How to Manage Pests
Pests in Gardens and Landscapes
Weed management in landscape plantings is often made difficult by the complexity of many plantings: usually more than one species is planted in the landscaped area and there is a mix of annual and perennial ornamentals. The great variety of ornamental species, soil types, slopes, and mulches creates the need for a variety of weed management options. There are also considerations regarding public concern about the use of chemicals to control weeds and their effect on water quality if the herbicide moves offsite through runoff. The choice of a specific weed management program depends on the weeds present and the types of turf or ornamentals planted in the area. Because of the many variables, weeds in landscape plantings are usually controlled by a combination of nonchemical and chemical methods.
Most landscape plantings include turfgrass, bedding plants, herbaceous perennials, shrubs, and trees. Use this publication as a practical review and guide to weed management options suited to planting beds and areas around trees and shrubs. Information on integrated pest management more directed towards turfgrass can be found in Pest Notes: Weed Management in Lawns.
WEED MANAGEMENT BEFORE PLANTING
An integrated approach, utilizing several methods, is the most economical and effective means of controlling weeds. Develop your weed management plan for landscapes before you plant by following these five basic steps:
WEED MANAGEMENT AFTER PLANTING Key to identifying weeds
When developing a weed management plan for an existing planting or after an installation is in place, consider the types of landscape plants present and the weeds present and their life cycles (annual, biennial, perennial) (Table 1).
Weed control options in the landscape include hand-weeding and cultivation, mowing, mulching, hot water treatments, and chemical control. All of these methods are used at one time or another in landscape maintenance operations (Table 2). For instance, after elimination by hand-pulling, cultivation, or a postmergent herbicide application, the subsequent growth of annual weeds can be discouraged with mulches and/or preemergent herbicides.
Herbicides should not normally be needed by home gardeners. Mulching, removal by hand, and proper placement of irrigation
water will be sufficient in most cases.
Hand-weeding and Cultivation
Cultivation (hoeing) and hand-weeding selectively remove weeds from ornamental plantings. If weeds are scattered throughout the site, hand-weeding may be the preferred management method. Hand-weeding can be time consuming, but should be included in all weed management programs to keep weeds from seeding. Hand-weeding must be repeated frequently until the plantings become established. Cultivation can damage ornamentals with shallow roots, bring weed seeds to the soil surface, and propagate perennial weeds. When cultivating, avoid deep tilling, as this brings buried weed seeds to the soil surface where they are more likely to germinate.
Perennial weeds are often spread by cultivation and should be controlled or removed by other methods. Frequent removal of weeds when they are small and have not yet set seed will rapidly reduce the number of annual weeds.
Young weeds in open areas also can be controlled with small flaming units. Propane burners are available to rapidly pass over young weeds to kill them. A quick pass over the plant is all that is necessary; do not burn the weed to the ground. Flaming is more effective on broadleaf weeds than grasses. Be careful not to flame dry vegetation, dry wood chips, or near buildings and other flammable materials, and don’t get the flame near desired plants.
Mowing can be used to prevent the formation and spread of weed seeds from many broadleaf weeds into cultivated areas by cutting off flower heads. However, weeds that flower lower than the mowing blade (such as spotted spurge or common woodsorrel) are not controlled. Repeated mowing tends to favor the establishment of grasses and low-growing perennial weeds. Mowing of some ground covers can rejuvenate them and make them more competitive against weeds.
The top growth of older weeds can be controlled by using a string trimmer. Annual broadleaf weeds are more effectively controlled than annual grasses because the growing points of grasses are usually below ground. Most perennial weeds are not controlled using string trimmers but will make them less noticeable in areas if you are limited in other methods for control.
Prevent repeated damage to trunks of established woody perennials from the string trimmer by placing a temporary shield around the trunk. Create your own shield by placing a segment of irrigation pipe that is several inches in diameter and split lengthwise around the base of the trunk.
