Studies with the asparagus "mother fern" culture in a temperate climate : a thesis presented in partial fulfilment of the requirements for the requirements for the degree of Master of Applied Science in Plant Science at Massey University
In temperate regions, asparagus is normally harvested in spring. An extended harvest season could prolong the supply of fresh asparagus and perhaps lead to an economic gain through high off-season prices. High production costs and low yield of some alternative strategies compared to the normal spring harvest seem to discourage their commercial use. However, field investigations on the mother fern system in a temperate climate have not been done. From 1995 through to 1996, field and controlled climate growth cabinets studies were conducted to evaluate the asparagus mother fern system in New Zealand. Separate field experiments, for UC 157 and Rutgers Beacon were carried out. Harvesting treatments were, normal spring harvest (September-early December) and two mother fern treatments, run from October-March and December-March. The carry-over effects of the experiment was determined in the spring of 1996, when the crop was harvested for one month (September-October) using the normal spring harvest system only. In the field study, peak spear production occurred in early December and mid-January, for normal spring harvest and mother fern treatments, respectively. Production of spears declined steadily from January to the end of March. The mother fern treatments resulted in a harvest season, which was 15 weeks longer than the normal harvest. However, the total-, marketable- and cumulative yields, and mean spear weight were significantly lower than for a 'normally' harvested crop. The normal spring harvest produced thicker and heavier spears than mother fern treatments. Spears from the latter were also more seedy than those from normal harvest. Environmental factors (insufficient moisture level, decreasing temperature) and possibly correlative inhibition may have been the causes of the reduced production of the mother fern system. UC 157 produced higher yields than Rutgers Beacon. The latter produced a large number of thin spears, which resulted in a high rejection rate. The follow up experiment did not show any marked treatment differences in total yield and number of spears. The experiment conducted in controlled climate growth cabinets studied the effects of temperature and harvesting systems (normal harvest and mother fern system) on spear and fern growth. Potted, one-year old plants, cvs. UC 157 and Jersey Giant, were grown at constant temperatures ranging from 15°C to 35°C at increments of 5°C. Spears (>8mm basal diameter, with closed tips) were harvested from these plants and used to visually assess postharvest shelf life at 20°C. The relative spear growth rate, spear production rate per plant increased with rising temperature from 15°C through to 30°C, beyond which they declined. Relative spear growth rate, spear production rate per plant and average basal spear diameter of mother fern plants were lower than for those under the normal harvest. Average spears diameter did not show any trend with respect to growing temperature. Correlative inhibition and respiratory activity of the fern, including the production of new roots and buds may have led to a reduction in reduced performance of the mother fern plants. The relative spear growth rate of Jersey Giant was higher than UC 157. The postharvest storage life of spears stored at 20°C in unperforated polythene bags averaged seven days. Growing temperature, harvesting system, cultivar did not influence the storage life of spears.