By then, they had four to five leaf pairs and were treated with high-intensity lighting for 16 hours at either 24 degsC or 17 degsC or were grown at 24 degsC with either low-intensity lighting for 16 hours or double that light intensity but over eight hours. After treatment, all plants were chilled (10 degsC or below) for eight weeks in the dark, before being grown to flower in 16-hour days at 18 degsC (later 21 degsC) with high-intensity lighting.
Plants grown at 17 degsC had begun to initiate flowers after three weeks of treatment and the flowers were fully developed by nine weeks. Flower initiation was much slower at 24 degsC and the plants developed few flowers on their lateral shoots.
Surprisingly, plants of 'Schneeball' grown in eight-hour days at 24 degsC flowered better than those grown in 16-hour days. Overall, the moderate temperature of 17 degsC produced compact plants that flowered early with many flowers on their lateral branches.
Temperature and Photoperiod Control of Morphology and Flowering Time in Two Greenhouse-Grown Hydrangea macrophylla Cultivars by Nordli, Strom and Torre (2011). Scientia Horticulturae 127 (3): 372-377. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti.