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Catherine M. Whitman and Erik S. Runkle

Flowering of the herbaceous perennial Aquilegia is generally considered to require vernalization after seedlings are mature, whereas photoperiod has little or no effect. We performed experiments to determine the flowering responses for two Aquilegia ×hybrida varieties, one of which reportedly has reduced cold requirements. Seedlings of Aquilegia ‘Origami Blue and White’ and ‘Winky Double Red and White’ with three to five leaves were either placed directly into a 5 °C cooler with low-intensity lighting for 9 hours/day or transplanted to 13-cm containers and grown (bulked) for 0, 3, or 6 weeks at 20 °C under 9-hour short days (SDs) or 16-h long days (LDs). Plants were then cooled at 5 °C for 0, 5, or 10 weeks and placed in a common forcing environment at 20 °C under LDs. Flowering response of the two cultivars differed markedly. All Aquilegia ‘Origami Blue and White’ plants placed directly into the forcing environment flowered and in a mean of 93 days. Flowering percentage of plants cooled in the plug tray decreased with increasing duration of cold treatment, and only 15% flowered after a 10-week cold treatment. All plants bulked for 3 or 6 weeks before cold treatment flowered after 25 to 36 days in the forcing environment. Adding bulking and forcing time together, time to flowering of ‘Origami Blue and White’ was complete and most rapid (62 days) when plants were bulked for 3 weeks under SDs and then forced under LDs. In contrast, no ‘Winky Double Red and White’ plants flowered without cold treatment, and 6 weeks of bulking followed by 10 weeks of cold was required for 100% flowering. These results indicate that ‘Origami Blue and White’ has a relatively short juvenile phase and flowering was promoted by SD bulking or cold treatment, whereas ‘Winky Double Red and White’ has a longer juvenile phase and requires cold for flowering.

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Erik S. Runkle, Catherine M. Whitman and Mike Olrich

Uniconazole is a plant growth regulator used to inhibit internode elongation on container-grown ornamental plants. Uniconazole is effective on a wide range of plants, but is not commonly used on bedding plants because of concerns about stunting and flowering delay. Our objective was to determine the effectiveness of uniconazole when used as a drench, eliminating the variability inherent in a spray application. Seedlings of Celosia argentea L. var. plumosa L. `Fresh Look Red', Petunia ×hybrida Vilm.-Andr. `Prostrate Wave Rose', Salvia splendens Sell ex Roem. & Schult. `Vista Red', and Tagetes erecta L. `Inca II Gold' in 288-cell plug trays were transplanted 2 days after arrival into 10-cm pots filled with a soilless medium containing no bark. Plants were placed in a greenhouse with a setpoint of 20 °C and under a 16-h photoperiod provided by high-pressure sodium lamps. A single drench application of 0, 0.04, 0.07, 0.15, or 0.30 mg active ingredient/pot was made 11 days after transplant. The uniconazole drench inhibited internode elongation in these species and higher rates provided a greater degree of response. At time of flowering, the 0.30-mg uniconazole drench inhibited shoot length in Celosia, Petunia, Salvia, and Tagetes by 36%, 23% 26%, and 13%, respectively. Drenches of 0.04 or 0.07 mg provided a desirable degree of height control for Celosia and Salvia. For vigorous species like Petunia or Tagetes, 0.15 to 0.30 mg may be more appropriate. We observed a 1- or 2-day delay in flowering of Salvia and Tagetes plants drenched with 0.30 mg, but no delays in Petunia flowering.

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Catherine M. Whitman, Erik S. Runkle and Arthur C. Cameron

Flowering of Aquilegia is generally considered to require vernalization, while photoperiod has little or no effect. The cold treatment is most effective when plants have passed the juvenile stage (often 12 to 15 leaves) prior to vernalization. We performed experiments on a cultivar reported to have a reduced vernalization requirement. Seedlings of Aquilegia ×hybrida Sims `Origami Blue and White' in 128-cell plug trays with four or five leaves were either placed directly into a 5 °C cooler or transplanted to 13-cm containers. Plants were grown (bulked) for 0, 3, or 6 weeks at 20 °C under 9-h short days (SD) or 16-h long days (LD) provided by incandescent lamps at 1 to 3 μmol·m-2·s-1. Plants had seven or eight leaves after 3 weeks bulking and 13 or 14 leaves after 6 weeks bulking. They were then cooled at 5 °C for 0, 5, or 10 weeks and placed in a common forcing environment of 20 °C under an LD provided by high-pressure sodium lamps. Aquilegia plants placed directly into the forcing environment flowered in 89 and 97 days in years 1 and 2, respectively. Flowering percentage of plants cooled in the plug tray decreased with increasing duration of cold treatment, and only 15% flowered after a 10-week cold treatment. All plants bulked for 3 or 6 weeks prior to cold treatment flowered, and in 26 to 35 days. Surprisingly, all plants that were moved directly from bulking treatments to the forcing environment (no cold treatment) flowered, and flowering was most rapid (36 days) in plants exposed to 6 weeks of SD before forcing. Therefore, our data indicate that SD can at least partially substitute for a cold treatment in this Aquilegia cultivar.

