When developing a weed management plan for an existing planting or after an installation is in place, consider the type of plants present (woody plants can tolerate herbicides that would injure an herbaceous plant) and the weeds present and their life cycles (annual, biennial, perennial).
Weed control options in the landscape include hand weeding and cultivation, mowing, mulching, and chemical control. All of these methods are used at one time or another in landscape maintenance operations. After elimination by hand-pulling, cultivation, or a post-emergence herbicide application, the growth of annual weeds can be discouraged with mulches and/or preemergence herbicides.
Cultivation and Hand-Weeding
Cultivation (hoeing) and hand-weeding selectively remove weeds from ornamental plantings. These methods are time-consuming and expensive, and 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 propagated by cultivation and should be controlled or removed by other methods.
Frequent hand removal of weeds when they are small and have not yet set seed will rapidly reduce annual weeds. If weeds are scattered at a site, hand-weeding may be the preferred management method. Hand-weeding can be time-consuming and costly but should be included in all weed management programs to keep weeds from seeding.
Young weeds 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 over dry vegetation or dry woods chips.
Mowing
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 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.
Mulches
A mulch is any material placed on the soil to cover and protect it. A mulch suppresses 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 materials. Other products that are used include paper; yard compost; hulls from pecan, cocoa, or buckwheat; municipal compost; and stones.
Organic mulches include wood chips, sawdust, yard waste (leaves, hardwood or softwood bark chips or nuggets. Bark chips have moderate particle size (1/5 to ½ inch) and moderate to good stability, while bark nuggets have larger particle sizes (1/2 to 2-1/2 inches) and excellent stability, over time. These materials can be used in landscape beds containing herbaceous or woody ornamentals.
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 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 too decomposed, the mulch may serve better as a weed propagation medium than as a means of prevention. Plan to periodically replenish landscape mulches, regardless of particle size, because of decomposition, movement, or settling. If seedlings germinate in mulches, a light raking or hoeing will remove the young weeds.
Inorganic mulches, which include both natural and synthetic products, 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) has been 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. For this reason, black plastic may not be the preferred long-term weed control method in landscape beds.
Herbicides for Landscape Plantings
Herbicides have been effectively used in many 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 mulch should provide sufficient control and avoid hazards to desirable plants associated with herbicide use. Some herbicides like metolachlor (Pennant) do not absorb well to soil organic matter. If there are large-sized particles in the mulch, the herbicide may not get to the soil under them. Products like oxadiazinon (Ronstar) and oxyfluorfen (Goal) that require a continuous surface layer must be placed on the soil surface under the mulch.
Avoiding 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. Granular formulations of preemergence herbicides are less likely to cause injury than sprayable formulations. Using a granular formulation reduces the potential for foliar uptake, but granules will injure plants if they collect in the base of leaves. Apply nonselective herbicides such as 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. 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 absorb the herbicide and minimize injury. Usually it just takes time for the herbicide 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. Herbicide injury symptoms vary according to plant species and the herbicide and can include yellowing (chlorosis) and/or bleaching, root stunting, distorted growth, and the death of leaves.