North Central Washington

A Comprehensive List of Vegetation Control Methods

by
George Wooten
Twisp, WA
November, 1999

The following outline is intended to help design and implement a comprehensive program of vegetation control. Such a program might be required in cases where large areas are involved, intractable species have invaded an area, control costs are potentially very high, or existing management techniques are ineffective or unacceptable.

I. Planning and technical analysis needs for a comprehensive vegetation control program.

A. Perform site analysis.

     1. Be site specific.

     2. Monitor ongoing projects and search for new invaders.

     3. Use site indicator species to understand ecological amplitude (Quarles,
     1999, Neal, 1993).

B. Consider invading species biology (Hobbs and Humphries, 1995, Hiebert
and Stubbendieck, 1993).

     1. Consider life form (annual, biennial, perennial, Quarles, 1999).
     With annuals and biennials prevention of seed formation is important; for
     perennials prevention of sprouting is important (Elmore, 1993a, Miller,
     1975, Quarles, 1999).

     2. Understand species performance (Luken, 1997, Marrs and Lowday, 1992).
     Some weed control methods can enhance the performance of new invaders,
     retarding establishment of desirable plant communities.

C. Analyze the disturbance regime (Quarles, 1999).

D. Identify operating systems and their controls.

     1. Identify economic factors, e.g. cost / benefit ratios (Luken, 1997,
     p. 133).

     2. Identify functioning and impaired biological and ecosystem processes.

          a. Identify desired future conditions.

          b. Identify biological and ecosystem control factors.

               (1) Facililtation controls (late-successional species response
               to environmental modifications (Connell and Slatyer, 1977).

               (2) Tolerance controls (late-successional species grow in the
               presence of early-successional species: Hils and Vankat, 1982,
               Walker and Chapin, 1987).

               (3) Inhibition controls (late-successional species establish and
               grow only after early-successional species become senescent or
               die: Hils and Vankat, 1982, Walker and Chapin, 1987).

          c. Identify effects of the proposed changes to the environment over
          time.

     3. Identify management limitations.

E. Perform integrated analysis

     1. Use an interdisciplinary approach to develop a broad range of reasonable
     options and alternatives, along with a realistic set of possible outcomes.

     2. Look at the big picture. "Attempts to control weeds without addressing
     the causes of the invasion are doomed because they treat symptoms rather
     than causes." - Hobbs and Humphries (1995). Avoid the urge to rush into a
     "crisis" situation, and instead, take time to plan a concerted effort to
     species.

     3. Use a decision-making system (Hobbs and Humphries, 1995, David Pimental,
     1999, David Pimentel in Vandenburg, 1996).

     4. Use ecosystem management (Daily et. al., 1999, Luken, 1997).

     5. Use Integrated Pest / Weed Management (IPM, IWM).

F. Implement a Plan

     1. Follow a timeline for achievement of goals.

     2. Insure that proper timing of control methods occurs.

     3. Monitor project and method effectiveness; include follow-up visits
     for the first three years.

     4. Practice adaptive management. Have a back-up plan. Be prepared to
     adapt management to changes in expectations.
 
 

II. Control Methods

A. Direct human intervention.

     1. Mechanical methods of removal ("mechanical controls").

          a. Hand-pulling ("manual control"). A new program of studies
          of the use and effectiveness of hand pulling to control noxious weeds
          has instituted at the University of Colorado by Professor Tim Seastedt
          (Boulder Camera, 8/22/99).

          b. Many specific tools exist for the control of specific invaders,
          such as the "Weed Wrench" type of implement (New Tribe, 5517
          Riverbanks Rd., Grants Pass, OR 97527, 541-476-9492). Educate yourself
          about the tool for the job and avail yourself of it.

          c. Mowing. A Colorado study (Lastoka Property, 1997-1998), found that
          mowing diffuse knapweed (Centaurea diffusa) reduced seed production
          85% in 1997 compared to unmowed areas. Mowing also helps prevent
          tumbleweed production. Timing, plant height and blade sharpness may be
          critical factors (Sam Lucy, Winthrop, personal communication).

