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UC Pest Management Guidelines


Field-grown snapdragon.

Floriculture and Ornamental Nurseries

Field-grown Flowers

(Reviewed 3/09, updated 3/09)

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

Most weeds are controlled in field-grown flower crops with cultivation, hand-hoeing, and preplant fumigation. Crops are planted in rows to make cultivation easier, and there is less crop damage from the cultivators. The crops may be direct seeded but are often transplanted as plugs or bulbs. 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 wheat, oats, or barley in winter and sudangrass or corn in summer.

Mulch 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 at 1.5 to 2 inches in depth 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 4 inches or more 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 (tubers, rhizomes, or bulbs).

Soil Solarization. Solarization is an effective method of controlling 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, ultraviolet (UV)-protected polyethylene 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, yellow and purple nutsedge, and sweet clover. In areas where radiation and temperatures are marginal for solarization, purslane is not controlled well.

Flaming. Shielded propane burners are effective in controlling 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 it controls grasses.

HERBICIDES

Most crops have one or more selective preemergent herbicides available for use. See Table 4 for selective herbicides that are registered on different flower crops. Postemergent herbicides can be used on a few flower crops. Most grass weeds can be controlled selectively with postemergent herbicides but broadleaf weeds cannot. Control weeds when they are small and before they flower and set seed. If a few weeds remain, rogue them out by handweeding.

Herbicides can be used before the crop is planted. The broad-spectrum fumigants metam sodium* (Vapam), metam potassium* (K-Pam), and dazomet (Basamid) can be used on prepared beds to control weeds and other 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.

Another method of using herbicides 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 postemergent 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 2 or more times before planting the crop to reduce the seed populations in the soil.

Preemergent 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 0.5 inch irrigation after application. See specific comments on the herbicides in the Treatment Table.

Postemergent herbicides (fluazifop-p-butyl, sethoxydim) are available to control most grasses in broadleaf 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, it will require more herbicide and some weeds may not be controlled. Broadleaf weed herbicides are generally not safe enough to use over flower crops. Even some selective grass herbicides may injure the crop if treated during flowering stage.

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, to keep the soil from eroding, reduce dust, and to maintain organic matter in the soil.

*Permit required from county agricultural commissioner.

[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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