Rate of nitrogen fertigation during vegetative growth and spray applications of urea in the fall alters growth and flowering of florists' hydrangeas

Plants of Hydrangea macrophylla 'Merritt's Supreme' were fertigated with 0, 70,140, 210, or 280 mg.L-1 nitrogen (N) from July to Sept. 2005 and sprayed with 0% or 3% urea in late October to evaluate whether plant N status during vegetative growth influences plant performance during forcing. In late November, plants were manually defoliated, moved into a dark cooler (4.4 to 5.5 degrees C) for 8 weeks, and then placed into a greenhouse for forcing. After budbreak, plants were supplied with either 0 N or 140 mg-L-1 N for 9 weeks. Plant growth and N content were evaluated in Nov. 2005 before cold storage and plant growth, flowering, and leaf quality parameters were measured in late Apr. 2006. Increasing N fertigation rate in 2005 significantly increased plant biomass by approximate to 14 g (26%) and plant N content by approximate to 615 mg (67%). Spray applications of urea (urea sprays) in the fall had no influence on plant biomass but significantly increased plant N content by approximate to 520 mg (54%). In general, plants grown with 210 and 280 mg-L-1 N during 2005 had the greatest growth (total plant biomass, height), flowering (number of flowers, flower size), and leaf quality (leaf area, chlorophyll content) during forcing in 2006. Urea sprays before defoliation increased plant growth, flowering, and leaf quality characteristics during forcing in 2006. Providing plants with N during the forcing period also increased plant growth, flowering, And leaf quality characteristics. Urea sprays in the fall were as effective as N fertilizer in the spring on improving growth and flowering. We conclude that both vegetative growth and flowering during forcing of 'Merritt's Supreme' hydrangea are influenced by both the N status before forcing and N supply from fertilizer during forcing. A combination of optimum rates of N fertigation during the vegetative stage of production with urea sprays before defoliation could be a useful management strategy to control excessive vegetative growth, increase N storage, reduce the total N input, and optimize growth and flowering of container-grown florists' hydrangeas.