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How to Manage Pests

UC Pest Management Guidelines

Ornamental nursery container-grown plants.

Floriculture and Ornamental Nurseries

Container Nurseries

(Reviewed 3/09, updated 3/09)

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

The growth and vigor of nursery stock can be reduced when weeds are allowed to grow in the container for any length of time. Slow-growing crops that do not cover the container quickly are particularly vulnerable to weed infestations. Managing weeds in container nurseries involves eliminating weeds and their seed and preventing the introduction of new weed seeds into the nursery. Although soil mixes are usually weed-free at planting, weed seeds can be blown in from other areas or may be brought in with the liner (transplant). Frequently, preemergent herbicides are applied to the potting mix surface in gallon or larger containers to prevent establishment of these weed seeds. Mulches may also be applied after canning or after weeding. After container plants are established, preemergent herbicides are applied one or more times per year for weed management. Hand-pulling of weeds that have escaped the herbicide treatments is necessary to prevent them from setting seed and reestablishing a weed population.

Most weeds in a container nursery come from contaminated liners; plants growing in, between, or near pots; potting mix (if it is stored uncovered where weed seed can blow in); irrigation water; vehicles; equipment; movement of soil; birds; and windborne seeds. Transplants produced in the nursery or purchased from others should be free of weeds and weed seed. Move gallon-size plants into larger, weed-free containers. Use of preemergent herbicides in and between the containers can reduce contamination or reinfestation but care must be taken that herbicides are not carried in water runoff.

Nursery Site Selection and Preparation. When selecting a new nursery site, choose one that is free of perennial weeds and away or upwind from sources of windborne weed seed. Some of the weed species that have easily wind-dispersed seed are sowthistle, common groundsel, horseweed, hairy fleabane, cudweed, and willowherb.

The most effective way to manage weeds is to start with a clean area and to keep it clean by creating a weed-free, well-drained site for containers. Covering the nursery site with gravel, concrete, or a geotextile (landscape fabric) helps control weeds under and between containers. Control perennial weeds before grading and installing irrigation equipment because they are nearly impossible to control after a nursery is established.

Soil Mixture. Although potting mix is usually weed-free, it can become contaminated with weed seed if stored uncovered where seeds can blow in from neighboring areas. Fumigate, steam sterilize, or solarize any seed-contaminated soil mix. Check the soil mix periodically for weed seeds by placing soil mix samples in a flat or two. Keep the flats moist and check for weed germination for 1 to 2 weeks. If weeds grow, consider fumigation or solarization of the soil mix.

For fumigation to be most effective, the soil mixture needs to be uniformly wet for 3 to 4 days before fumigation treatment so that the weed seeds imbibe water and begin to germinate. If the mix is too dry, too wet, or if there are large soil clods, fumigation will not be uniform. Fumigation is most successful when the soil is placed on a concrete pad or in a container and the steam or fumigant is introduced at several locations in the mix.

There are two main methods to fumigate a soil mix:

  1. Steam fumigation. The steam is usually mixed with air and injected into a loose soil mix to heat the mix to at least 140oF (46oC) for 30 minutes. Length of time and temperature are critical if weed seeds are to be controlled. Cover the pile so that the entire pile, including the outer edges, reaches 140oF. A major problem of steam fumigation is that equipment, such as a boiler blower, is required.
  2. Chemical fumigation. Vapam (metam sodium*), K-Pam (metam potassium*), or Basamid (dazomet) fumigation is sometimes used as a preplant treatment in potting mixes. Basamid is a dry formulation that must be mixed into the potting mix before wetting the pile. The pile is then covered for about 2 weeks as the Basamid degrades into the active fumigant, methyl isothiocyanate. The cover is removed and the soil allowed to air for 2 weeks before using the mix for potting. Vapam and K-Pam are liquids that can be applied in water to the mix and then tarped for 2 weeks. Air out the soil for 2 weeks before the crop is planted. Fumigants such as metam sodium or metam potassium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.

*Requires a permit from the county agricultural commissioner for purchase or use.

Although less commonly used, soil solarization can also be used to control weeds in the potting mix before planting. (See discussion of soil solarization in "General Methods of Weed Management" at the beginning of the weed section.)

