Commercial production of blackberries (Rubus L. subgenus Rubus, Watson) has increased rapidly in recent years from a total of 13,960 ha worldwide in 1995 to 20,035 ha in 2005 (Strik et al., 2007). Approximately 6000 ha are currently produced in the United States (U.S. Department of Agriculture, 2010a). More than half of the U.S. total is processed fruit (individually quick frozen, bulk frozen, puree, freeze-dried, canned, juice, and juice concentrate) produced primarily in Oregon and Washington (Strik et al., 2007). Organic production has also increased to greater than 2500 ha worldwide, but there was only 200 ha in the United States in 2008 (Strik and Finn, 2012; U.S. Department of Agriculture, 2010b). However, significant expansion in organic plantings is expected in the next 10 years as consumer demand for organic products increases and growers become more interested in targeting higher-value niche markets (Strik and Finn, 2012).
Guidelines for organic blackberry production are limited at the moment. General information is available for erect and semierect types used for fresh market production (Kuepper et al., 2003), but most of it is anecdotal and does not necessarily apply to organic production of trailing types used for production of processed fruit. Trailing blackberries ripen in midsummer in Oregon and Washington and are usually machine-harvested (Strik and Finn, 2012).
Weed management is considered critical for good production in berry crops (Barney et al., 2007; Bushway et al., 2008; Heiberg, 2002; Krewer et al., 2009; Pritts and Kelly, 2001; Zebarth et al., 1993). Blackberry plants are relatively vigorous, however, and therefore may tolerate more competition from weeds. A few commercial operations grow organic blackberries with no weed control and achieve reasonable yields (Strik, personal observations). The strategy is considered a holistic approach to weed management—maintaining good soil properties, providing a habitat for beneficial insects, and favoring healthy crop growth and high yield (Buhler, 2002; Marshall et al., 2003; Swanton and Weise, 1991)—but weeds left uncontrolled may interfere with machine harvest and produce seeds that become fruit contaminants.
Pre-emergent and contact herbicides are commonly used in conventional blackberry plantings (Barney et al., 2007; Bushway et al., 2008), but chemical options are limited for organic production. Perforated landscape fabric, often referred to as weed mat, is an alternative option and has been approved for use as a weed barrier in organic plantings (U.S. Department of Agriculture, 2011). Trailing blackberry may be well suited to weed mat because, unlike erect and semierect types, new canes, referred to as primocanes, emerge only from the crown or the base of the plants. Weed mat is currently used with success in other perennial cropping systems, including conventional and organic tree fruit orchards (Granatstein and Mullinix, 2008) and blueberry (Vaccinium corymbosum L.) fields (Julian et al., 2012).
The use of drip irrigation may also help reduce weed problems because, unlike sprinkler systems, water is delivered in small, focused areas along the row, reducing unwanted seed germination. Many organic growers irrigate by drip for this reason and often use it in combination with weed mat. Drip may also reduce disease problems compared with overhead irrigation by allowing leaves, flowers, and fruit to stay dry after each water application. Fertilizers are easily injected through a drip system, although information on injection of organic fertilizers is limited. Schwankl and McGourty (1992) tested two organic fertilizers, spray-dried fish protein and spray-dried poultry protein, and found that both products could be injected through microsprinkler, drip, and drip-tape irrigation systems with minimal clogging.
The objective of the present study was to evaluate the effects of three different organic weed management strategies, including weed mat, hand-weeding, and no weeding, on growth and early production of trailing blackberry. Plants in each treatment were irrigated by drip and fertilized using certified organic liquid fish products. Two popular cultivars, Marion and Black Diamond, were included in the study. Both are predominantly harvested by machine for high-value processed markets and together account for greater than 75% of the 2914 ha of blackberries produced in Oregon in 2012 (U.S. Department of Agriculture, 2013). Like all trailing types, the cultivars are perennial but the shoots are biennial, producing primocanes the first year, which then become floricanes with flowers and fruit the next year and then senesce after harvest. Mature plants will have both primocanes and floricanes in the same year in a typical annual or every-year production system (Julian et al., 2009; Strik and Finn, 2012).
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