Weed control conifers

Development of weed resistance to triazine herbicides has necessitated alternative weed control strategies. Rotation of triazines with other herbicides and combining triazines with other chemistries has been effective in ornamentals and Christmas tree plantations (Van Himme, 1989). Supplemental treatments in Christmas trees have been effective, using directed sprays of phenoxy herbicides or glyphosate before resistant weeds mature and produce seeds. Weed resistance from triazines is not a problem in most forest settings where the herbicide is applied only once or twice in a rotation, since 1 or 2 years of herbaceous weed control normally ensures survival and eventual dominance by conifers. When the conifer canopy closes, virtually all herbaceous vegetation is shaded out for some decades hence resistant weeds, if present, fail to survive from one generation to the next. 

Additional reasons to manage weeds in ornamental plants and conifer trees include fire protection, frost protection, rodent and disease mitigation, nutrient management, and aesthetics. Uncontrolled weeds and brush become serious fire hazards in the fall or early spring when herbaceous weeds are dry. Plantations of conifers have been lost because weeds were not controlled and fires started. Fires are less destructive when herbaceous vegetation is controlled. 

Simazine is widely used for preemergence control in field-grown nursery stock in North America. Selective use in container nurseries also has been demonstrated (Ahrens, 1972 Fretz, 1974 Wadsworth, 1975 Bing, 1983). It is less widely used in container-grown nursery production because of potential leaching (Elmore et al., 1976), but is especially effective when applied in the fall or winter to control winter annual weeds in containers. Simazine currently is registered in the United States for 50 species of woody ornamental nursery stock and Christmas trees. Tables 18.1 and 18.2 list selected species of conifers and deciduous ornamental plants, respectively, and their observed tolerance to simazine at rates of 2.2-3.3kg/ha. Information for these tables was obtained from personal observations and the literature, including those references by Ries et al. (1959) Ahrens (1961) Ticknor (1972) and Schubert et al. (1986). 

Triazine herbicides are primarily effective in controlling herbaceous weeds. A notable exception is the use of granular hexazinone for controlling woody invaders such as oaks (Quercus spp.) or aspen (Populus tremuloides) in conifer stands on medium- and fine-textured soils in the South or in boreal forests. Hexazinone also is used in the Maritime Provinces of Canada for controlling woody and herbaceous growth in natural Christmas tree stands (Townsend, 1995a). The impact of herbaceous plant cover is somewhat different from that of woody plant competitors, so control strategies on various forest conifer sites may differ. 

Oxyfluorfen is applied as a preemergence herbicide in soybeans, cotton, tomatoes, tobacco and green pepper at a rate of 0.2-0.4 kg active ingredient/ha. Grassy weeds are not well controlled by oxyfluorfen, but in the case of heavy grass weed infestation it can be tank—mixed with thiocarbamates, dinitroanilines, chloroacetamides, dalapon and paraquat. Oxyfluorfen is also recommended in tree fruits, vineyards and in conifer nurseries as an early postemergence treatment. 

Selective systemic herbicide. Inhibits photosynthesis. Used tor postemergence control of broad-leaved weeds and some grasses In brassicas, herbs, onions, leeks and conifer seed beds. Crystals mp = 84 - 86 soluble in H2O (0,058 g/100 ml). MeOH (30 g/100 ml), Me2CO (23 g/100 ml), CH2CI2 (20 g/1200 ml), C7H8 (20 g/100 ml), CeHiA (0.26 9/100 ml) LDso (rat orl) = 1390 mg/kg, (rat der) > 1000 mg/kg toxic to fish non-toxic to bees. Ciba-Geigy Corp. Syngenta Crop Protection. 

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