Monitoring Container Nurseries. Common weeds in container production nurseries are listed in Table 3. Because many of these weeds can germinate year-round in the nursery, check the containers regularly. Some weed species can flower and produce seed in only a month from seedling stage, so monitoring followed by hand-weeding is needed at least monthly to remove any weeds that were missed by herbicide treatments or from the last hand-weeding. It is essential to monitor for winter annual weeds germinating in late summer and for summer annuals germinating in late winter.

TABLE 3. Common Weeds in Container Production Nurseries.
Common name Scientific name
bittercress Cardamine spp.
cudweed Gnaphalium stramineum
groundsel, common Senecio vulgaris
lettuce, prickly Lactuca serriola
liverwort Marchantia polymorpha
pearlwort, birdseye Sagina procumbens
sowthistle, annual Sonchus oleraceus
spurge, prostrate or spotted Chamaesyce maculata
willowherbs Epilobium spp.
woodsorrel, creeping Oxalis corniculata

Identifying the weeds present in a given situation is an important factor in deciding which weed control strategy to take. Use the Grower's Weed Identification Handbook, UC ANR Publication 4030, Weeds of California, UC ANR Publication 3488, and the UC IPM Weed Photo Gallery, to help identify weeds. University of California Cooperative Extension farm advisors or county agricultural commissioners, botanic gardens, or arboretum personnel can also help with weed identification. Once the weed is identified, the herbicide susceptibility tables will help to determine the best herbicide or combinations of herbicides to supplement a weed management program and provide optimum control of the weed species present.

Herbicides. Preemergent herbicides are used extensively in container-grown ornamentals, usually in conjunction with handweeding, to control any weeds that escape the chemical treatment. The herbicides used depend on the weed species expected (see monitoring section), the time of year, the stage of the ornamental plants, and the tolerance of the ornamental plants to the herbicides. The weed species present at a particular site must be properly identified in order that the right herbicide is selected.

Because they are selective, the herbicides used in container production will not harm the ornamental species listed on the label if care is taken to use them properly. A number of factors determine if the ornamental plant will be adversely affected. These include:

  • Plant size. The smaller the plant the greater the sensitivity to herbicides, and therefore, the greater the likelihood of it being injured.
  • Degree of plant establishment. Newly-planted plants are more sensitive, because generally they have smaller root systems than established plants.
  • Soil texture and organic matter content. These properties can affect an herbicide's tendency to leach into the root zone. Some herbicides can be more strongly adsorbed on soil particles than others and on different soil fractions. The more clay and organic matter content, the greater the binding and usually the less leaching.
  • Rate of plant growth. Actively growing plants often are more sensitive to injury from certain herbicides than dormant plants.
  • Dosage of the herbicide. Higher dosages can cause crop injury; rates above the label are illegal. Use the lowest dose that will control the weeds in question.
  • Temperature. Temperature affects rates of chemical reactions in plants. Higher temperatures can greatly increase the speed of chemical reaction, which results in greater injury to plants as well as to weeds. Higher temperatures may also increase herbicide absorption through leaves and roots.
  • Tank mixing of products. Mixing wetting agents or another product that has wetting agents in it with an herbicide that has postemergent activity can greatly increase the activity and perhaps crop injury and affect selectivity.
  • Spray techniques. The method of application will affect distribution of the herbicide on the target. For most herbicides, the height of the spray boom should be adjusted so that the top of the ornamental plant receives uniform spray distribution. This means that with normal spray booms equipped with fan type nozzles, the nozzles should be at least 20 to 24 inches above the top of the plants. Spray booms adjusted too low can cause plant injury with certain herbicides. Individual nozzles should be checked for proper output.
  • Formulation. Granular formulations are generally safer with some products than the emulsifiable concentrate or wettable powder formulations. However, injury can result if the granules collect in the whorl of the plant.

Herbicide runoff can be a serious problem in some situations, so observe the following precautions to reduce runoff from occurring: (1) use herbicides with water solubility of less than 3.5 ppm; (2) spot treat; (3) use low-volume applications; (4) use only as much water as needed to move the herbicide into soil with the first irrigation following an application.

Container spacing can also affect herbicide loss when granular herbicides are applied. Tight spacing of containers can keep 50% more of the herbicide in the container rather than on the ground compared to containers that are spaced 8 inches apart. Additionally, herbicide loss can be reduced if drop spreaders are used rather than rotary spreaders. However, herbicide that falls on the ground is not totally lost because it helps control weeds between containers and thus contributes to the total weed management potential.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
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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