A mulch is any material placed on the soil to cover and protect it. Mulches suppress annual weeds by limiting light required for weed establishment. Many types of landscape mulches are available. The most common are bark and other wood products and black plastic or landscape fabric materials. Other products that are used include paper, yard compost, hulls from nuts (such as almonds) or cereals (rice), municipal composts, and stones.
Organic mulches include wood chips, sawdust, yard waste (leaves, clippings, and wood products), and hardwood or softwood bark chips or nuggets. Bark chips are moderate-sized particles (1/4 to 1/2 inch) and have moderate to good ability to withstand decompostion, while bark nuggets are larger in size (1/2 to 1 1/2 inches) and have excellent stability over time. All of these can be used in landscape beds planted with herbaceous or woody ornamentals. Larger mulch pieces (greater than 1 1/2 inch) do not provide good weed control the space between the pieces allows weeds to grow through.
The thickness or depth of a mulch necessary to adequately suppress weed growth depends on the mulch type and the weed pressure. The larger the particle size of the organic mulch, the greater the depth required to exclude all light from the soil surface. Coarse-textured mulches can be applied up to 4 inches deep and provide long-term weed control. Fine-textured mulches pack more tightly and should only be applied to a depth of about 2 inches. If the mulch is too decomposed, it is a weed propagation medium rather than a means of prevention. Plan to periodically replenish organic mulches, regardless of particle size, because of decomposition, movement, or settling. If seedlings germinate in mulches, a light raking, hoeing, or hand-weeding will remove the young weeds.
Including both natural and synthetic products, these mulches are generally more expensive and less widely used in the landscape. Natural inorganic mulches are stable over time and include materials such as sand, gravel, or pebbles. Most of these products are used in public and commercial plantings. If using a rock mulch, consider placing a landscape fabric underneath it. The fabric creates a layer between the mulch and soil, preventing rock pieces from sinking into the soil. The fabric prevents soil from moving above the rock layer, which would bring weed seed to the surface.
Black plastic (solid polyethylene) can be used underneath mulches to improve weed control. It provides excellent control of annual weeds and suppresses perennial weeds, but lacks porosity and restricts air and water movement. It also tends to tear and break apart rather quickly. For this reason, black plastic may not be the preferred long-term weed control method in landscape beds.
Manufactured materials, called geotextiles or landscape fabrics, have been developed to replace black plastic in the landscape. Geotextiles are porous and allow water and air to pass through them, overcoming the major disadvantage of black plastic. Although these materials are relatively expensive and time-consuming to install, they become cost-effective if the planting is to remain in place for 4 or more years. Geotextiles are used mainly for long-term weed control in woody ornamental trees and shrubs. Geotextiles should not be used where the area is to be replanted periodically, such as in annual flower beds or in areas where the fabric would inhibit the rooting and spread of ground covers. Tree and shrub roots can penetrate the materials and if the material is removed, damage can occur to the plant’s root system. This might be a concern if a fabric has been in place longer than 5 years. One geotextile fabric (BioBarrier) has an herbicide encapsulated in nodules on the fabric that reduces root penetration problems.
Placing a landscape fabric under mulch results in greater weed control than mulch used alone. There are differences in the weed-controlling ability among the geotextiles: fabrics that are thin, lightweight, or have an open mesh allow for greater weed penetration than more closely woven or nonwoven (spunbonded) fabrics.
To install a landscape fabric, you can plant first and then install the fabric afterwards using U-shaped nails to peg it down. After laying the cloth close to the ground, cut an “X” over the plant and pull it through the cloth. If laying down a fabric before planting, cut an “X” through the fabric and dig a planting hole. Avoid leaving soil from the planting hole on top of the fabric because this could put weed seeds above the material. Fold the “X” back down to keep the geotextile sheet as continuous as possible. Weeds will grow through any gap in the landscape fabric, so it is important to overlap pieces of fabric and tack them down tightly. Apply a shallow mulch layer (about 1 inch deep) to thoroughly cover the fabric and prevent photodegradation. If weeds grow into or through the geotextile, remove them when they are small to prevent them from creating holes in the fabric. Maintain a weed-free mulch layer on top of the fabric by hand-weeding or by applying herbicides. Use of a rock mulch above a landscape fabric can have greater weed control than fabric plus organic mulch combinations, but may stress plants in some situations. See Problems section following.
