Weed management in container production is one of the major problems facing the nursery industry. Annual weeds in particular have the ability to grow under a wide range of environmental conditions. As a result of their small size and limited reserves, successful germination of annual weed species occurs at the soil surface or at a shallow depth (Popay and Roberts, 1970). Annual bluegrass thrives best under cool, moist, and fertile conditions. In northern parts of the United States, it may be more of a summer annual than a winter annual. It is unable to tolerate extreme conditions and may die under such conditions (Holm et al., 1997). Common groundsel is a summer or winter dicotyledonous annual that reproduces by seed with optimal temperatures for germination ranging from 10 to 25 °C (Ren and Abbott, 1991). Extremely hot and dry conditions may lead to its death (Uva et al., 1997). Certain strains of common groundsel are found to be atrazine-resistant and glyphosate-resistant and are of great concern to the nursery industry (Mathers, 2002). Shepherd's purse is mainly a problem weed in temperate areas, and in the tropics it is found only at higher elevations. In the case of nondormant weed species exposed to light, increased seed germination is observed when the temperature fluctuates from a low of 15 °C to an optimal high of 25 to 30 °C (Popay and Roberts, 1970). Controlling weeds in container plants becomes essential as a result of the limited availability of nutrients, water, and space to the landscape plant. Weed occurrence in containers can cause growth reduction of woody ornamentals by nearly 50% in a single growing season (Fretz, 1972) and decrease the aesthetic value of the crop plant.
Hand weeding is not an economic solution to weed control in nursery containers as a result of the high cost of labor. Because nursery growers can spend up to $4000/acre on manual removal of weeds in containers depending on weed species (Mathers, 2003), chemicals are used to control weeds in a more cost-effective manner. Issues pertaining to direct herbicide application in container-grown ornamental plants include the need for repeated herbicide application, nonuniform applications of herbicide, herbicide loss to the environment, and phytotoxicity. Direct spray application of herbicides to containerized plants can result in leaf scorch, chlorosis on leaves and stems, and reduced plant growth (Adams, 1990). As much as 80% of broadcast herbicide may settle into spaces between containers (Gilliam et al., 1992) and leach or move offsite with runoff water. In many cases, uniform application of herbicides to the soil of containerized plants may not be possible because plant canopies irregularly cover the containers (Gorski, 1993).
As a result of increasing financial and environmental concerns, recent research focus has been on minimizing chemical use yet ensuring profitability in nursery crop production (Mathers, 2003). An integrated approach to controlling weeds that incorporates more than one control measure, including chemicals, thus becomes important. Herbicide carriers could help reduce the problem of quick release of herbicides into the environment. Geotextiles, when used in combination with preemergence herbicides, gave superior weed control (Appleton and Derr, 1990). Slow-release herbicide tablets were effective in reducing herbicide phytotoxicity to crop plants (Koncal et al., 1981). Discs made of fabric and plastic materials were equally effective or better in controlling weeds in containers than hand-weeding, herbicides, or geotextile fabrics (Chong, 2003). Organic mulches can be used to control weeds, conserve moisture in the soil, regulate soil temperature, and decrease weed seed germination, thus improving plant growth and reducing labor. Although the primary use of mulches in the horticulture industry is in the landscape, they can serve as herbicide carriers in container production. Oryzalin-treated douglas fir bark increased weed control by a factor of 1.8 and the duration of efficacy by a factor of 2.8 compared with bark alone. Compared with the application of the herbicide in water, the herbicide-treated bark also reduced phytotoxicity by 2.2 times and increased efficacy by 1.5 times (Mathers, 2003).
This study evaluated and compared rice hulls, landscape leaf-waste pellets, and pine bark as carriers for the preemergence herbicides oryzalin and diuron. The dinitroaniline herbicide group, which includes oryzalin, is the most widely used group of herbicides in nursery and landscape settings for the control of grassy weeds and small-seeded broadleaf weeds. Diuron is registered as a pre- and postemergent herbicide for a number of agronomic and fruit crops and has a half-life of 90 d (Monaco et al., 2002). It belongs to the urea group of herbicides that disrupt the photosynthetic process of susceptible plants. When applied postemergent, urea herbicides act as contact burners affecting only the stems and foliage, whereas the plant roots remain unaffected. The affected plants show yellowing of the foliage with eventual defoliation, leaving only the stem (Monaco et al., 2002). Diuron is potentially phytotoxic to spirea and several other ornamental plants (Simpson et al., 2004) making a good test system for the ability of organic mulches to reduce phytotoxicity.
Pine bark is a common and popular material used for mulching and as a media component. When used as mulch, pine bark can inhibit weed seed germination while allowing water to percolate through (Aaron, 1972). Pine bark is mixed with components like peat, sand, vermiculite, and perlite to form container media and is frequently used in container media to replace peatmoss, a relatively costly product (Laiche, 1990; Pokorny, 1966). Rice hulls are byproducts of the rice milling process and can be obtained inexpensively in large quantities from major rice-producing states including Arkansas, California, Louisiana, Mississippi, and Texas (Einert and Baker, 1973; Hough and Barr, 1956; Laiche and Nash, 1990). Rice hulls can be used to substitute for sand in containerized bare root whip production (Struve and McCoy, 1996) by improving the drainage and aeration of the media and adding manganese (Einert and Baker, 1973). Deciduous landscape leaves are a major waste product in suburban landscape settings. There is a potential use for this waste product that could improve nursery production efficiency and reduce environmental pollution. The objective of this study was to evaluate and compare rice hulls, landscape leaf-waste pellets, and pine bark as herbicide carriers for preemergence herbicides in the control of annual weeds in container-grown ornamental plants.
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