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Shi-Ying Wang, Royal D. Heins, William H. Carlson, and Arthur C. Cameron

Four herbaceous perennial species, Delphinium grandiflorum `Blue Mirror', Hibiscus xhybrida `Disco Belle Mix', Salvia xsuperba `Blue Queen', and Veronica longifolia `Sunny Border Blue' were forced in a glass greenhouse at 15, 18, 21, 24, or 27°C under long days. Before being forced, all tested species except H. xhybrida were exposed to 5°C for 12 weeks. Increasing forcing temperature generally promoted visible bud and flowering. However, visible bud and flowering of D. grandiflorum `Blue Mirror' and flowering of V. longifolia `Sunny Border Blue' were delayed at 27°C. Although the tested species tended to have more flower buds, bigger flowers, and greater height at lower forcing temperatures, the effect of forcing temperature on those characteristics was species-dependent. Temperatures as low as 15°C decreased bud number and flower size of H. xhybrida `Disco Belle Mix'. The base temperature (Tb) and cumulative thermal time (CTT) necessary to complete the indicated developmental stage were calculated from a linear regression: 1/f = a + bT. Based this equation, days to flowering (or visible bud) at certain temperatures or the temperature required for flowering within a certain number of days can be predicted.

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Cheryl Hamaker, William H. Carlson, Royal D. Heins, and Arthur C. Cameron

DIF is the difference between day (DT) and night (NT) temperatures. Temperature drop is a 2-hour temperature reduction at sunrise. DIF and temperature drop, which can be affected by light quality, are effective methods to control final plant height of many greenhouse crops. The effect of DIF and temperature drop on final height was determined for eight species of perennials. Durations for DIF temperatures were 12 hours for both DT and NT. Temperature alterations occurred at sunrise. Temperature treatments (DT/NT) consisted of zero DIF (20/20°C), negative DIF (16/24°C), or positive DIF (24/16°C), and a 2-hour drop (12.7/20.7°C). Long days (LD) were provided from 2200-0200 hr by either cool-white fluorescent (CWF) or incandescent (INC) lights. Data for days to visible bud and anthesis, bud number, and final height were collected. Positive DIF conditions enhanced elongation while negative DIF reduced it in all species. As DIF decreased from positive to negative, plant height was reduced 10%, 30%, 30%, and 20% in Coreopsis `Moonbeam' and `Sunray', Delphinium `Belladonna', and Scabiosa `Butterfly Blue', respectively. Negative-DIF responses were enhanced under CWF lights for some species. In negative-DIF conditions, Coreopsis `Moonbeam' and `Sunray' and Delphinium `Belladonna' were 10%, 10%, and 15% shorter, respectively, under CWF lights than INC lights.

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Cheryl Hamaker, William H. Carlson, Royal D. Heins, and Arthur C. Cameron

Twenty species of perennials were trialed to determine the effectiveness of five growth retardants on final plant height and flowering. Growth retardant treatments consisted of five sprays: 100 ppm ancymidol, 1500 ppm chlormequat, 5000 ppm daminozide, 30 ppm paclobutrazol, or 15 ppm uniconazole. Also included for comparison were two drenches of 15 ppm paclobutrazol or 7.5 ppm uniconazole. Spray treatments consisted of one application every 10 days until anthesis. Drench treatments consisted of one application only. Data for days to visible bud and anthesis, bud number, and final height were collected. Plant response varied significantly between growth retardant treatments. Sprays of ancymidol, chlormequat, daminozide, paclobutrazol, and uniconazole effectively controlled the height of 4, 3, 13, 4, and 12 species, respectively. Daminozide and uniconazole were the most effective sprays at controlling height on a broad range of species. However, daminozide delayed anthesis compared to control treatments of at least 5 species. Drench treatments of paclobutrazol and uniconazole were effective on 14 and 15 species, respectively. The number of responsive species increased significantly when paclobutrazol was used as a drench rather than a spray. All species tested were responsive to at least one growth retardant treatment.

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Emily A. Clough, Arthur C. Cameron, Royal D. Heins, and William H. Carlson

