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UC Pest Management Guidelines


Infestation of common groundsel, Senecio vulgaris (Compositae).

Floriculture and Ornamental Nurseries

General Methods of Weed Management

(Reviewed 3/09, updated 3/09)

In this Guideline: More about weeds in floriculture and ornamental nurseries:

Whether ornamentals are grown in containers, fields, or greenhouses, there are some control practices common to all methods of production that can reduce the impact of weeds on the crop.

Prevention. The most important factor in overall weed control is to prevent weeds from developing seed and perpetuating the weed problem. Sources of weed introduction include weedy stock, weed seeds in the growing area or nearby, or plant propagules in soil, manure, uncomposted yardwaste, or other organic matter sources. Many growers cultivate or treat the margins of the property with herbicides to reduce the number of windborne or water-carried seeds that can move to the growing area. Screens on open-water inflow sources can be installed to keep out water-borne seeds.

Cultivation. Weed management systems for field-grown ornamentals start with mechanical cultivation. Begin this process by irrigating the field to induce weeds to germinate and then cultivate the new seedlings. Alternatively, the field can be sprayed with an herbicide after weed emergence so that the soil will not be disturbed by cultivation before planting. Each time cultivation occurs, new weed seeds are brought close to the soil surface and they germinate. This method reduces the soil seed bank so fewer weed seeds will be present to germinate when the crop is planted.

After planting, herbicides can be used before weed emergence or the field can be cultivated between rows again after new weeds germinate. After harvest, cultivate again to kill emerged weeds so they do not seed and replenish the weed seed bank.

Cover Crops. Cover crops can be used between rows and at field edges to improve weed management and to allow for another crop to grow instead of weeds. The cover crop selected will depend on soil type, environmental conditions, and the ornamental crop. The cover crop can be a living mulch that is repeatedly mowed to minimize competition, or it can be sprayed with herbicides and used as a nonliving mulch. Certain cover crops can be hard to suppress with herbicides, such as white or strawberry clovers (see Table 2). An annual cover crop can be established and allowed to senesce naturally or be killed back by exposure to frost.

TABLE 2. Potential Cover Crops for Field-Grown Ornamentals.
DESIRABLE INTERMEDIATE LESS DESIRABLE
WINTER ANNUALS
beans, bell or fava clover, rose or crimson ryegrass, annual
bromegrass, blando fescue, zorro  
pea, Austrian winter field brome, California mustard, wild or black
rye, cereal oat (forage)  
vetch, hairy or purple    
SUMMER ANNUALS
beans, dry    
sorghum    
sudangrass    
sudangrass/sorghum hybrids    
PERENNIALS
perennial ryegrass/hard fescue orchardgrass, berber bermudagrass, clover,strawberry or white

Mowing. Mowing is used to prevent rampant growth of the weeds, reduce the formation of seed, and reduce the spread of weed seed into cultivated areas. Properly timed mowing operations can also suppress some perennial weeds such as established johnsongrass. However, repeated mowing over a period of time (seasons or years) without any other means of weed control tends to favor the establishment of low-growing perennial grasses, which are very competitive for water and nutrients. Also, species that have flower heads below the level of the blade are not effectively controlled.

Flaming. Flaming can be used before planting or on weeds between crop rows. To avoid injuring the crop, direct the flame at young weeds between the rows or use shields. Broadleaf weeds are controlled more effectively by flaming than grasses are, and young weeds are better controlled than older ones. Because of the cost of fuel, the time required to pass over the beds, and potential injury and fire hazard, flaming is not a widely used method of weed control for field grown flowers or nurseries.

Hand-removal. Hand-hoeing or hand-pulling of weeds is always a part of crop management because cultivation does not remove all of the weeds. In some crops there may not be any other method of control. By removing the few remaining weeds in the crop, not only will there be less competition, but fewer weed seeds will be produced.

Mulches. Bark (various kinds), composted yardwaste, and other organic material can be used to help suppress annual weeds by covering the soil surface and preventing weed seed germination and establishment. Fine organic mulch (finished yardwaste) may require only 2 to 3 inches of material to totally eliminate light and suppress growth of weeds. An advantage of the fine mulches is that after the crop is harvested, the mulch can be worked into the soil to improve soil structure, drainage, and water-holding capacity of the soil. A disadvantage of fine mulch is that weed seeds that fall on it will germinate and grow.

Coarse wood chips or bark may require 3 to 6 inches of material to eliminate light. Synthetic materials (geotextiles/landscape fabrics) made of polypropylene or polyester can also be used as mulches but because of cost they should only be used with perennial shrubs or trees or in containers. Because they last several years, they can be left on for the life of the tree or shrub, or they can be removed and reused. Dark plastic mulches can be used for weed control when using drip or furrow irrigation.

