Nitrogen Nutrition of Containerized Thuja x ‘Green Giant’1

Rooted stem cuttings of ‘Green Giant’ arborvitae (Thuja L. x ‘Green Giant’) were grown in 3.8 liter (#1) plastic containers containing a pine bark:sand (8:1 by vol) substrate. Plants were fertilized three times weekly for 15 weeks with a complete nutrient solution at N application rates (NARs) of 0, 10, 20, 40, 80, 160, or 320 mg/liter (ppm) supplied as ammonium nitrate (NH 4 NO 3 ). Both shoot and root dry weights were significantly affected by NAR. Maximum shoot weight, as predicted by a quadratic plateau model, was reached at a NAR of approximately 100 mg/liter (ppm), representing a 200% increase over controls [0 mg/liter (ppm) N]. Maximum and minimum root dry weights occurred at 0 mg/liter (ppm) N and approximately 50 mg/liter (ppm) N, respectively, representing a 37% decrease. Root length decreased 36% at a NAR of approximately 50 mg/liter (ppm). Root diameter (root area ÷ root length) increased before reaching a plateau at approximately 130 mg/liter (ppm) N indicating that root length decreased faster than root area at low N concentrations. Shoot concentrations of N, P, Mg, and S were maximized at approximately 71, 41, 48, and 52 mg/liter (ppm) N, respectively. Uptake of K and Ca were unaffected by N concentrations. Index words: arborvitae, carbon partitioning, container production, fertilization, mineral nutrition. Significance to the Nursery Industry ‘Green Giant’ arborvitae (Thuja L. x ‘Green Giant’) is an upright, pyramidal landscape tree that has attracted the attention of the nursery industry. Valued for its lustrous green summer foliage, rapid growth rate, and ease of propagation by stem cuttings, the tree is an attractive specimen when planted alone, or forms a dense evergreen screen when planted in rows. However, specific information pertaining to nutritional requirements for containerized culture of ‘Green Giant’ is unavailable. Our results indicate that applying N as ammonium nitrate (NH 4 NO 3 ) at rates >100 mg/liter (ppm) does not increase shoot dry weight or mineral nutrient uptake. Root dry weight and length do not continue to decrease at N application rates (NARs) >50 mg/liter (ppm). Increasing NAR beyond 100 mg/liter (ppm) N will neither improve shoot growth nor prove detrimental to root growth. Electrical conductivity (EC) of substrate solution for treatments that maximized growth were within the recommended range for container-grown nursery crops (13). Therefore, 100 mg/liter (ppm) N, applied three times weekly, is recommended for maximum growth of ‘Green Giant’.