Special consideration should be given to the fact that yellow nutsedge grows through all geotextiles. However, some fabrics are better at suppressing yellow nutsedge than others. For more information, see Pest Notes: Nutsedge.
Problems with Organic and Natural Inorganic Mulches
There are several problems associated with the use of organic and inorganic mulches. Perennial weeds such as field bindweed and nutsedges often have sufficient root reserves to enable them to penetrate even thick layers of mulches. Some annual weeds will grow through mulches, while others may germinate on top of the mulch as it decomposes. Weeds that are a particular problem are those that have windborne seeds such as common groundsel, prickly lettuce, and common sowthistle. Applying mulches at depths of greater than 4 inches may injure plants by keeping the soil too wet and limiting oxygen to the plant’s roots. Disease incidence, such as root or stem rot, may increase when deep mulches are maintained.
When mulches are too fine, applied too thickly, or begin to decompose, they stay wet between rains and allow weeds to germinate and grow directly in the mulch. For best weed control, use a coarse-textured mulch with a low water-holding capacity. When used alone, mulches rarely provide 100% weed control. To improve the level of weed control, one can apply preemergent herbicides at the same time as the mulch (see Table 3). Supplemental hand-weeding or spot spraying may also be needed.
Avoid mulches with a pH less than 4 or that have an “off odor” such as ammonia, vinegar, or rotten egg smell. These mulches were stored incorrectly and contain chemical compounds that may injure plants, especially herbaceous plants.
If using a composted mulch, temperatures achieved during the composting process should have killed most weed seeds. However, if the compost was stored uncovered in the open, weed seeds may have been blown onto the mulch. Be sure the mulch is not contaminated with weed seeds or other propagules such as nutsedge tubers.
Rock mulches should always be used with a landscape fabric underneath. Avoid getting soil or other growing media between the rocks or else weeds will grow there. Removal of these weeds by hand weeding or hoeing is very difficult. White rocks are very reflective and this increased light can damage sensitive plants. Dark colored rocks will retain heat and may also cause plant stress.
Hot Water or Steam Treatments
There are several machines currently available that use superheated hot water or steam to kill weeds. The equipment is expensive to purchase and maintain, so these machines are not appropriate for home use. However, commercial landscapers may find them useful in certain situations where the use of herbicides is not desired such as when line-marking playing fields, in playgrounds, around woody plants, for edging, and for weeds growing along fence lines. Some brands of equipment travel slowly (about 2 mile/hour) and are probably not cost-effective for weed control along roadsides. Because these methods employ boiling water or steam, workers must be adequately trained in the use of the machines to prevent severe burns. These machines are most effective on very young annual weeds or perennials that have recently emerged from seeds. The effect is similar to that of a nonselective, postemergent herbicide. Hot water and steam are not very effective on perennial weeds with established storage organs, such as rhizomes and bulbs, nor do they control woody plants. In general, broadleaf weeds are more easily controlled by this method than grasses A home gardener can apply hot water (greater than 200º F) and achieve a limited amount of weed damage. This method is not very effective because the water temperature rapidly drops once it is removed from the heat source.
Herbicides for Landscape Plantings
Herbicides have been effectively used in many types of landscape plantings and are most often integrated with the cultural practices discussed above. Generally, home gardeners should not need to apply herbicides to existing landscape plantings. Hand-weeding and mulching should provide sufficient control and avoid hazards to desirable plants associated with herbicide use. Many herbicides listed here are for use by professional landscape pest managers and are not available to home gardeners. To determine which herbicide(s) are in a product, read the active ingredients information on the label.
