Submodule 8: How Invasive Plants Are Controlled

Mechanical Control

Mechanical control of weeds can include a number of activities. While these methods can be quite specific for a particular target weed, they can also be quite expensive, time- and labor-intensive, and even very dangerous to life and limb. More than one treatment is usually required to contain an infestation. Native plants may also be removed in the process of removing targeted weeds. For these reasons, mechanical methods are generally favored for small infestations.

Let’s look at some examples of mechanical weed control:

Manual techniques that uproot, bury, cut, smother, or burn vegetation such as:

• Hand pulling
  This technique can be effective for single plants or small patches. It is labor-intensive requiring lots of hands, gloves, and bags. Since your goal is to remove the entire plant, roots and all, moist conditions or sandy soil can make the job easier. Dry conditions or heavy clay soils are more likely to cause roots to separate from plants when they are pulled. It is also easier to hand pull biennial plants during their second year after they have bolted (but before they flower and/or set seed), and not during their first year when they are in the rosette stage.
  
  Hand pulling is best for annual and biennial plants before they flower. Be sure to remove and destroy (preferably by burning) all plant materials.
  
  Hand pulling comes with numerous injury risks including muscle strains, scratches, cuts, and gouges from plants with shape leaf margins or spines. In addition, many invasive weeds can cause skin allergens and contact dermatitis.
  
  
• Removing weeds with hand tools:
  There are many tools on the market for weed removal including knives, shears, pruners, axes, machetes, shovels, and hoes. These tools can be quite dangerous and serious injury can result, especially if equipment is of low quality or is used incorrectly. This technique works best for annuals and biennials, but is less effective for perennials.
  
  A word of caution: Hand and tool-assisted plant removal can result in soil disturbance which can lead to new weed introductions. Soil disturbance can also lead to increased erosion of valuable topsoil.
  
  
• Cultivation, root plowing, chaining
  Cultivation, which generally requires frequent treatments, is used more in intensive agricultural settings than on rangelands. It can be very effective with small annuals and biennials that have not yet bolted. Cultivation may cause serious soil disturbance and spreading root fragments and seeds when used in some rangeland situations.
  
  Chaining and root plowing is more commonly used for removal of shrubs in rangeland settings. Vegetation is broken off at ground level or is pulled up by the roots.
  
  
• Mowing
  Weeds are commonly mowed along roadsides and around structures. This treatment is most suited to annual and biennial weeds before seed production and works best on plants with an erect growing pattern. Because of uncertain terrain along roadsides, it is imperative to use safety equipment including eye and ear guards. Mowing can be a viable pre-treatment before the application of herbicides.
  
  
• Fire
  Prescribed fires are ignited and allowed to burn areas infested by weeds. Prescribed fires can be very effective, but they can also be very dangerous and can get out of control if careful attention is not given to pertinent environmental conditions (e.g., wind speed, relative humidity, temperature, etc.). Careful supervision and appropriate permits are required.
  
  In some cases, however, fire can promote invasions if the target weeds have seeds or sprouting root systems that are particularly adapted to fire. In such cases, pre- and or post management actions must be taken to undo or reverse the promotion of invaders. Fires are most successful when they mimic or restore historical (natural) fire regimes, which had been disrupted by land use changes, urban development, firebreaks, or fire suppression practices. As with any weed management technique, prescribed fire is usually more effective when integrated with other management practices. Also, repeated burns may be necessary to successfully control weeds.
  
  Weeds can also be spot-burned with an instrument such as a propane torch. This is generally much cheaper and easier than implementing a prescribed fire, but is only effective when the infestation is small.

• Flooding
  For effective control by flooding, weeds usually must be submerged for 12-36 months. Therefore flooding is usually not an effective method to control weeds in arid and semi-arid regions. One exception is saltcedar. If small plants are completely covered for one to two years, most will die. For larger plants, the root crown and most shoots need to be submerged for several months to kill the plants.
  
  
• Smothering and Mulching
  Mulch is used to cover the soil surface so weed seeds and seedlings do not receive the light necessary for them to grow. Mulching is best used on relatively small areas since it also stunts the growth of native plants; for this reason, this method is used primarily for small, private lawns and gardens. It is less effective with perennials because their extensive root reserves help them survive as they grow up through the mulch. Mulch can consist of hay, grass clippings, wood chips or newspaper clippings.
  
  
• Solarization
  In solarization, soil is covered with black plastic to trap the sun’s heat, increasing soil temperatures to levels that kill plants, seeds, plant pathogens, and insects. In addition, sunlight is blocked, which can kill existing plants. Soil solarization, however, can cause long-lasting changes to the soil that can deter growth of desirable native species. The effectiveness of this method depends largely on how susceptible the weed is to high temperatures. For this reason it is most effective against winter annual weeds that germinate under cool conditions. Solarization can induce the release of nutrients from organic chemicals, making these nutrients more available to the plants. In California, solarization has been studied and applied to small-scale farms and organic operations. Weed biomass in parsley was reduced by up to 99% compared to untreated plots, In strawberry fields, solarization yielded good results for weed control and was significantly less expensive than chemical weed controls.