NITROGEN RATE AND TIMING OF NITROGEN APPLICATION IN POINSETTIA (EUPHORBIA-PULCHERRIMA WILLD EX KLOTZ)

'Celebrate 2' Poinsettias were grown for 8 weeks in a controlled-envir onment growth room until first signs of bract coloration. In growth st age I (GSI; weeks 1 through 4) low, medium, and high N rates (25, 75, and 125 mg N/liter respectively) were applied by subirrigation (no lea ching). Following floral induction [growth stage II (GSII), weeks 5 to 8], there were nine treatments: all possible combinations of the thre e N rates in GSI plus three rates (75, 125, and 175 mg N/liter)in GSII . Although >80% of shoot dry weight and >90% of total leaf area develo ped during growth GSII, reaching an acceptable plant size by week 8 de pended on receiving adequate fertilization in growth GSI. In contrast, leaf chlorosis, noted in plants receiving the lowest rate in GSI, was rapidly reversed by increasing the N rate in GSII. Quadratic regressi on equations fitted to shoot dry weight and leaf area data predicted t hat using 125 mg N/liter in both growth stages gave maximum responses at week 8. However, using 75 mg N/liter in GSI and 125 mg N/liter in G SII also produced acceptable growth in poinsettias. Our results sugges t that some growth restriction imposed by N availability during the fi rst 4 weeks of growth may be acceptable and perhaps desirable to reduc e growth regulator use and the environmental impact of overfertilizati on.