Most weeds are controlled in field-grown flower crops with cultivation, hand-hoeing, preplant fumigation, and herbicides where there is crop safety. Crops are planted in rows to make cultivation easier and to reduce crop damage from the cultivators. The crops may be direct seeded, but are often transplanted as bulbs or plugs; transplants are more tolerant to handling and herbicides than direct-seeded crops. After establishment, the crops can be cultivated two or three times before the crop canopy closes and the crop begins to compete with the weeds. Some handweeding is usually required to remove all of the weeds.
Crop rotation is beneficial to reduce weeds in the crop. When the same crop is grown year after year, the population of weed species that escape normal cultural practices increases. Good rotation crops are barley, oats, or wheat in winter and corn or sudangrass in summer.
Unless the crop is sprinkler irrigated, a dust mulch often is created that keeps the soil surface dry and reduces the germination and establishment of annual weeds. Organic mulches used around transplants can reduce weeds if the light cannot reach the soil. Fine mulches (composted yardwaste) applied 1-1/2 to 2 inches deep before the weed seeds germinate or when weeds are still in the seedling stage will control most of the weeds. If the mulch is coarse, 3 to 6 inches may be required to completely eliminate light from the soil, but unlike fine mulch, they do not allow weed seed germination in the mulch. Bulbs such as Dutch iris may be good candidates for mulching for weed control. They have a large food reserve to push through the mulch and may have better growth after mulching. With some crop species, having a fine mulch right around the base of the plant may result in disease damage to plants. Mulches should not be used if they contain weed seed or plant propagules (e.g., bulbs, rhizomes, or tubers).
Solarization effectively controls many weed species before planting. Solarization must be done during periods of high solar radiation and temperature. Before placing the plastic on the site to be treated, closely mow any established plants, remove the clippings, and then water the area well. Place clear polyethylene that is ultraviolet (UV) resistant over the area, extend it about 2 feet beyond the infested area on all sides, and pull it tightly 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, purple and yellow nutsedge, and sweet clover. In areas where solar radiation and temperatures are low and marginal for solarization, purslane is not controlled well.
Shielded propane burners effectively control young weeds between rows without disturbing the soil and bringing weed seeds to the soil surface as cultivation would do. Flaming controls broadleaf weeds better than grasses.
Most crops have one or more selective preemergence herbicides available for use. See the table TREATMENT FOR FIELD-GROWN FLOWERS for selective herbicides that are registered on flower crops. Postemergence herbicides (fluazifop-p-butyl, sethoxydim) can be used to control some grasses in broadleaved crops. These are applied after the weeds emerge but while they are still small (1–3 inches in height). If weeds are larger than 3 inches, more herbicide will be required and some weeds may not be controlled. Broadleaf herbicides are generally not safe enough to use over flower crops. Even some selective grass herbicides may injure the crop if treated during the flowering stage. Control weeds when they are small and before they flower and set seed. If a few weeds remain, hand weed them.
Herbicides can be used before the crop is planted. The broad-spectrum fumigants dazomet, metam potassium*, and metam sodium* can be used on prepared beds to control weeds and other soilborne pests. Depending on which fumigant is used, crops can be planted within days or in 2 to 4 weeks to allow time for the fumigant to dissipate. (* Permit required from county agricultural commissioner.)
Another method before planting is called a "stale seedbed" treatment. Beds are prepared for planting and irrigated to germinate weeds. After most of the weeds have germinated, a postemergence herbicide like diquat or glyphosate is applied to kill the weeds. Because no cultivation is done before planting, no new weed seeds are brought to the surface. This "irrigate, germinate, eliminate" approach may be repeated two or more times before planting the crop to reduce the seed populations in the soil.
Preemergence herbicides also can be applied after seeding or transplanting a crop but before the weeds emerge. These herbicides generally must be incorporated mechanically along the planted row or leached slightly into the soil by rainfall or 1/2 inch irrigation after application. See specific comments on the herbicides in the table TREATMENT FOR FIELD-GROWN FLOWERS.
After the annual flower crop has been harvested, clean up any weeds before they set seed. This can be done most effectively by cultivating. If the field is not needed until the next season, it may be beneficial to plant a cover crop to reduce weeds, keep the soil from eroding, reduce dust, and maintain organic matter in the soil.