Seasonal Growth and Nitrogen Uptake of Encore Azalea ‘Chiffon’ Affected by Nitrogen Availability and Containers

Abstract

Plant growth and nitrogen (N) uptake of Encore® azalea ‘Chiffon’ (Rhododendron sp.) grown in a traditional plastic container or a biodegradable container made from recycled paper were investigated over the 2013 growing season. Three hundred twenty 1-year-old azalea liners, grown in two types of containers, were fertilized twice weekly with 250 mL N-free liquid fertilizer with no N or 15 mm N from ammonium nitrate (NH4NO3). Biweekly from 10 May to 3 Dec., five plants from each N rate and container type were selected randomly to measure plant height, widths, and leaf chlorophyll content in terms of soil–plant analysis development (SPAD) readings, and were then harvested destructively for nutrient analyses. Leaf SPAD readings and tissue N concentration were influenced mostly by N rate rather than container type, with 15 mm N producing greater values than the no-N treatment. Leaf SPAD readings increased from May to August and decreased from September to December. Using 15 mm N, plastic containers generally resulted in similar or increased plant growth [plant growth index (PGI) and dry weight] and N uptake from May to August as in biocontainers, with greater SPAD readings, leaf and root dry weights, stem and root N concentrations, and leaf and root N content than biocontainers at some harvests. However, biocontainers resulted in greater PGI, dry weights, and N content (in leaves, stems, roots, and total plant) than plastic containers later in the season, from September to December. These differences appeared in September after plants grown in plastic containers ceased active growth in dry weight and N uptake by the end of August. Plants grown in biocontainers had extended active growth from 13 Sept. to 9 Nov., resulting in greater tissue N content and greater N uptake efficiency. The biocontainers used in this study produced azalea plants of greater size, dry weight, and improved N uptake by increasing growth rate and extending the plants’ active growth period into late fall. The beneficial effects likely resulted from greater evaporative cooling through container sidewalls and the lighter color of the biocontainers, and therefore led to lower substrate temperatures and improved drainage.