Improving the Yield of the Purple Asparagus Cultivar 'Purple Passion' by High Density Planting

Purple asparagus is becoming popular among consumers in Japan because of its unique appearance and good quality (thickness, sweetness and tenderness of spears). The growing area for purple cultivars ('Purple Passion' and 'Pacific Purple') is spreading, particular in the cold region of Japan. However, lower yields compared to those of the green cultivars ('UC157' and 'Gijnlim') may be a serious problem. Therefore, some growers hesitate to cultivate the purple cultivars. Between 1999 and 2005, we investigated high-density planting of the mother fern as a cultivation method to improve the yield of 'Purple Passion'. We also analyzed the polyphenol content that had been reported to be associated with the antioxidative capacity of asparagus. The yield of plots with conventional planting of 'Purple passion' was lower than that of 'UC157'. However, the yield of 'Purple Passion' in the plot with high-density planting showed yields as high as those with conventional planting of 'UC157', and the spears gained sufficient thickness. These findings suggest that 'Purple Passion' can achieve a sufficient yield when cultivated by high-density planting, and that a high concentration of polyphenol indicates consumption of purple spears may contribute to human health. The aptitude for high-density planting and nutrient advantage is key factors in achieving stable cultivation and promoting the consumption of purple varieties in Japan.