H. Pettersen
It is important to use young plants in studies concerning the influence of environmental factors on flower initiation in azaleas. Larger plants might show only small effects of daylength, especially when the plants have been given a "soft" pinch.

All cultivars reacted more or less as quantitative short day plants when grown at a high temperature. Continuous illumination (24 hours daylength) inhibited flowering to the greatest extent.

The effect of short day treatments in practice will depend very much on the prevailing daylength conditions.

There is an interaction of daylength and temperature on growth and development in azaleas. The azalea cultivars included in this study may be classified as day neutral or nearly so at a temperature of 15°C, while they are definitely short-day plants at temperatures of 20° or above. This classification holds true when it is based on days required from start of treatment till flowering and also when it is based on number of leaves initiated before bud initiation. Even after 30 weeks all the plants of 'Red Wing' and some of the 'Ambrosiana' were fully vegetative when grown at high temperature and 24 hours day. It may be possible to prevent flowering entirely in young plants under these conditions, at least for certain cultivars.

Pettersen, H. (1973). BUD FORMATION IN AZALEAS. Acta Hortic. 31, 27-34
DOI: 10.17660/ActaHortic.1973.31.3

Acta Horticulturae