How to Manage Pests
Pests in Gardens and Landscapes
For homeowners, the control of weedy or invasive woody species and large perennial grasses, such as bamboo, can be difficult (Table 1). Although many of these troublesome species are not closely related, they share one very important characteristic: following mechanical removal of shoot material, resprouting can occur rapidly from root crowns, rhizomes, or basal and low-growing stems. In many cases, the resprouting shoots will outnumber the original plants. This increases the difficulty of control.
Some of the species listed in Table 1, including poison oak, willow, chamise, cottonwoods (poplars), and certain wild blackberries, are natives and are not considered weedy in natural systems. However, in certain landscapes—for example, in urban settings—these species may become too dense and create a fire hazard or restrict movement of animals or humans through these areas.
NONCHEMICAL CONTROL METHODS
Mechanical techniques such as hand pulling or hoeing are rarely effective by themselves for the control of large shrubs, mature or resprouting trees, or perennial grasses. However, under some conditions, mechanical methods can control smaller shrubs or bunching perennial grasses, such as pampasgrass. For example, hand pulling, digging, or hoeing can be used to remove small shrubs or roots located in a yard or near houses. These procedures should be done in early spring or late fall when the soil is moist and the roots are easily removed. Digging when the soil is dry and hard usually breaks off the stems, leaving the stems and roots to resprout. Removing English or Algerian ivy with a shovel can be very effective if roots and stems are dug out. Cutting or mowing English or Algerian ivy followed by an application of glyphosate to the damaged leaves and cut stem tips can also provide effective control. In general though, mowing or cutting alone will not control the species listed in Table 1, unless performed repeatedly.
Root barriers can delay or reduce the growth of roots into areas where they are not wanted. A root barrier may consist of a hard wall, for example thick plastic, or of fabric impregnated with herbicide, such as Biobarrier. Root barriers may be installed to protect structures, or they can be installed at planting to direct root growth of young plants. In the latter case, a surround-type root barrier can be used, for example, a 15-gallon nursery pot with the bottom cut out. However, no type of root barrier gives complete control; eventually roots will grow under or through the barrier and upward toward the soil surface.
Although mulches are often used to control annual plants, they are not effective on herbaceous perennial or woody species that resprout from underground parts.
Burning is not effective for controlling resprouting shrubs and trees. In many cases, burning can increase the population of these species. In particular, burning is not recommended for poison oak because the smoke creates a serious health hazard.
Grazing by goats can provide control in small areas. Goats have been shown to vigorously feed on resprouting vegetation and shrubs, including poison oak. Overgrazing, however, can also damage desirable vegetation.
Biological control agents, such as insects or diseases which might attack the root stock of an undesirable plant, are not yet available for the control of any urban woody species. Because some of these plants are desirable ornamentals in many areas, there would be considerable opposition to the introduction of biological control agents. Furthermore, some of these weedy species, including poison oak, willow, chamise, cottonwoods (poplars), and certain wild blackberries, are natives with natural control agents already present. Consequently, biological control is not an option for their management.
Homeowners in California can purchase the postemergence herbicides fluazifop, triclopyr, glyphosate, and combinations of glyphosate with triclopyr or imazapyr for control of shrubs, mature and resprouting trees, and large perennial grasses. Depending on the compound, these chemicals can be used as a cut stump treatment, stem injection (frill or hack-and-squirt application), or as a foliar spray (applied to leaves) (Table 2). When using herbicides, extra care must be taken to keep the material from contacting desirable plants because these nonselective materials can cause serious plant injury. Also protect yourself by wearing appropriate protective equipment as stated on the herbicide label.
The effectiveness of foliar-applied herbicides in control woody plants or large perennial grasses depends on three factors:
Cut Stump or Stem Application
Stump treatments are most effective during periods of active growth. Stems of shrubs, trees, or bamboo should be cut close to the soil surface. Immediately after cutting, herbicide should be applied with a paint brush or with a plastic squeeze bottle. Delaying application will result in poor control. For small stumps, completely cover the cut surface; for large stumps, it is only necessary to wet the cambium (the outer ring of wood, next to and including the bark). For vines or small-stemmed shrubs, stems can be cut with loppers or clippers and herbicide solution painted or sponged onto the cut end.
Treatment solutions should contain 8% to 10% triclopyr (the 8% material available to homeowners in nurseries and other stores will work fine, undiluted) or 8% to 10% glyphosate. (If using a brand that has 18% glyphosate listed in the active ingredients, make a 1:1 solution of the product and water. If the product contains 41% glyphosate, use 1 part product and 3 parts water.)
Regrowth from cut stumps can be sprayed when leaves fully expand. Cut stump application of glyphosate can sometimes injure nontargeted plants of the same species in close proximity to the treated plant. This occurs via herbicide translocation through root grafts. This type of root grafting damage depends on the species. Rarely, if ever, does root grafting occur between plants of different species.
Stem Injection (Frilling or Hack-and-Squirt)
A hatchet or machete can be used to partially (hack) or completely (frill) girdle the trunk of a tree or the stems of a large shrub, using downward strokes to flare out the bark and cambium. Apply triclopyr, glyphosate, or glyphosate mixtures undiluted into the frill or hack marks. For most effective control, it is best to completely girdle the stems. As with cut stump treatments, similar root grafting injury can occur with stem injection treatment.
Brush Control. 1997. J. W. Everest and M. Patterson. Alabama Cooperative Extension Pub. ANR-1058. Auburn, AL. Available online.
Pest Notes: Invasive Weeds. Nov. 2007. C. E. Bell, et al. Oakland. Univ. Calif. Agric. Nat. Res. Publ. 74139.
Pest Notes: Poison Oak. May 2000. J. M. DiTomaso and W. T. Lanini. Univ. Calif. Agric. Nat. Res. Publ. 7411.
Pest Notes: Pesticides: Safe and Effective Use in the Home and Landscape. Apr. 2006. C. A. Wilen, et al. Oakland. Univ. Calif. Agric. Nat. Res. Publ. 74126.
Pest Notes: Wild Blackberries. Apr. 2002. J. M. DiTomaso. Oakland. Univ. Calif. Agric. Nat. Res. Publ. 7434.
Weeds of California and other Western States. 2007.J. M. DiTomaso and E. A. Healy. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3488.
Authors: J. M. DiTomaso, Plant Sciences, UC Davis; G. B. Kyser, UC Cooperative Extension, UC Davis. This article
is a revision of DiTomaso, J.M. 1997. Woody weed invaders—controlling bamboo and unwanted trees and shrubs.
Proc. California Weed Science Society. 49:14-16.
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