NURSERY RESPONSE OF ACACIA KOA SEEDLINGS TO CONTAINER SIZE, IRRIGATION METHOD, AND FERTILIZATION RATE

Planting koa (Acacia koa A. Gray) in Hawai'i, USA aids in restoration of disturbed sites essential to conservation of endemic species. Survival and growth of planted seedlings under vegetative competition typically increases with initial plant size. Increasing container size and fertilizer rate may produce larger seedlings, but high fertilization can lead to surface and groundwater pollution and relatively low fertilizer use efficiency. Subirrigation systems may help mitigate this problem. Our study objective was to evaluate koa seedling growth with overhead or subirrigation over a range of container volumes (50 to 656 mL) and fertilizer rates (0 to 9.6 kg center dot m-3). Increasing container volume from 50 to 656 mL yielded koa seedlings with 200% more height and stem diameter growth. Subirrigation resulted in less nutrient leaching losses and yielded seedlings of similar vigor as overhead irrigated seedlings. Subirrigation helps optimize fertilizer delivery, which may improve fertilizer use efficiency and reduce environmental contamination.