Herbicide use on desert, mulch, or non-turf landscape areas with desirable vegetation

The most common situation in desert landscapes is a rock-mulch or non-turf area with desirable perennial ornamental plants. The most labor efficient weed control program for such an area is to use a preemergence herbicide to kill germinating weed seedlings and then use a postemergence herbicide to control weeds that escaped the preemergence herbicide treatment. If you object to the use of herbicides and are willing to do the work, read the section: Mechanical weed control.

Preemergence herbicides can be applied twice a year; once in the fall (September to late October) to prevent the emergence of winter weeds and again in early spring (February to March) to control summer weeds. Preemergence herbicides usually last up to six months. However, some will provide adequate weed control for almost a year when applied at higher rates depending on the amount of rainfall received in a given year. Postemergence herbicides are used to control weeds that escape preemergence herbicide treatments. If preemergence herbicides are not used, postemergence herbicides (or hand pulling) will have to be used to remove weeds after every significant rainfall event which becomes time consuming and expensive.

Liquid preemergence herbicides suggested for landscape use include dithiopyr, oryzalin, pendimethalin, and prodiamine. Most herbicides are colorless but some herbicides such as oryzalin, pendimethalin, and trifluralin stain soil and rock surfaces a yellow-orange color that dissipates when the herbicide is incorporated with water. Preemergence herbicides in granule form such as Amaze (benefin plus oryzalin), Pendulum 2G (pendimethalin) or Snapshot 2.5TG (trifluralin plus isoxaben) are also effective and are sometimes easier to apply to the landscape. Another advantage of granule herbicides is that the granules will fall through the foliage of desirable plants to the soil surface underneath. Thus herbicides can be applied to areas that could not be sprayed with a liquid. These herbicides control winter and summer annual grasses and small-seeded broadleaf weeds (e.g., pigweed and nettleleaf goosefoot). They do not kill many broadleaf weeds with larger seed. Isoxaben, metolochlor and simazine are other preemergence herbicides used in desert landscapes to increase the number of broadleaf weeds that are controlled and are sometimes mixed with the herbicides mentioned above. Metolochlor and simazine have some mobility in soil in contrast to the other herbicides mentioned so they should not be used around desirable plants in sandy soils. All preemergence herbicides will require follow up treatment (i,e., hand pulling or postemergence herbicides) to control weeds that escape preemergence herbicide treatments. Two or more years of continuous preemergence herbicide use will result in better weed control than a single year of use because continuous use reduces the number of weed seeds in the soil. Control of 98% of 10,000 weed seedlings per 1000 square feet is not acceptable (200 weed seedlings survive) but 98% control of 200 weed seedlings per 1000 square feet would be acceptable (4 seedlings survive).

Preemergence herbicides are only effective if they are incorporated into the soil using at least ½ inch of rainfall or irrigation water shortly after application. Preemergence herbicides applied to rock must be washed or carried through the rock mulch and into the soil in order to be effective. Preemergence herbicides cannot be washed off of organic mulches (e.g., wood chips) and should not be used on these types of mulches. Similarly, for best results, preemergence herbicides should be applied to rock or soil surfaces free of organic debris such as leaves. It is also a waste of time and money to apply preemergence herbicides if they are not incorporated into the soil shortly after application. This is because many preemergence herbicides breakdown in sunlight, or dissipate by turning into a gas (i.e., volatilize), or undergo both processes. Several types of sprinklers can be used to slowly irrigate a landscape area to incorporate preemergence herbicides. Place straight sided cans in several places in the herbicide treated area and water until at least ½ inch of water accumulates in the each can. Avoid applying water too fast because this results in runoff rather than the movement of water and herbicide movement into the soil.