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Catherine M. Whitman, Royal D. Heins, Arthur C. Cameron and William H. Carlson

The influence of cold treatments and photoperiod on flowering of 8- to 11-node and 18- to 23-node Lavandula angustifolia Mill. `Munstead' plants from 128-cell (10-mL cell volume; P1) and 50-cell (85-mL cell volume; P2) trays, respectively, was determined. Plants were stored at 5 °C for 0, 5, 10, or 15 weeks, then forced under a 9-h photoperiod (SD), or under a 4-h night-interruption (NI) (2200 to 0200 hr) photoperiod at 20 °C. Percentage of plants flowering, time to flower, and plant appearance were evaluated. Increasing duration of cold treatment was associated with an increase in flowering percentage in plants from both cell sizes. More plants flowered under NI than SD except in P2 cooled for 15 weeks, where all plants flowered. Average time to visible bud (VB) and to opening of the first flower (FLW) generally decreased with increasing duration of cold treatment. Inflorescence count in P2 plants increased with increasing duration of cold treatment. To determine the relationship between forcing temperature and time to flower in L. angustifolia `Munstead', three sizes of plants were exposed to 5 °C for 13 weeks and then forced under a 4-h NI (2200 to 0200 hr) at 15, 18, 21, 24, or 27 °C. Plants generally flowered more quickly at higher temperatures, time to FLW decreasing from 77, 71, and 60 days at ≈15.6 °C to 46, 40, and 36 days at ≈26 °C for P1, P2, and 5.5-cm (190-mL pot volume) (P3) plants, respectively. Generally, P1 plants flowered 5 to 10 days later than P2, and P2 flowered 5 to 10 days later than P3.

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Catherine M. Whitman, Royal D. Heins, Arthur C. Cameron and William H. Carlson

The influence of cold treatments on flowering in Campanula carpatica Jacq. `Blue Clips' was determined. Plants with 10 to 12 nodes (P1) and 12 to 16 nodes (P2), in 128-cell (10-mL cell volume) and 50-cell (85-mL cell volume) trays, respectively, were stored at 5 °C for 0, 2, 4, 6, 8, 10, 12, or 14 weeks under a 9-hour photoperiod. They then were transplanted and forced in a 20 °C greenhouse under a 9-hour photoperiod with a 4-hour night interruption (NI) (2200 to 0200 hr). Time to visible bud and to flowering in P1 decreased slightly as the duration of cold treatment increased. Flowering was hastened by ≈10 days after 14 weeks at 5 °C. Cold treatments had no significant effect on time to visible bud or flower in P2. The number of flower buds on P1 did not change significantly in response to cold treatments, while flower bud count on P2 increased by up to 60% as duration of cold treatments increased. Final height at flowering of both ages decreased 10% to 20% with increasing duration of cold exposure. To determine the relationship between forcing temperature and time to flower, three plant sizes were forced under a 9-hour photoperiod with a 4-hour NI (2200 to 0200 hr) at 15, 18, 21, 24, or 27 °C. Plants flowered more quickly at higher temperatures, but the number and diameter of flowers were reduced. Days to visible bud and flowering were converted to rates, and base temperature (Tb) and thermal time to flowering (degree-days) were calculated. Average Tb for forcing to visible bud stage was 2.1 °C; for forcing to flower, 0.0 °C. Calculated degree-days to visible bud were 455; to flower, 909.

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Catherine M. Whitman, Royal D. Heins, Arthur C. Cameron and William H. Carlson

The effectiveness of cool-white fluorescent, high-pressure sodium, incandescent, and metal halide lamps for inducing flowering through daylength extensions in Campanula carpatica Jacq. `Blue Clips', Coreopsis grandiflora Hogg ex Sweet `Early Sunrise', and Coreopsis verticillata L. `Moonbeam' was compared. Lighting was delivered as a 7-hour day extension with photosynthetic photon flux (PPF) ranging from 0.05 to 2.0 μmol·m-2·s-1 following a 9-hour natural daylength. Threshold irradiance values for flowering ranged from <0.05 to 0.4 μmol·m-2·s-1, depending on species. Saturation irradiance values for Campanula carpatica `Blue Clips' and C. grandiflora `Early Sunrise' were between 0.2 ± 0.2 and 0.7 ± 0.5 μmol·m-2·s-1, and did not differ between lamps. An irradiance of 1.0 μmol·m-2·s-1 from any lamp was adequate for flowering in Coreopsis verticillata `Moonbeam'. Time to flower at irradiances above the saturation points did not differ significantly between lamp types for all species tested. Campanula carpatica `Blue Clips' and Coreopsis grandiflora `Early Sunrise' plants had significantly longer stems under incandescent lamps than in any other treatment. Coreopsis verticillata `Moonbeam' plants grown under cool-white fluorescent lamps had stems ≈10% longer than those grown under high-pressure sodium or incandescent lamps.