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.
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.
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.
When weeds escape preemergent herbicides or geotextile fabrics, postemergent herbicides can be used to control established weeds. Postemergent 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).
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.