     2. Physical methods of plant control.

          a. Hot water / steam treatment (Waipuna International, Ltd.
          1999, Chapel Hill Newspaper, 1999, contact Allen Spalt, Dir.,
          Agricultrual Resources Center, PESTED, 115 W. Main, Carrboro, NC
          27510 (919-967-1886).
    
          The town of Carrboro, North Carolina is killing weeds with hot water
          from a special machine. The hot water removes the waxy cuticle coating
          plant leaves and stems, causing rapid death. The method is not plant
          specific, but can be made more effective through combination with a
          surfactant (Havey, 1999).

          b. There was an article on p.28 of the May 19 Capital Press re: Greg
          Prull in Eugene, Oregon, with a new "wet infrared" system for
          killing weeds. Built the first prototype in 1996.

     3. Chemical methods of plant control.

          a. Herbicide application. Herbicides work through several modes
          of action, including the inhibition of electron transport, growth
          regulation auxin/cytokinin mimicry, amino acid synthesis inhibition,
          synthesis inhibition, seedling growth inhibition, photosynthesis
          inhibition, cell membrane disruption, and pigment inhibition
          (Anderson, 1994). As in medicine, there is no "silver bullet" that
          will cure the problem without side effects. Herbicides remain general
          in their specificity, with a host of side effects and application
          problems. Claims of herbicides effective in extremely low quantities
          are merely more potent, and in fact such herbicides can be more
          hazardous in the case of spills. The utility of herbicides is
          not the subject of this paper; they are mentioned as a category for
          the sake of completion.

          b. Herbicide amplification (not a desirable technique, in my humble
          opinion; see above - GW). An example of how herbicide use can be
          increased if pelletized seed is coated with an herbicide-absorbing
          charcoal layer (Clearwater Seed Company factsheet on Clearbon (TM)
          Seed Pellets, Clearwater Seed Company, Lewiston, ID. Still, the
          concept of using chemical absorbents value as a control method.
          Charcoal in forest fires has been  shown to absorb allelopathic
          chemicals produced by late-seral heathers, the growth of early seral
          species.

          c. Sunlight-activitated tetrapyroles ("Laser herbicides") can be
          applied to kill plants (developed by Univ. Ill. biochemist C.A.
          Rebeiz).

          d. Cnicin, a knapweed extract is being tested under an EPA grant to
          the University of Colorado as a chemical agent to increase the
          effectiveness of biological control insects on knapweeds (Centaurea
          spp.).

          e. Surfactants and wetting agents. Wetting agents have shown genuine
          effectiveness in weed control, although their use is primarily as
          adjuvants of herbicides. They have also been shown to increase the
          effectiveness of hot water treatments (Havey, 1999).

          f. Corn gluten. In a recent comparison, corn gluten meal, a byproduct
          of the corn milling process, was more effective in garden weed
          inhibition than chemical herbicides (Consumer Reports, March, 1999).
          Corn gluten is also a source of nitrogen, but it has the disadvantage
          that it inhibits grass germination. Information is available at the
          University of Iowa.

          g. Peach oil. Charles Wilson, a government scientist at the
          Agricultural Research Service's Appalachian Fruit Research
          Station in Kearneysville, W. Va. have found that the bencaldehyde
          in peach oil is effective at controlling fungal attacks (Rocky
          Mountain News, Mar. 14, 1999).

          h. Pathogen attractants to encourage beneficial insects. Application
          of substances that enhance animal herbivory e.g., sugars, proteins,
          etc. (Grossman and Quarles, 1992).

B. Biological controls - through disruption of growth processes and
reproduction.

     1. Reproduction intervention.

          a. Pollination controls. As a rule, invasive species tend to
          be generalist-pollinated, thus examples of successful control
          through pollinators is lacking. Encouragement of biologically
          diverse habitats, which should have increased pollinator
          diversity, has been shown to be more drought-resistant,
          however the reason is still poorly understood.
         
          b. Seed bank controls (Quarles, 1999, Luken, 1990, Luken, 1997).
          Many weed species produce large quantities of seed, which can
          survive in soils for centuries. Control of seedbanks is therefore
          an important and necessary control method in the management of
          invasive species.
        
          c. Factors of seedbank quantity, quality and viability:

               (1) Local vegetation sources.