When weeds have been removed from an area, preemergent herbicides can then be applied to prevent the germination or survival of weed seedlings. Preemergent herbicides must be applied before the weed seedlings emerge. Examples of preemergent herbicides include: DCPA (Dacthal), dithiopyr (Dimension), isoxaben (Gallery), napropamide (Devrinol), oryzalin (Surflan, Weed Impede), oxadiazon (Ronstar), oxyfluorfen (Goal), pendimethalin (Pendulum, Pre-M), and prodiamine (Barricade). DCPA, dithiopyr, oryzalin, napropamide, pendimethalin, and prodiamine control annual grasses and many broadleaf weeds and can be used safely around many woody and herbaceous ornamentals. Isoxaben is used for control of broadleaf weeds.
Match herbicides with weeds present, and consider using herbicide combinations. Combinations of herbicides increase the spectrum of weeds controlled and provide effective control of grasses and many broadleaf weeds. Commonly used combinations include tank mixes of the materials listed above or isoxaben/trifluralin (Snapshot), oryzalin/benefin (XL2G), oxyfluorfen/oryzalin (Rout), and oxyfluorfen/pendimethalin (Ornamental Herbicide 2). Check the label to determine which ornamental species the material can safely be used around and which species of weeds are controlled.
Timing of a preemergent herbicide application is determined by when the target weed germinates, or by when the weed is in the stage that is most sensitive to the herbicide. In general, late summer/early fall applications of preemergent herbicides are used to control winter annuals, while late winter/early spring applications are used to control summer annuals and seedlings of perennial weeds. If heavy rainfall occurs after preemergent herbicide application or if a short residual (short acting) product was applied, a second preemergent herbicide application may be needed. Generally, herbicides degrade faster under wet, warm conditions than under dry, cool conditions. Some preemergent herbicides, such as surflan, need to be applied at a higher rate if the soil is high in organic matter due to binding with the organic matter. Check the herbicide label to recommendations about what rate to use depending on your soil type.
Cultivation and Preemergent Herbicides
No cultivation should occur after an application of oxyfluorfen; however, shallow cultivation (1 to 2 inches) will not affect the activity of napropamide, pendimethalin, or oryzalin. Also, soil type and pH can affect the activity of some herbicides. Use the information contained in herbicide labels and from your local county Cooperative Extension office to determine the tolerance of an ornamental plant species to a given herbicide.
Herbicide and Mulch Placement
The placement of a preemergent herbicide in relation to an organic mulch can affect the herbicide’s performance. Additionally, the characteristics of organic mulches can affect how herbicides work. Mulch that is made up of coarse particles will have little effect on herbicide activity. A mulch that primarily consists of fine particles can reduce the availability of some herbicides. The finer the organic material (compost or manure, compared to bark), the greater the binding of the herbicide. Many preemergent herbicides are tightly bound by organic matter, and while the binding minimizes leaching, it can also minimize an herbicide’s activity.
Another important factor is the depth of the mulch. An herbicide applied on top of a thin mulch may be able to leach through to where the weed seeds are germinating, but when applied to the top of a thick layer of mulch it may not get down to the zone of weed seed germination. Products like oxadiazon (Ronstar) and oxyfluorfen (Goal) that require a continuous surface layer must be placed on the soil surface under the mulch. Suggestions for use of mulch with herbicides are given in Table 3.
When weeds escape preemergent herbicides or geotextile fabrics, postmergent herbicides can be used to control established weeds. Postmergent herbicides control existing plants only and do not give residual weed control. Their primary function is to control young annual species, but they are also used to control perennial species. Some herbicides are systemic, that is, they are translocated throughout rapidly growing plants and can kill the entire plant. Common systemic herbicides include herbicides clethodim and fluazifop which selectively control most annual and perennial grasses, 2, 4-D which selectively controls many annual and perennial broadleaf plants and glyphosate (Roundup Pro and others) which is nonselective and effective on most annual and perennial weeds. Glufosinate (Finale), diquat (Reward), pelargonic acid (Scythe), and the new herbicides containing clove oil (Matran, EcoEXEMPT) are nonselective, contact herbicides that kill or injure any vegetation they contact. They kill small annual weeds, but only “burn off” the tops of perennial weeds.
