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Hannah Mathers, Ohio State University Extension, Horticulture and Crop Science |
Herbicides are applied either to the foliage of growing weeds (postemergence) or to the soil to prevent germination (preemergence). Postemergent foliar herbicides are either contact or systemic chemicals.
Contact herbicides weaken and disorganize the plant cell membranes causing leakage and eventual localized death. Contact herbicides are generally most effective against annuals. Complete coverage is essential in weed control with contact herbicides. An example of a contact herbicide is Gramoxone.
Systemic herbicides include the phenoxy herbicides (e.g., 2,4-D) and dicamba, picloram, amitrole, and glyphosate. Systemic herbicides are translocated throughout the plant to their sites of physiological action. Translocated herbicides are effective against all types of weeds; however, they have their greatest advantage when used to control established perennials. Complete coverage is not required with translocated materials; however, uniform applications are critical.
Postemergents are generally best when applied to young plants and may take several days to work. The older formulations of Roundup, for example, would take 10 to 14 days to show activity. The newer Roundup Ultra formulation works much faster.
Postemergent contact with active growth of the ornamental plant, including green bark, will result in injury. So contact with the ornamental plant or conditions that could promote contact, such as high winds, should be avoided. Poor results can occur with postemergent applications if the weed is under stress at the time of application, if rainfall occurs within six hours, or if heavy rainfall occurs within two hours.
Preemergent herbicides (e.g., Casoron, Surflan, Ronstar) are applied either to the soil or growing medium surface and are usually absorbed by root systems, or by emerging shoot tips as they make their way through the soil surface during seed germination. Most must be dissolved in the soil/medium to work. Generally, one-half inch of irrigation or rainfall after application of a preemergent is required to activate the herbicide. Soil type (sand, gravelly soils, clay), organic matter content, and temperature are all important in determining the effect and activity of a preemergent application. Because they are preemergents, they must be applied to weed-free surfaces. Generally, nurseries make two applications of preemergents per year, during the fall and spring.
There are also chemicals called soil fumigants. Soil fumigants are used to kill weeds and underground plant parts, but they are also used to control nematodes and disease-causing pathogens in the soil. Soil fumigants are used before planting high-value crops, for seedbed preparation, and sometimes before planting high-value landscapes or treating potting soil (Kuhns etal., 1998).