Soil Solarization. Heating soil to high temperatures can kill many weed seeds. Solarization is done by covering bare soil with clear plastic during periods of high solar radiation and temperature (in California's interior valleys, this is generally during June to August). Before placing the plastic on the site to be treated, cultivate or closely mow any established plants and remove the clippings, then smooth the soil surface and irrigate the area well. Place clear, ultraviolet (UV)-protected polyethylene over the area and extend it about 2 feet beyond the infested area on all sides and pull it tightly so it is close to the soil. The plastic must be left in place and maintained intact for 4 to 6 weeks for control of weeds. Many annual weeds can be controlled using this method. Weeds not well controlled include field bindweed, yellow and purple nutsedge, and clovers.

Media for containers or for use in greenhouses can be solarized using clear bags or flats or small low mounds of soil covered with clear polyethylene. In greenhouses, beds can be solarized before planting. See Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377 and UC IPM Pest Notes: Soil Solarization for Gardens and Landscapes, UC ANR Publication 74145, for more details on soil solarization.

Transplants. Using transplants rather than direct-seeding a crop allows the crop to establish more quickly and be more competitive with weeds. Also, a transplant is generally more tolerant to soil-applied herbicides than new seedlings.

Chemical Control. Herbicides are used in many ornamental production areas as an economical option to control weeds. By using herbicides before the weed emerges, weed competition with the ornamental crop can be reduced or eliminated, resulting in higher quality ornamental plants and less labor costs.

Herbicides are generally classified according to when they are used in relation to crop and weed growth stage. A preplant herbicide is applied before planting. These herbicides are used before the desirable plants are present because they control both germinating seedlings and established plants. Preemergent herbicides kill weeds at the seed germination stage. These herbicides are applied before weeds emerge. The third type of herbicide, the postemergent herbicide, is applied after the weed has emerged. Preemergent and postemergent herbicides may be applied before or after the crop is planted depending on the crop and the herbicide selected. The section on "Susceptibility of Weeds to Herbicide Control" outlines the susceptibility of weeds to different herbicides.

Preplant herbicides. Herbicides that are applied before planting the crop may be fumigants, nonselective postemergent materials, or selective preemergent products. The fumigant herbicides, such as metam sodium or K-Pam, are often applied as an injection to cultivated soil. They are generally covered with a polyethylene tarp to seal in moisture and keep gas from escaping immediately. Dazomet (Basamid) is a powder that is incorporated into the soil. These materials must be applied by licensed applicators. Nonselective postemergent herbicides are applied to young weeds before planting. For example, glyphosate is applied to perennial weeds generally when they are growing vigorously and are beginning to flower to obtain the best control. Other nonselective herbicides include diquat (Reward), pelargonic acid (Scythe), plant oil-based herbicides, and crop oil. A selective preemergent herbicide can be applied and incorporated mechanically into soil before direct seeding or transplanting.

Preemergent herbicides. Preemergent herbicides comprise the largest number of herbicides, because the weed seedling stage is the easiest part of the plant cycle to interrupt, and these herbicides are generally safest for the crop. Examples of these herbicides are oryzalin (Surflan), napropamide (Devrinol), oxadiazon (Ronstar), trifluralin (Treflan), pendimethalin (Pre-M, Pendulum), prodiamine (Endurance, Barricade), oxyfluorfen (Goal), isoxaben (Gallery), and flumioxazin (Broadstar or Sureguard). There are a number of preemergent herbicides sold as combinations such as Snapshot (oryzalin/isoxaben), Rout (oxyfluorfen/oryzalin), OH2 (oxyfluorfen/pendimethalin), and others.

Apply preemergent herbicides to the soil after hand-weeding or cultivating to remove emerged weeds. Follow the application with an irrigation or rain to settle the soil. Some preemergent herbicides can also be mechanically incorporated. However, read the label to know if doing so will affect the chemical barrier. For example, oxadiazon and oxyfluorfen are taken up by the seedling as it emerges; disturbing the chemical may create some gaps where a seedling can avoid growing through the herbicide. These two materials depend on a barrier of herbicide on the soil surface to be effective. A second handweeding 7 to 10 days after an herbicide application may be needed to ensure elimination of previously germinated seedlings.