               (2) Seed germination rates.

               (3) Seed decay rates and physical destruction of seeds.

               (4) Seed predation.

               (5) Reseeding efforts.

          d. Reproductive spread of seeds and vegetative parts (see under
          prevention).

     2. Allelopathy. Allelopathy is the production of plant growth and
     germination inhibitors by other plants has been documented in
     natural systems (Aldrich, 1987; Harrison and Peterson, 1991).
     The benefits of allelopathic plants has been documented (Schmidt,
     1980; Jarvis et. al., 1985), as has the occurrence of allelopathic
     root exudates (Rice, 1987, Saggese, 1985). Controlled allelopathy
     is possible through planting of known allelopathic plants,
     application of the allelopathic chemcal or genetically modifying
     a plant to produce the allelopathic substance.

     3. Genetic methods.

          a. Hybridization - Invading species may evolve or adapt to
          a less pathogenic form over time. Occasionally adaptation is
          toward a more pathogenic form, but the observation has been
          repeatedly made that well-adapted invading species are less
          lethal to their host ecosystem, e.g., a parasite that kills its
          host ecosystem is not likely to be successful in the long run.
          This effect represents an interaction between the newly
          invaded ecosystem as well as within the invading organism.

          b. Biologically engineered hybrids. Genetic engineering has the
          potential to change the fitness of invading species through
          genetic intervention, through the planned introduction of
          lethal mutations, sterile pollen, sterile or unfit seeds, etc.

C. Site modification.

     1. Hydrologic regime. Availability of moisture, e.g., through
     irrigation, is perhaps the single most effective control measure
     for  diffuse knapweed in Okanogan County, Washington (George Wooten,
     personal observation).

     2. Soils.

          a. Soil chemistry - amendments, fertilization.

               (1) Nutrient availability and nutrient cycling is an extremely
               important consideration in the management of invading species,
               which tend to be less dependent on specific soil nutrients.

               (2) Soil pH. Many species favor high or low soil pH, e.g.,
               blueberries and other Ericaceous species are acid-loving,
               whereas species such as some bluegrasses, junipers, etc.
               favor basic soils. Such amendments can favor desirable
               species, affecting vegetation dominance.

               (3) Timing of fertiliziation encourages certain species (Deal,
               1966).

          b. Soil texture.

               (1) Removal of rocks from soils can have a beneficial effect on
               ecosystem health. Rocks and rocky areas are drier microsites that
               absorb sunlight, and allow runoff to occur faster. Removing rocks
               from soils makes a site more homogenous, and soils are finer,
               benefitting certain species, e.g. bluegrasses, which can help 
               outcompete weeds (George Wooten, personal observation).

     3. Shading. Amount and timing of shade, and controlling whether vegetation
     is deciduous or not is an effective tool for vegetation management (Elmore,
     1993b).

     4. Mulching. Examples of effective mulching controls

          a. Straw mulch 3.5" thick gave 98 percent control of yellow
          star-thistle (Centaurea solstitialis) - Dremann (1996).

          b. Greenwaste materials (Elmore, 1996).

          c. Cover-cropping (Weston, 1996).

          d. Synthetic mulches (Elmore and Tafoya, 1993; Walker and Prather,
          1996). Recent studies show the synthetic mulches to be less effective
          than natural materials at reducing runoff (Cathleen Hapeman in Science
          News, Vol. 156 (1999).

          e. Allelopathic mulching (Putnam and Weston, 1986; Altieri and Doll.
          1978; Quarles, 1999).

     5. Topographic factors.

          a. Slope and aspect affect how water, sunlight and soil interact
          with an area. Slight changes in slope affect how weeds invade an area,
          e.g., cultivation of the Palouse loess results in flattening of soils
          through time, causing a more homogenous substrate for growth.
          Techniques such as terracing are widely used to grow crops on steep
          slopes.

D. Disturbance regime.

     1. Soil disturbance.

          a. Frequent cultivation can discourage perennials (Quarles,
          1999).

          b. Crop rotation discourages weeds (Dunham, 1973; Kempen, 1992).

