Hooker’s evening primrose is a biennial to short-lived perennial native to areas as far north as Oregon, south as Panama, east as Texas, and west as coastal California (Dietrich et al., 1997). Plants are often found in full sun along streams and rocky slopes as well as in meadows and disturbed areas (Arnold, 2008; Dietrich et al., 1997). Hooker’s evening primrose shoots (1.5 to 8 ft tall) emerge from the center of a basal rosette at about the 20- to 50-leaf stage (Dietrich et al., 1997; Harte, 1994). Leaves are dull green to gray-green with bluntly dentate or subentire margins (Dietrich et al., 1997). Showy yellow flowers (1.2 to 1.4 inches wide) usually only occur once at sunrise or sunset during periods of long days and short nights (Correll and Johnston, 1970; Dietrich et al., 1997; Harte, 1994). Flowers are pollinated by hawk moths (family Sphingidae) and give rise to seed capsules measuring 0.8 to 2.6 inches long (Dietrich et al., 1997). Each capsule splits in four places and may contain up to 500 seeds (Harte, 1994).
Drought tolerance and floral attributes of hooker’s evening primrose have led to its use as an ornamental plant in xeriscapes throughout arid and semiarid regions of the United States (Arnold, 2008). High levels of γ-linolenic acid, an omega-3 fatty acid essential for human metabolism, present in seed oil has further increased interest in the cultivation of this plant as an agronomic crop (Balch et al., 2003). However, because of poor field germination, hooker’s evening primrose must first be propagated in the greenhouse and transplanted into the field following maturation to the rosette stage (Murphy et al., 1999).
The presence of weeds in agricultural fields often increases the time and costs of crop production as well as reduces crop yields and quality (Anderson, 1996). Bridges (1992) estimated that the average annual monetary loss because of weed competition in 46 U.S. grown crops was $4.1 billion in 1991. Losses are directly proportionate to the amount of light, water, and nutrients that weeds intercept from the agronomic crops they infest (Buchanan and Burns, 1970). The critical weed free period for many crops exists during early establishment. The critical weed free period for sorghum (Sorghum bicolor), beet (Beta vulgaris), and field bean (Phaseolus vulgaris) to prevent yield loss is 4, 9 to 12, and 5 to 7 weeks, respectively (Burnside and Wicks, 1967; Dawson, 1964, 1965). Early critical weed-freeperiods and phytotoxicity concerns associated with many postemergence herbicides make preemergence herbicide applications even more important.
Few research trials have focused on the phytotoxic effect of preemergence herbicides on evening primrose species. Richardson and West (1986) observed no reduction in common evening primrose fresh shoot weight grown from seed 42 DAT in response to isoxaben (0.075 kg·ha−1) and trifluralin (1.0 and 2.0 kg·ha−1). However, applications of simazine (0.25 and 0.5 kg·ha−1), chlorsulfuron (0.01 and 0.02 kg·ha−1), and metsulfuron (0.00375 and 0.0075 kg·ha−1) were lethal to common evening primrose at 42 DAT. Stringer et al. (1985) reported 91% to 100% survival of seeded common evening primrose at 68 DAT in response to trifluralin (0.8 kg·ha−1), EPTC (2.0 kg·ha−1), propachlor (4.0 kg·ha−1), and linuron (0.25 kg·ha−1). However, when applied at a higher rate, linuron (1.0 kg·ha−1) and lenacil (0.5 and 2.0 kg·ha−1) reduced common evening primrose survival below 9% at 68 DAT.
No research has investigated the phytotoxic response of hooker’s evening primrose plug transplants to preemergence herbicides. Varied phytotoxic responses of common evening primrose to preemergence herbicides suggest that options may exist for the safe control of weeds present within hooker’s evening primrose when grown as an agronomic field crop. Enhanced weed control during early establishment may reduce competition for water and nutrients as well as increase seed yield and oil content. Therefore, the objective of this research was to determine the phytotoxic effect of preemergence herbicides on hooker’s evening primrose plug transplants grown in the greenhouse.
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