The Influence of Greenhouse Environmental Factors on Forcing Dahlia variabilis Willd.1

Abstract

Dutch-grown tuberous-rooted dahlias, ‘Kolchelsee’ and ‘Park Princess’, were forced from January to June in 1975 and 1976. In all experiments, plant height was controlled by applying 0.5 mg/pot of ancymidol 2 weeks after planting. Fertilization was essential and an application of one of several slow-release formulations of Osmocote or weekly applications of 20N—8.8P—16.6K (200 ppm N) as a soluble fertilizer produced high quality plants which flowered in approximately 70 days. The highest quality plants and best height control was obtained with 25°C day and 16° night temperatures. Flowering was delayed at 24/12° day/night temperatures, and although flowering was accelerated at 28/17° and 29/20° day/night temperatures plant quality was adversely affected. Natural spring photoperiods which increased from 10 to 14 hours were optimal for forcing. Long days given either as a 16-hour photoperiod or as a 4-hour night break delayed flowering slightly. Dahlias grown under an 8-hour photoperiod flowered the earliest but not all plants flowered. Dahlias required high light intensities during forcing. Under 50% shading plants were too tall for pot plant use even after treatment with ancymidol.

Comparative Inflorescence Development of Two Cultivare of Forced Tuberous-rooted Dahlias1

“…Krijthe (14) reported that in 'L' Innocence', a mignon dahlia which produced only 1 row of RF, the DF were well formed with various organ primordia visible before it was covered by the subtending CB. Durso and De Hertogh (10) have shown that short days caused 'Park Princess' to produce flowers with few RF and many DF. In flowers with open eye centers, we observed a development of the CB and DF very similar to that described by Krijthe (14).…”

“…2 size tuberous root clumps produced in The Netherlands. Clumps were shipped on Dec. 17, 1975 and Jan. 10,1977, respectively, and received on Jan. 10, 1976 andFeb. 15, 1977, respectively.…”

“…There has been some confusion over the correct scientific name for cultivated dahlias. Many American authors have desig nated them as D. pinnata (5 ,7 , 12, 19), while others have used D. variabilis (3,4,6,10,17). D. variabilis is an incorrect desig nation, because it is a synonym for D. pinnata (21,22).…”

Pinching Forced Tuberous-rooted Dahlias1

“…Recent efforts to develop techniques for forcing dahlias from tuberous-root clumps of asexually propagated, established garden cultivars have resulted in several cultivars being sug gested for commercial use (5). Proper fertilization and tempera ture regimes, along with ancymidol for height control, are required to produce quality plants (6,7). The naturally in creasing spring photoperiods are optimal for flower initiation and development (7).…”

“…Proper fertilization and tempera ture regimes, along with ancymidol for height control, are required to produce quality plants (6,7). The naturally in creasing spring photoperiods are optimal for flower initiation and development (7).…”