Oenothera fruticosa L.`Youngii-Lapsley' and Stokesia laevis L'Hér. `Klaus Jelitto' are two hardy herbaceous perennials with great potential as pot crops. The vernalization and photoperiod requirements were examined for each species. Plants were cooled for 0, 3, 6, 9, 12, or 15 weeks at 5 °C with a 9-h photoperiod. After cold treatment, plants were forced in greenhouses at 20 °C under a 16-h photoperiod using high-pressure sodium lamps. The photoperiod requirement was determined by forcing plants at 20 °C with and without a 15-week cold treatment at 5 °C under 10-, 12-, 13-, 14-, 16-, 24-h and 4-h night interruption using incandescent lamps. Plants of Oenothera fruticosa `Youngii-Lapsley' cooled for 0 weeks did not flower. All plants cooled for 3 weeks flowered and time to flower decreased from 53 to 43 days as duration of cold increased from 3 to 15 weeks. `Youngii-Lapsley' flowered under every photoperiod, but time to flower and number of flowers decreased from 54 to 40 days as photoperiod increased from 10 to 24 h. Percentage flowering of Stokesia laevis `Klaus Jelitto' increased from 50 to 100, and time to flower decreased from 112 to 74 days as duration of cold increased from 0 to 6 weeks. Without a cold treatment, plants of `Klaus Jelitto' flowered only under daylengths of 12, 13, and 14 h. After cold treatment, plants flowered under every photoperiod except 24 h, and time to flower decreased from 122 to 65 days as photoperiod increased from 10 to 16 h. Additional aspects of flowering and the effect of different forcing temperatures will be discussed.

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Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

`Snowcap' Shasta daisy [Leucanthemum ×superbum Bergmans ex. J. Ingram (syn: Chrysanthemum ×superbum, C. maximum)] was grown under various photoperiods and temperatures to determine their effects on flowering. In the first experiment, plants were held for 0 or 15 weeks at 5 °C and then were grown at 20 °C under the following photoperiods: 10, 12, 13, 14, 16, or 24 hours of continuous light or 9 hours with a 4-hour night interruption (NI) in the middle of the dark period. Without cold treatment, no plants flowered under photoperiods ≤14 hours and 65% to 95% flowered under longer photoperiods or NI. After 15 weeks at 5 °C, all plants flowered under all photoperiods and developed three to four or 10 to 11 inflorescences under photoperiods ≤14 or ≥16 hours, respectively. To determine the duration of cold treatment required for flowering under short photoperiods, a second experiment was conducted in which plants were treated for 0, 3, 6, 9, 12, or 15 weeks at 5 °C, and then grown at 20 °C under 9-hour days without or with a 4-hour NI. Under 9-hour photoperiods, 0%, 80%, or 100% of plants flowered after 0, 3, or ≥6 weeks at 5 °C, and time to flower decreased from 103 to 57 days as the time at 5 °C increased from 3 to 12 weeks. Plants that were under NI and received ≥3 weeks of cold flowered in 45 to 55 days. For complete and rapid flowering with a high flower count, we recommend cold-treating `Snowcap' for at least 6 weeks, then providing photoperiods ≥16 hours or a 4-hour NI during forcing.

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Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

Phlox paniculata Lyon ex Pursh `Eva Cullum' plants were grown under seven photoperiods following 0 or 15 weeks of 5 °C to determine the effects of photoperiod and cold treatment on flowering. Photoperiods were a 9-hour day extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled plants remained vegetative under photoperiods ≤13 hours; as the photoperiod increased from 14 to 24 hours, flowering percentage increased from 20 to 89. Flowering of noncooled plants took 73 to 93 days. Flowering percentage was 19, 50, or 100 when cooled plants were held under photoperiods of 10, 12, or ≥13 hours or NI, respectively. Time to flower in cooled plants progressively decreased from 114 to 64 days as the photoperiod increased from 10 to 24 hours. Reproductive cooled plants had at least three times more flowers, were at least 50% taller, were more vigorous, and developed seven or eight more nodes than did noncooled plants. Photoperiod had no effect on height of flowering plants.

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Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

To determine the flowering requirements of Rudbeckia fulgida Ait. `Goldsturm', plants were grown under 9-hour photoperiods until maturity, then forced at 20 °C under one of seven photoperiods following 0 or 15 weeks of 5 °C. Photoperiods consisted of a 9-hour day that was extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled `Goldsturm' remained vegetative under photoperiods ≤13 hours, and essentially all plants flowered under photoperiods ≥14 hours or with a 4-hour NI. Flowering percentages for cooled plants were 6, 56, or ≥84 under 10-, 12-, or ≥13-hour daylengths and NI, respectively. Critical photoperiods were ≈14 or 13 hours for noncooled or cooled plants, respectively, and base photoperiods shifted from 13 to 14 hours before cold treatment to 10 to 12 hours following cold treatment. Within cold treatments, plants under photoperiods ≥14 hours or NI reached visible inflorescence and flowered at the same time and developed the same number of inflorescences. Fifteen weeks of cold hastened flowering by 25 to 30 days and reduced nodes developed before the first inflorescence by 28% to 37%. Cold treatment provided little or no improvement in other measured characteristics, such as flowering percentage and uniformity, flower number, plant height, and vigor.

