Floricane-fruiting blackberry do not produce fruit in the Northern Great Plains region of the United States unless the primocanes are protected by some method such as laying canes parallel to the ground using a rotating cross-arm system and rowcovers to protect from low winter temperatures (Mettler and Hatterman-Valenti, 2018). Even under these protected conditions, fruit production was generally low, regardless of the cultivar. However, FF blackberry could be grown in an annual production system using dormant hardwood cuttings when they have achieved their chilling requirement for breaking physiological dormancy. If hardwood cuttings could be successfully rooted, then greenhouse growers could produce miniature (≈30 cm long stems) blackberry plants that develop a flower shoot and fruit within several months. Cuttings would be taken during the dormant season to root, flower, and fruit for an annual high-density (≈75,000 potted plants/acre) production system that should produce ≈5000 lb/acre local fruit during an off-peak time (F. Takeda, unpublished data).
Blackberry is typically propagated from leaf-bud cuttings during the summer and by tip-layering primocanes during late summer (Hartmann et al., 2011; Takeda and Soria, 2011). Other successful methods of commercial blackberry propagation include root cuttings and tissue culture (Caldwell, 1984; Hartmann et al., 2011). Attempts to root hardwood cuttings of FF blackberry have produced variable success rates (Bray et al., 2003; Gonçalves et al., 2012; Lopez-Medina and Moore, 1997; Takeda et al., 2011; Zimmerman et al., 1980). There has been no consensus regarding the best method of rooting hardwood cuttings. Root initiation on blackberry cuttings appears to be cultivar-dependent regardless of the methods used (e.g., cutting material, time of collection, and auxin application). Bray et al. (2003) used three thornless cultivars (Apache, Arapaho, and Navaho) and found that the rooting response to auxin application to the cut end of hardwood cuttings was variable among cultivars. For ‘Apache’, the application of 0.3% indole-3-butyric acid (IBA) in water as a 1- to 3-s quick dip caused an increase in root formation; however, for ‘Arapaho’ and ‘Navaho’, the treatment decreased root formation compared with that of untreated cuttings. The study also found cuttings rooted well using a method similar to that of other studies involving hardwood cuttings and placing them in a peat perlite medium mix with intermittent misting. In contrast, Takeda et al. (2011) developed a protocol for rooting one-node hardwood cuttings without using intermittent mist in a greenhouse setting. Hardwood cuttings were taken from ‘Siskiyou’ and ‘Triple Crown’ laterals after the canes had undergone an outdoor dormancy period and axillary buds had undergone floral bud initiation and differentiated floral bud primordia. One-node leafless cuttings were wrapped with moistened paper towels and placed in a sealed plastic bag under low light at room temperature with weekly hand misting. Both cultivars produced adventitious roots within 10 to 14 d, with most of the roots forming at the base of the bud. Soon after root formation, a flower shoot emerged from the axillary bud; ≈2 months later, the flowers on the resulting miniature plants (e.g., 30- to 50-cm-tall plants with a single flower shoot) bloomed. Similarly, Gonçalves et al. (2012) used two-node floricane cuttings from ‘Arapaho’, ‘Ouachita’, and ‘Karaka Black’ to examine the cane position (top, middle, and bottom) influence on rooting and subsequent flower and fruit production. These findings indicated that cuttings of FF blackberries taken during early winter or even after flower bud differentiation had occurred could successfully produce a plant that initiates flowers and then fruit.
The method of rooting hardwood blackberry cuttings developed by Takeda et al. (2011) would be ideal for annual production of FF blackberry in the Northern Great Plains region, where winter temperatures cause severe cane injury and dieback. By using one-node cuttings from plants grown in nursery pots and stored in a cooler, one could obtain miniature plants for an annual production system that is feasible in northern climates. The poor rooting success of hardwood cuttings could be overcome by the application of auxin, because it has been reported to benefit rooting in some cases (Bray et al., 2003; Lopez-Medina and Moore, 1997).
Although previous studies have researched the use of leafless one-node or two-node floricane cuttings for the purpose of subsequent flowering and fruiting, the use of auxin has not been fully explored in these instances (Gonçalves et al., 2012; Takeda et al., 2011). There are currently no widely accepted recommendations for the use of auxin for hardwood cuttings of blackberry, and minimal research exists regarding the appropriate form and concentration. Studies involving auxin used on hardwood cuttings reported that it was applied with an intermittent mist in a greenhouse setting in the form of a powder or liquid comprising 0.3% IBA (Bray et al., 2003; Lopez-Medina and Moore, 1997). The roles of different rates of IBA or applications of a different auxin hormone group, such as 1-naphthaleneacetic acid (NAA), have not been investigated for hardwood blackberry cuttings.
The objective of this study was to evaluate several methods of rooting hardwood cuttings of FF blackberries to be used in an annual production system. Specifically, we examined the effects of auxin, rooting environment, cultivar, and age/growing location of the mother plant (established plants in-ground vs. young plants in a cooler).
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