Experimental Kūmara Gardens at Whatarangi, Palliser Bay and Robin Hood Bay, Marlborough, New Zealand: Results after 23 Years

Two experimental gardens of kūmara (Ipomoea batatas), one on each side of Cook Strait, were planted and harvested without fertiliser for a period of 14 and 23 years respectively. The mean annual yield for the garden on the north side of Cook Strait was 10.2 tonne/ha and 7.5 tonne/ha for the southern garden. More than 90% of tubers harvested weigh less than 100g. Yields fluctuated considerably, but did not decline over time. We monitored rainfall, sunshine hours, air and soil temperatures, and several soil chemical characteristics. Some correlations with yield were found, but do not account for the high degree of variation in annual yield. Between plant variation in yield was a similar order of magnitude as annual variation. No correlation was found between seed size and individual plant yield. Several soil nutrients decline significantly over time. Conversely, phosphorus consistently rises over time in both gardens, starting about 10mg/L and rising to c.50 mg/L after 12 years. This is attributed to vesicular-arbuscular mycorrhizae (VAM) around the root system of kūmara. A minor sub-project with taro, Colocasia esculenta, shows success on both sides of Cook Strait. Results from the research were modeled to estimate pre-European population size in Palliser Bay