Postemergence herbicides complement the use of mulches and preemergence herbicides by providing a way to kill emerged weeds without disturbing the soil or mulch surface. They also prevent weed seed production and reduce the labor required for weed control. If only annual broadleaf and grass weed seedlings are present, either a non-selective, contact herbicide such as diquat, glufosinate, or pelargonic acid can be used, or a non-selective, translocated herbicide such as glyphosate can be used for weed control. Since these three herbicides are non-selective, they will kill almost all herbaceous annual weeds. Annual grass weeds can be removed from and around ornamental trees, shrubs and flowers using clethodim, fluazifop and sethoxydim. Unlike glyphosate, clethodim, fluazifop and sethoxydim are selective, translocated herbicides that control grasses but not nutsedge or broadleaf species (including desirable broadleaf ornamentals). Interestingly, clethodim is far more active on annual bluegrass than the other grass herbicides. Clethodim, diquat, fluazifop, glufosinate, glyphosate, pelargonic acid and sethoxydim have no soil activity so they can be used near or under ornamental trees and shrubs. Inadvertently spraying desirable foliage with diquat, glufosinate or pelargonic acid is less serious than inadvertently spraying glyphosate on an ornamental because diquat and glufosinate are not translocated away from the sprayed foliage and do not injure unsprayed parts of the plant.

Foliar-applied, translocated herbicides should be used to control perennial weeds that have underground structures such as roots, rhizomes and tubers that can generate new shoots after shoots are removed mechanically or with contact herbicides. After being absorbed into sprayed leaves, foliar-applied, translocated herbicides are translocated out of these leaves with food materials to other parts of the weed including underground parts. Glyphosate is a non-selective, foliar-applied, translocated herbicide that can be used to control nutsedge, grass, and broadleaf weeds near and underneath woody ornamental species in desert landscapes. The glyphosate spray should be carefully directed to avoid contact with desirable plants. Since roses are particularly sensitive to glyphosate, a different selective herbicide or a contact herbicide should be carefully used around roses. Halosulfuron is a selective, foliar-applied, translocated herbicide that is very effective on nutsedge species, suppresses some broadleaf weeds, and does not cause severe injury to roses. It is commonly used in turfgrass to remove nutsedge species from the grass but it can also be used around established woody ornamentals in desert landscapes. A mixture of glyphosate and halosulfuron is especially effective for controlling nutsedge species around woody perennials. Clethodim, fluazifop and sethoxydim are selective, foliar-applied, translocated herbicides that control grasses but not nutsedge or broadleaf species. They are especially effective for removing perennial grasses such as bermudagrass and fountain grass from trees, shrubs and flowers. All of the translocated, foliar-applied herbicides discussed above are slow acting, typically taking up to three weeks to kill perennial plants. Large perennial weeds often survive the initial herbicide treatment and must be retreated when weed regrowth appears to be normal, which usually occurs in two to three months.

All foliar-applied, translocated herbicides work best when sprayed on weeds that are actively growing, are not water stressed, are not growing slowly due to cold temperatures or stressed due to extreme temperatures. A rain or irrigation free period of 6 hours is needed after application to avoid washing herbicides off treated leaves. Most postemergence herbicides require the addition of a surfactant or crop oil concentrate to the spray solution to facilitate the absorption of the herbicide through the leaf surface (follow the directions on the herbicide label or the instructions on the product). Postemergence herbicides are usually applied as spot treatments (see directions on herbicide label or instructions on the product) where weed foliage is sprayed to the point of being wet but not to the extent that herbicide solution runs off of the treated foliage. Spray runoff of weed foliage wastes herbicide and necessitates more frequent mixing and filling of the herbicide sprayer. The sprayer nozzle used to apply postemergence herbicides should spray a medium to course spray and should not produce a fine mist. Fine mists consist of very small spray droplets that can drift a considerable distance and affect desirable plants that were not sprayed. Herbicides, especially postemergence herbicides, should not be sprayed or applied when it is windy or hot because this enhances the unintended drift or movement of herbicide spray droplets. During hot weather, it is best to spray herbicides early in the morning when temperatures are lower and relative humidity is higher.

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