● April–June 2005 15(2) 370 PRODUCTION & MARKETING REPORTS Comparison of Flurprimidol to Ancymidol, Paclobutrazol, and Uniconazole for Tulip Height Control Brian A. Krug, Brian E. Whipker 1 , Ingram McCall, and John M. Dole ADDITIONAL INDEX WORDS. A-Rest, Bonzi, plant growth retardants, Sumagic, Topflor SUMMARY. Preplant bulb soaks of ancymidol, flurprimidol, paclobutra- zol, and uniconazole; foliar sprays of flurprimidol; and substrate drenches of flurprimidol, paclobutrazol, and uniconazole were compared for height control of ‘Prominence’ tulips (Tulipa sp.). Height control was evaluated at anthesis in the greenhouse and 10 days later under postharvest condi- tions. Substrate drenches of ancymi- dol, flurprimidol, and paclobutrazol resulted in adequate control using concentrations of 0.5, 0.5, and 1 mg/pot a.i. (28,350 mg = 1 oz), respectively. At these concentrations, ancymidol drenches cost $0.06/pot and paclobutrazol drenches $0.03/ pot. Since flurprimidol is not yet available and no price is available, growers will need to assess the cost compared to ancymidol and pa- clobutrazol. Flurprimidol foliar sprays at <80 mg·L –1 (ppm) were ineffective in controlling height during green- house forcing, but during postharvest evaluation 80 mg·L –1 resulted in 14% shorter plants than the untreated con- trol. Preplant bulb soaks of flurprimi- dol, paclobutrazol, and uniconazole at concentrations of 25, 50, and 10 mg·L –1 , respectively, effectively con- trolled plant height. Preplant plant growth regulator soaks are a cost-ef- fective method of controlling plant height of tulips because of the limited amount of chemical required to treat a large quantity of bulbs. P otted tulips are popular green- house crops due to their mini- mal spacing requirements and short benching time (Miller, 2002). DeHertogh (1996) suggested that an optimal height for pot tulips is 20 to 25 cm (7.9 to 9.8 inches), excluding the pot, to meet shipping requirements. A potential postharvest problem is the downward curvature of the flower scape due to excessive postharvest elongation of the stem and the heavy inflorescence. To control height, plant growth regulators (PGRs) are often required. Dole and Wilkins (1999) recommended an ancymidol (A-Rest; SePRO, Carmel, Ind.) drench of 0.125 to 0.5 mg/pot a.i. with higher concentrations being used for more vigorous cultivars, while Barrett (2002) recommended ancymidol drenches of 1 to 4 mg·L –1 per pot within the first 2 d of greenhouse forcing. Paclobutra- zol (Bonzi; Syngenta, Greensboro, N.C.) label recommendations call for drenches of 0.31 to 2.5 mg/pot a.i. or 1-h preplant soaks in a 2 to 5 mg·L –1 solution. Tulips are listed on the uniconazole (Sumagic; Valent USA, Marysville, Ohio) label, but no recom- mended concentrations are provided. The recommendation for flurprimidol (Topflor; SePRO Corp., Carmel, Ind.) in Europe is two foliar applications of 0.25 to 0.5 mg·L –1 , 1 week apart (M. Bell, personal communication). Flurprimidol is available in Europe as a 1.5% concentrate, but the formulation being introduced into the U.S. will be a 0.38% concentrate. This study was conducted to compare the efficacy of flurprimidol preplant soaks, foliar sprays, and substrate drenches with the current commercial recommendations of ancymidol, paclobutrazol, and uni- conazole for tulip height control. Materials and methods Noncooled Tulipa ‘Prominence’ bulbs were planted in standard 10.2- cm-diameter (4 inches) round plastic pots with a volume of 575 mL (0.6 qt) on 25 Oct. 2002 with three bulbs per pot. The root substrate was Berger BM 6 (Berger Peat Moss, St. Modeste, Que.), which contained 75% to 80% Canadian sphagnum peat and 20% to 25% perlite. For 10 weeks the cooler temperature set point was 5.0 °C (41 °F). On 4 Jan. 2003 the cooler tem- perature set point was reduced to 1.1 °C (34 °F) until 8 Feb. 2003. Greenhouse forcing began on 8 Feb. 2003 with day/night set points of 20.0/17.8 °C (68/64 °F). Plants were forced under natural day length. Pots were fertilized with nitrogen at 150 mg·L –1 using 15N–0P–12.5K once per week. P LANT GROWTH REGULATOR TREATMENTS. On 25 Oct. 2002, preplant bulb soaks applied for 10 min included: flurprimidol (0.38%) at 25, 50, 100, 200, or 400 mg·L –1 ; paclobutrazol at 25, 50, 100, 200, or 400 mg·L –1 ; and uniconazole at 5, 10, 20, 40, or 80 mg·L –1 . The PGR substrate drenches were applied within 24 h of when forcing began (9 Feb.); plants were between 7.6 and 10.2 (3 to 4 inches) cm tall at application time. Substrate drenches were ap- plied at 59.1 mL (2 fl oz) per pot and included: flurprimidol at 0.25, 0.5, 1, 2, or 4 mg/pot a.i.; paclobutrazol at 0.25, 0.5, 1, 2, or 4 mg/pot a.i.; and ancymidol at 0.0625, 0.125, 0.25, 0.5, or 1 mg/pot a.i.. A foliar spray of flurprimidol at 5, 10, 20, 40, or 80 mg·L –1 was also applied based on the application rate of 203.7 mL·m –2 (0.5 gal/100 ft 2 ). An untreated control was also included. The experiment was a completely randomized design with six pot replications with three plants per pot for each of the 36 treatments. At anthesis (all petals fully colored and beginning to separate), plant height (measured from the soil line to the uppermost part of the inflorescence), was recorded. POSTHARVEST. Four pots, ran- domly selected, from each treatment were placed in a growth chamber at 20.0 °C after anthesis. Light was provided by fluorescent bulbs at 24 to 75 μmol·m –2 ·s –1 . Plant height was recorded 10 d after anthesis. DATA ANALYSIS. Data were tested by analysis of variance (ANOVA) using general linear model (SAS Institute, Cary, N.C.) and means were sepa- rated by least significant differences (LSD) at P ≤ 0.05. Plant heights at the end of forcing and postharvest were regressed using the PROC REG to determine the best-fit, linear or qua- dratic, model. Terms of the model were evaluated for significance based on a comparison of F values at α = 0.05. PROC NLIN in SAS, as modified by Department of Horticulture Science, Box 7609, North Carolina State University, Raleigh, NC 27695-7609 1 To whom reprint requests should be addressed; e-mail: bwhipker@ncsu.edu Use of trade names in this publication does not imply endorsement of products named nor criticism of similar ones not mentioned. Thanks to Berger for the root substrates, Scotts for the fertilizer, Dillen Products for the pots, and SePRO for grant support.