Weed control in matted-row strawberry culture systems of the Pacific Northwest (PNW) is a difficult challenge. Like in most of the northern United States, PNW strawberries are usually produced in a three-year cycle with tillage used for weed and runner control between the rows. Matted-row culture without weed control caused a 14% reduction in ‘Jewel’ strawberry yield during the second year and 51% reduction during the third (Pritts and Kelly, 2004). Kelly et al. (2007) reported that finger weeders and brush hoes may improve in-row weed control in newly transplanted strawberry, although their use is not widespread in the PNW. Organic strawberry production currently relies nearly exclusively on hand weeding and the use of plastic mulch (Pritts and Kelly, 2001). When available, hand weeding is usually expensive, accounting for an estimated 75% of labor costs the first year. Weeding crews may disturb rooting of daughter plants within the row as weeds are removed, potentially delaying row closure and strawberry establishment. Plastic mulches are widely used for weed control in annual strawberry production (Berglund et al., 2006; Johnson and Fennimore, 2005), but they prevent daughter plant establishment and therefore are not suitable for matted-row systems (Kelly et al., 2007). In effort to improve in-row weed control in organic matted-row strawberry, natural weed control products and practices have been evaluated.
Corn gluten meal has shown promise as an organic amendment. It contains ≈10% nitrogen (≈47% crude protein) and has been used as both a fertilizer and weed control product (Christians, 1993). In strawberry, 90% reduction in cover of common lambsquarters (Chenopodium album), common purslane (Portulaca oleracea), redroot pigweed (Amaranthus retroflexus), large crabgrass (Digitaria sanguinalis), and green, giant, and yellow foxtail (Setaria viridis, S. faberi, and S. pumila, respectively) was reported from application of 490 g·m−2 (Nonnecke and Christians, 1997). Applied at renovation in July, CGM increased weed cover and weed density by August, but if applied in August, CGM reduced weed cover and dicot weed density by September (Nonnecke and Christians, 2002). Conversely, CGM applied in July reduced dicot weeds by August in two of three years, although strawberry yield increased in only one year (Dilley et al., 2002). Strawberry yield was also decreased if CGM was applied too often in a single growing season, likely as a result of excess nitrogen contributing to abundant foliar growth (Nonnecke and Christians, 1997). In Turkey, CGM at 4 t·ha−1 applied in August before transplanting of strawberry resulted in control of shepherd’s-purse (Capsella bursa-pastoris), annual bluegrass (Poa annua), and prostrate knotweed (Polygonum aviculare) (80%, 74%, and 49%, respectively) by May (Albay and Boz, 2003). Like CGM, WG inhibited weed seedling growth (Gough and Carlstrom, 1999), but there are no reports for use of WG in strawberry.
Mustard seed meal contains glucosinolates, which break down in soil to isothiocyanate in moist soil (Borek and Morra, 2005; Brown et al., 1991; Vaughn and Berhow, 2005; Vaughn et al., 2006). Boydston et al. (2011) demonstrated that much of the herbicidal activity of MSM is the result of its glucosinolate content with effective doses for 90% redroot pigwweed control of 14.5 and 3.2 g·m−2 for two high-glucosinolate MSM, although low-glucosinolate MSM has also suppressed growth of common purslane and redroot pigweed (Bañuelos and Hanson, 2010). High-glucosinolate MSM (yellow mustard, Sinapis alba) provided up to 98% control of annual bluegrass, 74% control of common chickweed (Stellaria media), 95% of creeping woodsorrel (Oxalis corniculata), and 97% control of liverwort (Marchantia polymorpha) with no observable injury to container-grown ornamentals (Boydston et al., 2008). High-glucosinolate MSM (S. alba) applied up to 4.5 t·ha−1 reduced counts of common lambsquarters, redroot pigweed, and henbit (Lamium amplexicaule) by up to 50% but also reduced onion bulb yield 29% when applied three times at 2.2 t·ha−1 (Boydston et al., 2011). Rice et al. (2007) reported 72% to 99% control of common lambsquarters and redroot pigweed with high-glucosinolate MSM (S. alba) but also 80% injury to seeded vegetables in one year out of two. The pesticidal effects of Brassicaceous seed meals on insects and soilborne pathogens has been reported, but effects have been variable (Borek et al., 1997; Mazzola et al., 2001; Smolinska et al., 1997). High-glucosinolate MSM (brown mustard, Brassica juncea) has been shown to be toxic to dagger nematode (Pratylenchus penetrans) at rates of 0.05% to 0.1% (w/w) in soil with no reduction in strawberry shoot or root dry weight (Qing et al., 2007).
Flame has been used for selective weed control in crop production since the early 1940s (Anderson, 1977). Flame weeding is most commonly used as part of a stale seedbed strategy; crops are seeded several days after final seedbed preparation and weeds removed with flame before crop emergence. Organic vegetable growers have used flaming coupled with a stale seedbed for many years (Cramer et al., 1991; Stopes and Millington, 1991; Wookey, 1985). These techniques reduced density and biomass of common purslane and common chickweed in simulated vegetable seedings (Caldwell and Mohler, 2001). A single flaming 4 d after seedbed preparation and 1 d before transplanting lettuce seedlings reduced weed density by 62% (Balsari et al., 1994). Although propane flame applied at summer renovation is sometimes used to reduce disease inoculum and mite infestations in strawberry (Daugaard, 1999; Meshcheryakova, 1986; Titov, 1986), flaming for weed control in this crop has not been widely investigated. The usefulness of flame for weed control in newly planted strawberry has been noted, but only as a factor in reducing a reservoir for other plant pests (Gengotti and Lucchi, 2000).
Corn and wheat gluten products, MSM products, and propane flaming may offer additional weed control options for both organic and conventional strawberry producers in the PNW. Because neither crop safety of these treatments nor their efficacy on the weed species that flourish in the maritime climate of western Washington state had been previously documented, a trial was conducted to evaluate efficacy and crop safety of organic amendments and propane flaming in matted-row strawberry.
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