Early Forcing of Tulip Using a Soil Cooling System

Bulbs of‘Gander’ tulip subjected to usual precooling (15°C 3W+5°C 8W) were planted in beds with or without soil cooling tubes in early autumn. Soil cooling at 15°C reduced bud blasting and increased cut flower length and weight, but delayed flowering. When the bulbs were planted in late September and their temperatures were varied by changing the distance between bulbs and cooling tubes, the bulbs kept at 18-20°C resulted in 100% flowering in mid November without delay. Exposure of the bulbs to ethylene just after the lifting ensured high flowering percentages and improved cut flower quality even when they were grown under high soil temperature conditions. The bulbs planted in late September, kept below 20°C by soil cooling for 2 weeks and then transferred to an ambient condition resulted in the same flowering percentage and cut flower quality as those kept below 20°C by continuous soil cooling until their flowering.