Managing weeds in ornamental plant production, whether in field soil, greenhouses, or outdoor containers, can be difficult but is essential to successful production. Weeds not only compete with the crop for plant nutrients and sunlight but are also unsightly and do not meet clean nursery quality standards. In addition, ornamental plants infested with certain noxious weeds cannot be sold because of quarantine regulations. Because of the high value of ornamental crops and the limited number of herbicides available, growers often resort to costly hand-weeding. However, many of the strategies used in vegetable row crops or tree crops can be adapted for use in field-grown trees and cut flower production. For example, planting in rows allows the field to be more easily cultivated by hand or mechanically. The use of drip irrigation in tree or shrub production greatly reduces excessively wet areas, thus reducing the germination and growth of weeds.
Whether ornamentals are grown in containers, fields, or greenhouses, there are some control practices common to many methods of production that can reduce the impact of weeds on the crop as listed below in no particular order.
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 manure, soil, 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. When using fine-mesh screens, increasing the surface area of the water intake and periodic debris removal may be needed to avoid clogging of the water flow.
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 a postemergence herbicide after weed emergence so that the soil will not be disturbed by cultivation before planting. Each time cultivation occurs, new weed seeds can be brought close to the soil surface and germinate. This method reduces the soil seed bank so fewer weed seeds will be present to germinate when the crop is planted.
After planting (and crop emergence if a seeded crop), preemergence herbicides can be used before weed emergence or the field can be cultivated between rows after new weeds germinate or both. 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. See the Potential Cover Crops for Field-Grown Ornamentals for the desirability of species for cover crops. 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. An annual cover crop can be established and allowed to senesce naturally or (where freezing temperatures occur) be killed back by exposure to frost.
| DESIRABLE | INTERMEDIATE | LESS DESIRABLE |
|---|---|---|
| WINTER ANNUALS | ||
| beans, bell or fava | clover, rose or crimson | mustard,1 wild or black |
| bromegrass, blando | fescue, zorro | ryegrass, annual |
| pea, Austrian winter field | brome, California | |
| 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 | ||
| 1 Mustards can be undesirable cover crops because they can be invasive weeds in wildlands. | ||
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 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. If performed after seed set, mowing can spread weed seed and exacerbate weed problems.
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 than grasses by flaming, and young weeds are better controlled than older ones. Because of the cost of fuel, the time required to pass over the beds, potential injury to workers, 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
Various kinds of bark, 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. Only 2 to 3 inches of fine organic mulch (finished yardwaste) may be required to totally eliminate light on the soil and suppress the 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 drainage, soil structure, 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 generally are used only with perennial shrubs or trees or under 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, moist 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 polyethylene that is ultraviolet (UV) resistant 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 clovers, field bindweed, and purple and yellow nutsedge.
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.
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 (preemergence) herbicides than are germinating crop seeds.
Herbicides
Herbicides are used in many ornamental production areas as an economical option to control weeds. By using herbicides before weeds emerge, 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. Preplant herbicides are applied before planting. These herbicides are used before the desirable plants are present because some can control both germinating seedlings and established plants. Preemergence herbicides kill weeds at the seed germination stage. These herbicides are applied before weeds emerge. Postemergence herbicides are applied after the weeds have emerged. Preemergence and postemergence herbicides may be applied before or after the crop is planted depending on the crop and the herbicide selected. See the section SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL to guide herbicide choice based on the weed species present.
Preplant herbicides
Herbicides that are applied before planting the crop may be fumigants, nonselective or selective postemergence herbicides, or preemergence herbicides, which are selective because they are generally safe for use around established plants. The fumigant herbicides, such as metam potassium* and metam sodium*, are often applied as an injection to cultivated soil. They are generally covered with a polyethylene tarp to seal in moisture and slow the escape of fumigant gas. Dazomet is a powder that is incorporated into the soil. All of these materials must be applied by licensed applicators. Nonselective postemergence herbicides can be used preplant as well. Certain preemergence herbicides can be applied and incorporated mechanically into soil before direct seeding or transplanting if the crop is tolerant to that herbicide. (*Requires a permit from the county agricultural commissioner for purchase or use.)
