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How to Manage Pests

Pests in Gardens and Landscapes

Woody Weed Invaders Published 1/08 In this Guideline:
Nonchemical control methods Chemical control About Pest Notes Publication Glossary

Himalaya blackberry, Rubus armeniacus, fruit and whitish color on back of leaflet.

Perennial grasses like this weedy bamboo can escape gardens, becoming established in natural habitats.

Big periwinkle, Vinca major, foliage and flowers.

Tree-of-heaven, Ailanthus altissima, plants.

Algerian ivy, Hedera canariensis, leaves.

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 Control

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.

Cultural Control

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

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.

CHEMICAL CONTROL

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.

Foliar Application

The effectiveness of foliar-applied herbicides in control woody plants or large perennial grasses depends on three factors:

1. Application at proper growth stage. Postemergence applications are most effective after the leaves are fully developed and when the plant is actively growing. This is often in late summer or early fall. Avoid making applications too early in the spring or summer or too late in the fall, after the leaves have begun to turn color and senesce (age).
2. Spray-to-wet coverage. All leaves and stems should be glistening following herbicide application. However, coverage should not be to the point of runoff. In many cases, one application of herbicide does not completely control these species. Retreatment should be made when new leaves are fully expanded. Treated areas should be watched closely for at least a year and retreated as necessary.
3. Proper concentration. Generally a higher rate of herbicide is required to control shrubs, large grasses, and resprouting trees than is required to kill seedlings of herbaceous plants. With most of these herbicides, a solution of 1% to 2% active ingredient is appropriate. Too high a rate may kill the conducting tissues in the plant before the herbicide reaches the below-ground buds. This may result in killing the aboveground portion of the plant, but allow recovery of underground reproductive parts such as rhizomes.

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.

WARNING ON THE USE OF CHEMICALS

SUGGESTED READING

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.

PUBLICATION INFORMATION

Pest Notes: Woody Weed Invaders
UC ANR Publication 74142        

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.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616

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