- There is limited information available on production of trailing blackberry, particularly certified organic plantings, which are of interest to growers as there is increased consumer demand and a price premium over conventionally-produced fruit. Various production strategies were evaluated for their effect on yield, biomass production, carbon (C), and nutrient concentrations and content in a certified organic trailing blackberry field grown at the North Willamette Research and Extension Center in Aurora, OR. The planting was irrigated by drip and fertigated with an Organic Materials Review Institute-listed fish hydrolysate and fish emulsion fertilizer. The study was conducted over two complete years and the planting was machine-harvested for the processed market. Treatments used in the study were: cultivar ('Marion' and 'Black Diamond'), irrigation strategy [no irrigation after the final fruit harvest in July (no postharvest) and continuous summer irrigation (postharvest)], weed management strategy [nonweeded (weeds left to grow in the row), hand-weeded (weeds hoed as needed throughout the season), and weed mat (a porous landscape fabric)], and primocane training time (August and February).
The best performing organic production systems did not depend on irrigation strategy, utilized weed mat, and used February-training (for 'Marion' only). When the plantings were mature, 'Marion' and 'Black Diamond' yielded as much as 9 and 11 t∙ha⁻¹, respectively; similar to what would be expected in conventional production. The use of weed mat consistently increased yield and vegetative growth, even when compared to hand-weeded (13% increase). 'Black Diamond' plants did not compete as effectively with weeds as 'Marion' and were more readily infested by raspberry crown borer (Pennisetia marginata Harris) which likely reduced yield. Unlike 'Black Diamond', 'Marion' was negatively affected by an unusually cold winter in 2014. In that year, August-trained 'Marion' plants had 1 kg/plant less yield than February-trained plants, as well as less biomass.
Soil pH, organic matter content, and soil ammonium-nitrogen (NH₄-N), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), copper (Cu), manganese (Mn), and zinc (Zn) concentrations were greater under weed mat than in hand-weeded plots. Several nutrients were below recommended standards in both the soil or primocane leaf tissue, including soil K, soil boron (B), and primocane leaf N, phosphorus (P), K, Ca, Mg, S, B, and Zn concentrations in at least one year or cultivar. 'Black Diamond' tended to have higher floricane and fruit nutrient concentrations than 'Marion'. Use of weed mat often led to the highest nutrient concentrations in the soil, leaves, and fruit, while withholding irrigation postharvest had limited effects, and the impact of primocane training time varied among years, nutrients, and plant parts.
Aboveground dry biomass production in the planting averaged 5.75 t·ha-1, approximately 50% of which was comprised of C. Floricanes, primocanes, and fruit comprised 45%, 30%, and 25% of aboveground plant biomass, respectively. The average aboveground C stock of the planting was 0.75 t·ha⁻¹ in late winter. The treatment with the largest impact on dry biomass and nutrient content was weed management. Weeds reduced aboveground plant dry biomass, primocane, floricane, and fruit nutrient content, and annual gain. Using weed mat for weed control generally led to the largest dry biomass and nutrient content. February-trained 'Marion' plants lost more of most nutrients in 2014 than the year prior, although nutrient gain was not affected by cultivar. Both cultivars lost the most N in harvested fruit when weed mat was used (22 t·ha⁻¹, as compared with 18 t·ha⁻¹ with hand weeding and 12 t·ha⁻¹ with weeds present in 2013), although 'Black Diamond' with weed mat lost 6 t·ha⁻¹ more N through fruit removal than 'Marion' in 2014. Continuous summer irrigation resulted in plants that gained more dry biomass, N, K, Mg, S, B, and Cu than those that received no irrigation after fruit harvest in one or both years. Nitrogen, K, and B were lost at higher rates than what was applied through fertilization, which would eventually lead to the depletion of those nutrients in the planting.
Both cultivars appear to be well suited for organic production, although each had their own challenges. Allowing weeds to grow in the row reduced yield, dry biomass, and nutrient concentrations and content, while both hand weeding and the use of weed mat resulted in increased growth and yield. Weed mat improved production even over hand weeding and reduced labor, making it an ideal choice in this organic system. Withholding irrigation after harvest reduced water use by an average of 44% each year without adversely affecting yield or nutrient concentrations in either cultivar, although it did reduce dry biomass and some nutrient gains. Training time mainly affected 'Marion', which had reduced growth and yield when primocanes were trained in August.