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 surface of the media in 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 growing medial is usually weed-free at planting, weed seeds can be blown in from other areas or may be brought in with the liner (transplant). Frequently, preemergence herbicides are applied to the surface of potting mix in containers of 1 gallon or larger to prevent the establishment of weed seeds. Mulches may also be applied after canning or after weeding. After container plants are established, preemergence 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
- irrigation water
- movement of soil
- plants growing between, in, or near pots
- potting mix, if it is stored uncovered where weed seed can blow in
- windborne seeds
Transplants produced in the nursery or purchased from others should be free of weeds and weed seed and moved into larger, weed-free containers. Use preemergence herbicides in and between the containers to reduce contamination or reinfestation, but take care so that herbicides are not carried off-site in water runoff.
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 concrete, a geotextile (landscape fabric), or gravel 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.
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, solarize, or steam sterilize any seed-contaminated soil mix. Check the soil mix periodically for weed seeds by placing samples of soil mix 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 absorb water and begin to germinate. If the mix is too dry or too wet or there are large clods of soil, fumigation will not be uniform. Fumigation is most successful when the soil is placed on a concrete pad or in a container and the fumigant or steam is introduced at several locations in the mix.
There are two main methods to fumigate a soil mix:
- Steam fumigation. The steam is usually mixed with air and injected into a loose soil mix to heat the mix to at least 140°F 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 140°F. A major problem of steam fumigation is that equipment, such as a boiler and blower, are required.
- Chemical fumigation. Fumigation with pesticides such as dazomet (Basamid), metam potassium*, or metam sodium* is sometimes used as a preplant treatment in potting mixes. For recommended fumigants, see MANAGEMENT OF ROOT-KNOT AND OTHER SOIL-DWELLING NEMATODES. Dazomet is a dry formulation that is mixed into the potting mix before wetting the pile. The pile is then covered for about 2 weeks as the dazomet 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. Metam potassium and metam sodium 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 planting the crop. Fumigants such as metam potassium and metam sodium are sources 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 the discussion of soil solarization in GENERAL METHODS OF WEED MANAGEMENT at the beginning of the weeds section.
Monitoring Container Nurseries
See the Common Weeds in Container-Production Nurseries table for links to photographs and more information on these species. 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 the 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.
|Common name||Scientific name|
|groundsel, common||Senecio vulgaris|
|lettuce, prickly||Lactuca serriola|
|pearlwort, birdseye||Sagina procumbens|
|sowthistle, annual||Sonchus oleraceus|
|spurge, prostrate or spotted||Chamaesyce maculata|
|woodsorrel, creeping||Oxalis corniculata|
Identifying the weeds present in a given situation is an important factor in deciding which weed control strategies to employ. Use the Weeds of California, UC ANR Publication 3488, and the UC IPM Weed Photo Gallery to help identify weeds. University of California Cooperative Extension advisors or county agricultural commissioners, botanic gardens, or arboretum personnel can also help with weed identification. Once the weed is identified, the herbicide labels and tables of SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL will help you determine the best herbicide or combinations of herbicides to supplement a weed management program and provide optimum control of the weed species present.
Preemergence 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 to select the effective herbicide. Apply the herbicide as soon as possible to achieve optimal weed control but keep in mind that some plants may be injured if applied before the soil has settled around the roots.
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 the
- Degree of plant establishment. Newly-planted plants are more sensitive because generally they have smaller root systems than established 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 targeted. Note that using too low of rate can promote the development of resistance to herbicide.
- 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.
- Plant size. The younger the crop plant the greater the sensitivity to herbicides, and therefore, the greater the likelihood of it being injured.
- Rate of plant growth. Actively growing plants often are more sensitive to injury from certain herbicides than dormant 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. As clay and organic matter content increase, binding increases and usually there is less leaching.
- 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 flow rate and spray pattern.
- Tank mixing of products. Mixing in wetting agents (chemicals that reduce the surface tension of liquids) or another herbicide that has wetting agents in it with an herbicide that has postemergence activity can greatly increase the activity and perhaps also increase crop injury and reduce selectivity. Check the label and follow any instructions regarding adding adjuvants or tank-mixing products.
- Temperature. Temperature affects rates of chemical reactions in plants. Higher temperatures can greatly increase the speed of chemical reactions, which can result in greater injury to plants as well as to weeds. Higher temperatures may also increase herbicide absorption through leaves and roots.
Herbicide runoff can be a serious problem in some situations, so observe the following precautions to reduce runoff:
- Spot treat.
- Use herbicides with water solubility of less than 3.5 ppm as listed in the Herbicide Handbook and certain online guides (PDF).
- Use low-volume applications. 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.