     2. Fire.

          a. Controlled burns have a multitude of effects, including
          the death of trees, increases in shade, sprouting of shrubs
          and herbaceous perennials, temporary reduction in competition,
          changes in seed germination, and changes in nutrient availability,
          to name a few. Vegetation can be categorized as early-, mid-,
          or late-seral in respect to fire occurrences, and expected
          changes in vegetation make-up can be used to favor or disadvantage
          species makeup.

          b. Flame weeding is an effective weed control method, it is
          inexpensive and is not labor-intensive. Flame weeders are
          available at hardware stores. The principle is to burst
          plant cells, causing loss of fluids and thermal denaturation.
          A further advantage is that mineral soil is exposed by the process.

          c. Fire suppression is a factor that affects many forests,
          particularly in the interior western US. Effects of this
          anthropomorphic change to landscapes has both positive and
          negative aspects, including increased shading, increased
          dominance by late-seral species, increased competition for
          nutrients and sunlight, decreased germination, etc.
          Early-seral species, including many weedy ones, can be
          expected to decrease over time as an area revegetates
          following fire.

     3. Climatic and diurnal factors.

          a. Wind entrainment. Barrier or "snow" fences have been
          succussfully used to capture "tumbleweed" forms of weeds,
          preventing their spread, and allowing easier control by
          burning or plant removal.

          b. Night-time cultivation. Germination of the seeds of some
          weedy species is enhanced by exposure to sunlight, which may
          signal the enzymes in the seed coat that the time is ready
          for sprouting. Night-time tractor cultivation can cut weed
          infestations by 80 percent (Hartmann and Nezadal, 1990).

     4. Protective measures. The use of set-aside preserves where management
     is minimal can have positive or negative effects on invasive species.
     In healthy ecosystems, there is a strong ecological "inertia" that
     can help maintain a weed-free system. In contrast, when left to
     themselves, some infestations, e.g. leafy spurge (Euphorbia esula),
     continue to worsen.

E. Ecological controls - factors influencing the interaction of species.

     1. Plant competition.

          a. Restoration grass seeding - Leafy spurge (Euphorbia esula)
          decreased 67 percent two years after grass seeding in Minnesota
          (Biesboer et. al, 1994).

          b. Restoration plantings - used to control species make-up of an area.

          c. Natural regeneration (Friedman 1987, Muller 1971, Hanawalt 1971)
          - useful in public forestry where money is available for silvicultural
          design considerations, and where revegetation processes can operate
          naturally.

          d. Competitor enhancement - improving the chances for success of a
          competitor.

               (1) Pruning of desirable species (Elmore, 1993a).

               (2) Watering to benefit desirable species (Quarles, 1999). Water
               infrequently and deeply to encourage depth rooting.

     2. Animal controls

          a. Regime - rotation, fencing, seasonality, etc. Grazing can
          have side-effect benefits and risks, including soil aeration,
          fertilization for the former, and unintended weed spread and
          soil disturbance for the latter.

          b. Grazing system examples.

               (1) Goats (Owsley, 1999; Nelson, 1999; Jackson, 1998). Angora
               goats reduced leafy spurge (Euphorbia esula) biomass by 44% in
               3 years in North Dakota (Sedivic and Maine, 1993).

               (2) Sheep. Five years of sheep grazing on experimental plots
               in Canada reduced leafy spurge (Euphorbia esula) by 93 percent
               (Johnston and Peake, 1960).

     3. Invertebrate biological controls (Delfosse, 1994; Kelleher, J. S., M.

          a. Hulme, eds. 1984; Piper, Gary L. 1985). Many insect and microbial
          controls are now being developed by the USDA and universities.
          Research requires time and a quarantine before release, however
          both leafy spurge (Euphorbia esula) and the knapweeds (Centaurea
          spp.) have become the poster childs of a large USDA effort to
          bring them under control using several dozen insect and microbial
          agents. Such intensive programs will eventually succeed, as
          the originating countries of origin for most species have a large
          number of parasites. The research and release of agents carries
          with it the risk that unintended hosts will be attacked and
          decimated, and notable examples already indicate this has occurred.
          Although safeguards are thorough, not every native plant and
          growing environment can be tested. Examples of invertebrate biological
          controls:

          b. Natural insect enemies (Hobbs and Humphries, 1995).

