The Effect of Nitrogen and Potassium Fertilization on Growth and Nutrient Content of Container Grown Dahlia 'Magic Moment'

Dahlias are a popular floriculture crop and are grown as herbaceous perennials, potted plants, cut flowers and for tuber production. Specific nutritional guidelines have not been developed in order to optimize container production. Proper information on fertilization rates and tissue nutrient concentrations are required to maximize productivity and quality for growers.  The objectives of this research were to establish base line nitrogen (N) and potassium (K) fertilization rates as well as tissue nutrient concentrations to maximize plant quality while minimizing over fertilization.  Dahlia ‘Magic Moment’ plants were field produced from cuttings in 15 L containers outdoor in Kennett Square, PA, starting May 13, 2010, for 106 days.  On weekdays, plants received fertilizer treatments, which were a factorial arrangement of three N rates (50, 200, and 350 ppm) and three K₂O rates (50, 200, and 350 ppm). There was also a control treatment of 0 N and 0 K giving a total of 10 treatments with 4 replications.  Tissue N concentration increased and tissue K concentration decreased as N fertilization rate increased.  This is likely the result of nutrient antagonism where higher concentrations of positively charged ammoniacal N in the fertilizer suppress uptake of K.  Tissue K concentration increased significantly as K rate increased.  The effect of K fertilization rate on tissue N concentration was non-significant.  Shoot fresh weight, tuber fresh weight, and the shoot to tuber weight were all unaffected by K.  As N fertilization rate increased, shoot weight increased significantly.  However, just the opposite occurred for tubers, where tuber weights increased 170% and 487% when N rate decreased from 350 ppm to 200 and 50 ppm, respectively.   When comparing the results for tubers with tissue N concentrations, there is an apparent shift in resource allocation when tissue N drops below approximately 4%.  This is further supported by the shoot to tuber ratio increase of 687% and 791% when fertilization rate increase from 50 ppm to 200 and 350 ppm, respectively.  Further research is needed to determine the critical tissue concentration of N where dahlias shift growth partitioning towards or away from tubers.  However, this data does indicate fertilization of dahlias with high N can reduce tuber production while limiting N can promote tuber production.  There was no effect of three K rates on growth of dahlia shoots or tubers, which indicates regular fertilization with 50 to 100 ppm K₂O should supply sufficient K for production.