fall into moving water or are carried to new locations by animals or people.

Plants that are able to reproduce vegetatively can present complex management challenges. For example, yellow toadflax (Linaria vulgaris) plants produce rhizomes, so pulling toadflax is often ineffective because new plants can sprout from rhizome fragments left in the soil.

Seeds

Photo of Scotch broom flowers and seed pod.
Scotch broom seed pod and flowers. Photo credit: E Coombs/Oregon Dept. Ag., www.forestryimages.org

Seeds are another way plants reproduce. Seeds are sometimes contained within fruits, which are formed after flowers are pollinated. Plants with showy, fragrant flowers attract insects or birds to serve as pollinators. Plants that do not have flowers (such as pine trees) produce seeds in cones and are pollinated by the wind.

Seeds are uniquely designed for various means of dispersal. Some seeds are light with feathery structures that allow them to be carried long distances by the wind. Other seeds are prickly, sticky, or tasty so that they will be transported by attaching to an animal’s fur or feet, or passing through the digestive tract. Plants that grow near or in water may have seeds that float along until they land on the bank or shore, or sink to the bottom to germinate. People also play a role in spreading seeds both accidentally and intentionally.

Seed characteristics and dispersal methods present a particular challenge for managing invasive plants like Scotch broom (Cytisus scoparius). Scotch broom seed pods typically burst apart into spiral halves, ejecting seeds only a short distance from the parent plant. However, seeds disperse to greater distances with water, soil movement, vehicle tires, human activities, and animals. Even if Scotch broom plants are controlled, seeds in the soil may survive for up to 30 years.

PLANT LIFE CYCLES AND STRATEGIES

A manager’s knowledge of the processes by which a plant completes its life cycle (germinates, grows, reproduces, and dies) can help identify vulnerable life stages and determine the best timing for control. It is also important to know when native or desired plants are vulnerable so that control methods can be used in a way that will not cause injury.

Plant Life Cycles

Learning to recognize plants when they are in different stages of their life cycle can help with identification for surveying, mapping, or control. For example, if management methods target plants before they flower, it is necessary to be able to identify plants when they are in the vegetative growth stage. Plant life cycles vary, but there are some general phases that most plants—native or nonnative—go through.

Diagram of a plant life cycle.

Life History Strategies

The life history of a plant refers to the period in which the life cycle is completed. Some plants grow rapidly, produce seeds, and die in one season, whereas others may live for several years and produce seeds many times. Depending on the amount of time required to complete their life cycle, plants are categorized by three life history strategies: annual (one year), biennial (two years), and perennial (many years).

The types of plant life history strategies are associated with different growth forms. For example, all woody plants (trees, shrubs, and vines) are perennial; grasses are either annual or perennial; and herbaceous forbs can be annual, biennial, and/or perennial.

Understanding different growth forms and associated strategies by which plants complete their life cycle can help managers select and time control methods that target invasive plants when and where they are most vulnerable.

Diagram of annual life history strategy.
Annuals

Annuals live for one year and their primary strategy for long-term survival is producing large numbers of seeds or seeds that survive for several years before they germinate.

Unlike deep-rooted perennials, shallow-rooted annual grasses such as Japanese stilt grass (Microstegium vimineum) are susceptible to manually pulling as a control method. If these plants have been at the site for years, there are likely to be substantial reserves of seeds in the soil.

Diagram of biennial life history strategy.
Biennials

Biennials usually take two years to complete their life cycle. During the first growing season, biennials grow leaves, stems, and roots and then enter a period of dormancy. The next spring, biennials grow rapidly, producing flowers, fruits, and seeds before they finally die after the second growth season.

Biennial invasive plants are often targeted for control during their first year of development because they have not yet produced and dispersed their seeds and are more susceptible to some methods, such as herbicides. Houndstongue (Cynoglossum officinale), for example, has a stout taproot that is difficult to pull out by hand. Even when the top portion of the plant is removed, biennials like houndstongue can initiate growth from the top of the root, so effective control can require using a method that also damages the root below the soil.

Diagram of perennial life history strategy.
Perennials

Perennials can live for many years, generating more vegetative growth and creating a stronger plant that can withstand environmental stresses (and control methods). Although perennials may not produce as many seeds as annuals and biennials, they produce seeds year after year. Often their fewer seeds are more likely to germinate and develop into new plants than the seeds of annuals or biennials.

Perennial invasive plants are usually more difficult to control than annuals or biennials because they spread both vegetatively and by dispersing seeds. Stems and roots store food and produce new shoots whenever top growth is killed. Complete control of perennial invasive plants requires killing all plant parts that are capable of producing new shoots or seeds.