FLOWERING IN BEGONIA X HIEMALIS FOTSCH AS AFFECTED BY DAYLENGTH AND TEMPERATURE
The first cultivar mentioned in the literature is 'John Heal' from 1883. This cultivar was a result of a cross between B.socotrana and B. x tuberhybrida 'Viscountess Doneraile'.
Since then a number of cultivars has been introduced and cultivated. Names as 'Ensign', 'Elatior', 'Exquisite' and 'Baardse' belong to the hybrid-group B. x hiemalis, and some of them have also been used, and are still in use as a name for the whole group. Elatior-begonia and Baardse-begonia are names commonly used in Europe. In USA the name Holland-begonia is used.
From a previous work by Roodenburg (3), it is commonly assumed that Begonia x hiemalis form flowers when the length of day is between 12 and 14 hours. Roodenburg states: "The main flower-forming period is situated in the late summer and the early autumn. It is the result of the shortening of the days in August and September, for daylengths of 14 to 12 and less are favourable for flower bud initiation in these begonias. Later on, in the much shorter days of November and December, the growth gradually comes to a standstill."
To prevent this standstill during winter months, artificial lengthening of the days has been practicised on mother plants in order to provide cutting-material for propagation. On the other hand, shortening the days in the late spring and summer is used to initiate flowers on plants for sale (4) (6).
However, experiences from cultivation of B. x hiemalis in Norwegian nurseries indicates a more complex reaction than set up by Roodenburg, and there is reason to assume that temperature and daylength interact in the flower formation. In order to obtain additional information on the flowering in B. x hiemalis a series of experiments were carried out in 1966 and 1967 (5).
Two experiments were carried out in growth rooms with artificial lights only. Plants delivered from a commercial propagator were placed in rooms with 12, 15, 18, and 24°C. The plants were illuminated for 10, 13 and 16 hours per day with fluorescent tubes. There were about 200 watt per sq. m. or approximately 5000 lux on the top of the plants.
In the first experiment in 1966, plants of the cultivars 'Frau Gertrude Rieger', 'Rose Queen' and 'Novemberglut' were given these conditions for 6 weeks and then moved to a greenhouse with natural daylength and a night temperature of 18°C.