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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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Genhua Niu, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

Pansy [Viola ×wittrockiana Gams. `Delta Yellow Blotch' (Yellow) and `Delta Primrose Blotch' (Primrose)] plants were grown in a greenhouse under two CO2 concentrations [ambient (≈400 μmol·mol-1) and enriched (≈600 μmol·mol-1)], three daily light integrals (DLI; 4.1, 10.6, and 15.6 mol·m-2·d-1), and nine combinations of day and night temperatures created by moving plants every 12 h among three temperatures (15, 20, and 25 °C). Time to flower decreased and rate of flower development increased as plant average daily temperature (ADT) increased at all DLIs for Yellow or at high and medium DLIs for Primrose. Increasing the DLI from 4.1 to 10.6 mol·m-2·d-1 also decreased time to flower by 4 and 12 days for Yellow and Primrose, respectively. Both cultivars' flower size and Yellow's dry weight [(DW); shoot, flower bud, and total] decreased linearly as plant ADT increased at high and medium DLIs, regardless of how temperature was delivered during day and night. DW in Yellow increased 50% to 100% when DLI increased from 4.1 to 10.6 mol·m-2·d-1 under both CO2 concentrations. Flower size in Yellow and Primrose increased 25% under both CO2 conditions as DLI increased from 4.1 to 10.6 mol·m-2·d-1, but there was no increase between 10.6 and 15.6 mol·m-2·d-1, regardless of CO2 concentration. Plant height and flower peduncle length in Yellow increased linearly as the difference between day and night temperatures (DIF) increased; the increase was larger under lower than higher DLIs. The ratio of leaf length to width (LL/LW) and petiole length in Yellow increased as DIF increased at medium and low DLIs. Carbon dioxide enrichment increased flower size by 4% to 10% and DW by 10% to 30% except for that of the shoot at medium DLI, but did not affect flower developmental rate or morphology. DW of vegetative and reproductive parts of the plant was correlated closely with photothermal ratio, a parameter that describes the combined effect of temperature and light.

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Emily A. Clough, Arthur C. Cameron, Royal D. Heins, and William H. Carlson

Influences of vernalization duration, photoperiod, forcing temperature, and plant growth regulators (PGRs) on growth and development of Oenothera fruticosa L. `Youngii-lapsley' (`Youngii-lapsley' sundrops) were determined. Young plants were vernalized at 5 °C for 0, 3, 6, 9, 12, or 15 weeks under a 9-hour photoperiod and subsequently forced in a 20 °C greenhouse under a 16-hour photoperiod. Only one plant in 2 years flowered without vernalization, while all plants flowered after receiving a vernalization treatment, regardless of its duration. Thus, O. fruticosa had a nearly obligate vernalization requirement. Time to visible bud and flower decreased by ≈1 week as vernalization duration increased from 3 to 15 weeks. All plants flowered under 10-, 12-, 13-, 14-, 16-, or 24-hour photoperiods or a 4-hour night interruption (NI) in a 20 °C greenhouse following 15-weeks vernalization at 5 °C. Time to flower decreased by ≈2 weeks, flower number decreased, and plant height increased as photoperiod increased from 10 to 16 hours. Days to flower, number of new nodes, and flower number under 24 hour and NI were similar to that of plants grown under a 16-hour photoperiod. In a separate study, plants were vernalized for 15 weeks and then forced under a 16-h photoperiod at 15.2, 18.2, 20.6, 23.8, 26.8, or 29.8 °C (average daily temperatures). Plants flowered 35 days faster at 29.8 °C but were 18 cm shorter than those grown at 15.2 °C. In addition, plants grown at 29.8 °C produced only one-sixth the number of flowers (with diameters that were 3.0 cm smaller) than plants grown at 15.2 °C. Days to visible bud and flowering were converted to rates, and base temperature (Tb) and thermal time to flowering (degree-days) were calculated as 4.4 °C and 606 °days, respectively. Effects of foliar applications of ancymidol (100 mg·L-1), chlormequat (1500 mg·L-1), paclobutrazol (30 mg·L-1), daminozide (5000 mg·L-1), and uniconazole (15 mg·L-1) were determined on plants vernalized for 19 weeks and then forced at 20 °C under a 16-h photoperiod. Three spray applications of uniconazole reduced plant height at first flower by 31% compared with that of nontreated controls. All other PGRs did not affect plant growth. Chemical names used: α-cyclopropyl-α-(4-methoxyphenyl)-5-pyrimidinemethanol (ancymidol); (2-chloroethyl) trimethylammonium chloride (chlormequat); butanedioic acid mono-(2,2-dimethyl hydrazide) (daminozide); (2R,3R+2S,3S)-1-(4-chlorophenyl-4,4-dimethyl-2-[1,2,4-triazol-1-yl]) (paclobutrazol); (E)-(S)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-pent-1-ene-3-ol (uniconazole).