Most of the specific herbicides listed here are not available for home gardener use and should only be applied by professionals. The exceptions are dithiopyr (in Greenlight Crabgrass Preventer, some formulations of Preen, and some “Weed and Feed” mixes); fluazifop (in Ortho Grass B Gon, Greenlight Grass Out, and others) glufosinate (Finale and others), pelargonic acid (Scythe), clove oil (Burnout and others), and glyphosate (Roundup and others).
Avoid Herbicide Injury
Because of the close proximity of many different species of plants in the landscape, herbicide injury may occur, resulting in visual plant damage. Herbicide injury symptoms vary according to plant species and the herbicide and can include the following.
Granular formulations of preemergent herbicides are less likely to cause injury than sprayable formulations. Using a granular formulation reduces the potential for damage by foliar uptake, but granules of oxadiazon (Ronstar) or oxyfluorfen (Goal) mixtures can still injure plants if they collect in the base of leaves or adhere to wet leaves. Apply nonselective liquid herbicides such as diquat, pelargonic acid, or glyphosate with low pressure and large droplets on a calm day. Use shielded sprayers when making applications around ornamentals to avoid contact with nontarget plants.
Herbicide injury to established plants from soil-applied chemicals is often temporary but can cause serious growth inhibition to newly planted ornamentals. Herbicides that contain oryzalin or isoxaben are more likely to cause this injury. Injury may result when persistent herbicides are applied to surrounding areas for weed control in turf, agronomic crops, or complete vegetative control under pavement. Activated charcoal incorporated into the soil may adsorb the herbicide and minimize injury. Usually it just takes time for herbicide residues to completely degrade. To speed degradation, supplement the organic content of the soil and keep it moist but not wet during periods of warm weather.
Derr, J. F. et al. Feb. 1997. Weed Management in Landscape and Nursery Plantings, from Weed Management and Horticultural Crops. WSSA/ASHS Symposium.
DiTomaso, J. M. and E. A. Healy. 2007. Weeds of California and Other Western States. Oakland: Univ. Calif. Agric Nat. Res. Publ. 3488.
Dreistadt, S. H. 1992. Pests of Landscape Trees and Shrubs. Oakland: Univ. Calif. Agric Nat. Res. Publ. 3359.
Elmore, C.L. 1997. Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. Oakland: Univ. Calif. Agric. Nat. Res. Publication 21377.
Fischer, B. B., ed. 1998. Grower’s Weed Identification Handbook. Oakland: Univ. Calif. Agric Nat. Res. Publ. 4030.
UC Statewide IPM Program. Integrated Pest Management Weed Photo Gallery, an online photographic reference to many weeds commonly found in California. Accessed Jan. 16, 2007.
UC Statewide IPM Program.The Pest Notes series has many relevant titles: Annual Bluegrass. Bermudagrass. Common Knotweed. Common Purslane. Crabgrass. Creeping Woodsorrel/Bermuda Buttercup. Dandelion. Dodder. Field Bindweed. Green Kyllinga. Kikuyugrass. Mistletoe. Nutsedge. Plantains. Poison Oak. Russian Thistle. Spotted Spurge. Weed Management in Lawns. Wild Blackberries. Oakland: Univ. Calif. Agric Nat. Res.
UC Statewide IPM Program. Soil solarization method. Accessed Jan. 16, 2007.
Whitson, T.D. ed. 2004. Weeds of the West. Western Society of Weed Science.
Pest Notes: Weed Management in Landscapes
Authors: C. A. Wilen, UC Statewide IPM Program, San Diego Co.; C. L. Elmore, Weed Science/Vegetable Crops Emeritus, UC Davis
Produced by UC Statewide IPM Program, University of California, Davis
Produced by University of California Statewide IPM Program
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