Olive (Olea europaea L.) orchard plantings have increased in Texas over the past 10 to 15 years primarily southwest of San Antonio and in the Hill Country (Stein et al., 2013). Olives have historically been produced under dryland conditions with trees spaced widely apart (173 trees/ha) to reduce intraspecific competition for stored soil water reserves (Rapoport et al., 2004; Vossen, 2007). In the early 1980s, irrigated orchards were planted in high densities (617 trees/ha) in attempts to reach full production quicker, reduce alternate-bearing behavior, and increase olive yields (Beede and Goldhamer, 1994; Pastor, 1994; Tous et al., 1999). Varieties used in high-density olive orchards are pruned to a central leader and staked with a pole for support (Stein et al., 2013; Vossen, 2007). The tree canopy starts ≈0.6 m above the orchard floor with plants reaching a maximum height of 2.1 m (Stein et al., 2013; Vossen, 2007). Although high-density systems increase olive yields, trees may still require 8 to 10 years to reach full production, harvest costs are high, mechanical hedging often reduces tree vigor, and pest control is more difficult in such a compact system (Civantos Lopez Villalta, 1998; Vossen, 2002).
Proper weed control practices are critical in olive orchards to reduce competition for valuable inputs such as water, nutrients, and sunlight, as well as to reduce the likelihood of negative impacts from pests (insects, nematodes, and pathogens) residing on the orchard floor. Goff et al. (1991) reported less K, Ca, and Mg concentrations in pecan [Carya illinoinensis (Wangenh.) C. Koch] tree leaves growing in plots infested with weeds compared with plots where weeds were controlled through use of herbicides or cultivation practices. Less disease was observed in tomato (Solanum lycopersicum L.) production when weed control practices were implemented (Wszelaki et al., 2005). Killian and Meyer (1984) observed that the number of peaches [Prunus persica (L.) Batsch] damaged by cat-facing insects in herbicide-treated plots was significantly less than that of fruit sampled from nontreated control plots.
Plant competition can be most detrimental during the first few years of orchard establishment. Weed control efforts must be focused within close proximity to the base of olive trees. However, few preemergence herbicides are available for the effective control of weeds in olive orchards. Montemurro et al. (2002) observed adequate control of several broadleaf and grass weeds with preemergence applications of oxyfluorfen at 0.6 kg a.i./ha. Treated plots exhibited higher crop yields than weedy control plots with minimal olive tree phytotoxicity (Montemurro et al., 2002). Flumioxazin, isoxaben, indaziflam, oryzalin, oxyfluorfen, pendimethalin, diuron, and simazine are labeled for preemergence applications to orchards, but several of these herbicides may only be applied to nonbearing olive trees or during orchard establishment (UC Statewide IPM Program, 2014).
Weed control is often achieved or enhanced with applications of organic mulches (cereal straw, green waste, or composted wood chips) or synthetic mulches (polyethylene, polypropylene, or polyester) around the base of trees (UC Statewide IPM Program, 2014). Applications of cattail (Typha latifolia L.) and rice (Oryza sativa L.) mulch to the floor of a mandarin orange (Citrus reticulata Blanco) grove reduced weeds 85% to 98% compared with nonmulched plots (Abouziena et al., 2008). Burkhard et al. (2009) observed weed biomass reductions of 55% to 73% when highbush blueberries (Vaccinium corymbosum L.) were mulched with pine needles. Mulches not only reduce weed germination and growth by blocking light to the soil surface, but may aid in soil moisture retention during droughty periods of the growing season (Moody et al., 1963; Ramakrishna et al., 2006; Teasdale and Mohler, 1993; UC Statewide IPM Program, 2014; Wesson and Wareing, 1967). Cregg and Schutzki (2009) reported that mulch (pine bark, hardwood fines, cypress mulch, and color-enhanced ground pallets) applications increased soil moisture retention in several shrub species compared with no mulch + weed control treatments. Furthermore, covering preemergence applications with mulch may reduce photodegradation, volatilization, and/or runoff of herbicides that remain at the soil surface, especially when irrigation or rainfall is limited (Mickelson et al., 2001). Wooten and McWhorter (1961) observed better weed control with subsurface applications of S-ethyl dipropylthiocarbamate using a horizontal blade applicator compared with surface applications of the same herbicide. Less herbicide runoff of atrazine, metolachlor, and cyanazine were observed in response to herbicide incorporation into the soil compared with no-till surface applications (Mickelson et al., 2001). Therefore, further investigation of preemergence herbicides in combination with mulch for weed control in olive orchards is warranted. Identification of improved weed control techniques will enhance olive tree health and annual olive yield.
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