          c. Introduced invertebrate control species - examples.

          d. Flea beetles (Aphthona spp.) reduces leafy spurge (Euphorbia esula)
          control from 4000 ac to 400 acres (Rocky Mountain News, 7/25/99, and
          cut states costs in half. In Alberta, Canada spurge was reduced 99% five
          years after release (Harris, 1991).

          e. Root-boring insects (Delfosse, 1994).

          f. Seedhead insects (Delfosse, 1994).

          g. Musk thistle (Carduus nutans) control has shown promise using the
          head weevil, Rhinocyllus conicus.

          h. Genetically altered (transgenic or designer) insects may eventually
          be introduced for weed control, but none are currently in planning
          stages.

     4. Microbial biological controls.

          a. T.W. Anderson at Montana State University in Bozeman has
          established field plots that indicate the microbe Sclerotinia
          sclerotinium is capable of controlling up to 90% of mature spotted
          knapweed (Centaurea maculosa). Although the soil-forne fungus has
          a broad host range that limits its release, it demonstrates that
          it may be possible to harness the auxotrophic control mechanism
          (Grossman, 1999).

          b. The rust Puccinia lagenophora reduces groundsel (Senecio
          vulgaris) populations through photosynthesis reduction
          (Grossman, 1999).

III. Prevention methods

     According to Missoula County Extension Agent Jerry Marks, "Prevention
     is the most important tool. We are constantly asked why we didn't
     control this [weed] 20 years ago." - article by Daryl Gadbow in
     Experts depend on mixture of methods to control weeds, The Montana
     Missoulian, June 23, 1999. Examples of prevention methods include:

A. Prioritize new invaders.

B. Use signage along infested areas. The public will avoid
transporting weeds to and from those areas.

C. Control seed transport, intentional and otherwise, that can cause
the spread of invaders. Examples of how seed transport may be prevented
or reduced include the following

     1. Contaminated seeding mixtures (Quarles 1999); use only 100% noxious-weed free seed.

     2. Avoid contaminated hay. Use weed-free hay and grain pellets (Olivarez, 1995).

     3. Avoid contaminated mulch.

     4. Avoid contaminated topsoil (Quarles, 1999).

     5. Avoid contaminated road gravels.

     6. Avoid contaminated nursery stock (Quarles, 1999).

     7. Avoid contaminated manure (Quarles, 1999). 

D. Control road use through infested areas. Noxious weeds often grow
along roadsides where the disturbance of the roadside environment is favorable
to their growth. This enhances the spread of invaders along roads, exactly
where the transport mechanism occurs, and also where their subsequent spread
can do the most damage. Roadside weed control and prevention of spread
should be a high priority for road managers.

E. Use quarantines and vehicle washing, e.g., of tractors, cattle and
logging trucks that may have just passed through a weed-infested site and
are planning to go to a new site.

F. Eliminate the cause, not the symptoms, of the spread of invaders,
i.e., control soil disturbances such as logging, grazing, and mining, until
weed spread is stopped.

IV. Education

     According to Missoula County Extension Agent Jerry Marks,
     "Education is at least half the effort."
     - from an article by Daryl Gadbow in "Experts Depend on
     Mixture of Methods to Control Weeds" - The Montana Missoulian,
     June 23, 1999.
   
     Examples of methods of educating about invasive species include:

A. Make a list of targeted user groups.

B. Provide weed identification information distribution at central
locations.

C. Post public relations / media / local displays at central locations.
Sponsor a "Weed of the Week" in a local newspaper.

E. Establish a weed sighting report form.

F. Sponsor research projects that study invasive species (Morrison,
1997). Projects should include a set of study goals; replicate sampling;
randomization; controls; preliminary sampling; and sampling authentication.
Projects might include setting thresholds for measurement; coherent problem
questions; area division; appropriate sample sizes; data distribution tests.

References to Comprehensive Vegetation Control Methods