Preemergent herbicides must be applied before the weed seeds germinate. Because of the varied germination periods of the weed species and the selectivity of the herbicides, it is usually necessary to use different herbicides at different times of the year to achieve the best control. For example, common groundsel and lesser-seeded bittercress can germinate at almost any time during the year, but their maximum germination in a field situation occurs in a cool, moist environment. Thus, a late summer herbicide treatment for control of fall- and winter-germinating seedlings is most desirable. For summer weeds such as crabgrass and purslane, apply herbicides in late winter. Keep in mind that where artificial conditions for germination can occur, such as in a container nursery where irrigation may occur daily, these weeds can germinate at any time. Also, herbicides may not control weeds for as long as stated on the label because of frequent irrigation.

Postemergent herbicides. Postemergent herbicides are applied after weeds have emerged. They often are very selective and control only a narrow range of weed species. Examples of selective postemergent herbicides include sethoxydim (Vantage), fluazifop-p-butyl (Fusilade), and clethodim (Envoy). Sethoxydim and fluazifop-p-butyl control most annual grasses, except annual bluegrass and fine fescue. Clethodim will control annual bluegrass as well as other grasses. Products containing the phenoxy group of herbicides, such as 2,4-D, will selectively control broadleaf weeds in monocots but will injure a broadleaf crop. There are no selective postemergent herbicides that can be used over a wide spectrum of ornamental species for broadleaf weed control. Nonselective herbicides are those containing glyphosate (Roundup Pro, Touchdown, etc.), diquat (Reward), pelargonic acid (Scythe), glufosinate (Finale), and plant oils such as euginol. Nonselective herbicides can be used around the field to keep weeds from seeding but must be kept away from the crop.

Apply postemergent herbicides when weeds are in the seedling stage, which is the stage when they are the most susceptible and require the least amount of herbicide for effective control. Most postemergent herbicides need an addition of an adjuvant (surfactant or nonphytotoxic oil) for maximum control. Glyphosate may not need an additional surfactant depending on the formulation, but fluazifop-p-butyl and clethodim need a nonionic surfactant and sethoxydim needs an oil additive for best results. Check the herbicide label for information about which type of surfactant to use and the rate.

In some field-grown flowers, shrubs, or trees, control of grasses with the postemergent herbicides sethoxydim, fluazifo-p-butyl, or clethodim can be very effective. Since sethoxydim or fluazifop-p-butyl will not control annual bluegrass, the field may not look good aesthetically, but annual bluegrass does not compete heavily with the crop. Most broadleaf ornamentals tolerate these herbicides. There are no broad-spectrum broadleaf herbicides that are safe to use on a wide variety of ornamentals.

By using preemergent or postemergent herbicides or, where possible, mulches instead of hand-weeding or cultivation, the root system of desirable plants is not disturbed. Roots are not cut off with a hoe or plants pulled accidentally, and because new weed seeds are not brought to the soil surface, as would be with cultivation, fewer weeds will germinate to start a new weed crop.

Equipment for Application of Herbicides. Choice of application equipment depends on the product formulation and the location and size of areas to be treated. In greenhouses or small farm operations, a backpack hand-pump sprayer may be effective. In field nurseries, a small pull-behind or tractor unit may be more desirable. In large nurseries, over-the-top sprayers to cover full beds may be best. To get the most uniform distribution of preemergent liquid herbicides (either wettable powder (WP), water dispersable granules (WDG) or other liquid formulations), use flat fan nozzles evenly spaced on a boom. If spraying foliage with a postemergent herbicide, hollow or solid cone nozzles can be used.

Most herbicides are applied at 20 to 60 gallons of solution per acre at pressures of 30 to 40 pounds per square inch (psi). Applying liquids with a single nozzle hand wand does not give as uniform distribution as multiple nozzles on a boom. Because the effectiveness of preemergent herbicide applications is highly dependent on the concentration applied to the soil, make applications as uniformly as possible. When applying to containerized plants be aware that the media surface may be blocked by plant foliage. Where the crop makes it difficult for the herbicide to reach the media, a granular product would be more effective.

Dry materials (granules) are applied by common types of spreaders. The distribution of granules by the drop-type applicator is probably the most accurate, although the side-throw-type of spreader is easier to use. In large container operations, applicatyors such as a Gandy spreader may be a worthwhile investment. Granule size and weight differs among herbicides. Also, size of granule may not be uniform within a single formulation, making uniform distribution from one herbicide to another is difficult with the same spreader. As with liquid formulations, calibration of the equipment is essential for proper application. In contrast to liquid applications, granules can cause severe wear to the application equipment, so the equipment will need to be calibrated more frequently.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Weeds
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego County
Acknowledgment for contributions to Weeds:
C. L. Elmore, Vegetable Crops/Weed Science, UC Davis

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