Preemergence herbicides
Preemergence herbicides must be applied before the weed seeds germinate. These herbicides comprise the largest number of herbicides available to ornamental growers because they are generally safest for the crop and the weed seedling stage is the easiest part of the plant cycle to interrupt. Examples of these herbicides are dimethphenamid-d, flumioxazin, indaziflam, isoxaben, napropamide, oryzalin, oxadiazon, oxyfluorfen, pendimethalin, prodiamine, and trifluralin. There are a number of preemergence herbicides sold as combinations such as dimethenamid-p/pendimethalin (Freehand), oryzalin/isoxaben (Snapshot), oxyfluorfen/oryzalin (Rout), and oxyfluorfen/pendimethalin (OH2).
Apply preemergence herbicides to the soil after cultivating or hand-weeding to remove emerged weeds. Follow the application with an irrigation or rain to move the herbicide in the top 1/2 inch of media or soil where the seeds are germinating. A second handweeding 7 to 10 days after an herbicide application may be needed to ensure elimination of previously germinated seedlings. However, read the label to learn if doing so will affect the chemical barrier. For example, oxadiazon and oxyfluorfen are taken up by the seedling as it emerges; disturbing the soil may create some gaps in the herbicide barrier.
Because of the varied germination periods of the weed species and the selectivity and sometimes limited persistence of the herbicides, it can be necessary to use different preemergence herbicides at different times of the year or repeat application of a particular herbicide 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 winter annual 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. The length of time a preemergence herbicide stays active is also an important consideration in application frequency and timing. For example, because of frequent irrigation herbicides may not control weeds for as long as stated on the label.
Postemergence herbicide
Postemergence herbicides are applied after weeds have emerged. Some are very selective and control only a narrow range of weed species. Examples of selective postemergence herbicides include clethodim, fluazifop-p-butyl, and sethoxydim. Fluazifop-p-butyl and sethoxydim control most annual grasses, except annual bluegrass and fine fescue. Clethodim will control annual bluegrass as well as other grasses. Products containing phenoxy herbicides, such as 2,4-D, will selectively control broadleaf weeds in monocots but will injure a broadleaf crop. There are no selective postemergence herbicides that can be used over a wide spectrum of ornamental species for broadleaf weed control. Nonselective herbicides are those containing diquat, glufosinate, glyphosate, pelargonic acid, and plant oils such as eugenol. Nonselective herbicides can be used around the field to keep weeds from seeding but must be kept away from the crop.
Apply postemergence contact herbicides when weeds are in the seedling stage, the stage when they are the most susceptible and require the least amount of herbicide for effective control. Translocated herbicides such as glyphosate and the grass selective herbicides can be effective on bigger weeds. In some field-grown flowers, shrubs, or trees, control of grasses with the postemergence herbicides clethodim, fluazifop-p-butyl, or sethoxydim can be very effective. Many postemergence herbicides need addition of an adjuvant (surfactant oil) for maximum control. Check the herbicide label for information about whether a surfactant should be added and which type of surfactant to use and the rate.
By using preemergence or postemergence herbicides or, where possible, mulches instead of cultivation or hand-weeding, the root system of desirable plants is not disturbed. Roots are not cut off with a hoe or crop plants pulled accidentally. And because new weed seeds are not brought to the soil surface, as they would be with cultivation, fewer weeds will germinate to start a new weed crop.
Application of Herbicides
Calibration of the equipment is essential for proper application regardless of whether the herbicide is sprayed or applied dry as granules. Granules and wettable powder formulations can cause severe wear to the application equipment, so the equipment will need to be calibrated more frequently.
Most liquid 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 preemergence herbicide applications is highly dependent on even soil coverage, make applications as uniformly as possible. When applying to container 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 can be more effective. Granules are applied to dry foliage and if the foliage restricts the movement to the media surface, the foliage can be lightly brushed or shaken with a broomstick or similar tool to dislodge granules. Recalibration of the application equipment is important when changing herbicides as granule size and weight differ among herbicides.