This post is part of the series Simulating Climates in Growth Chambers.

Much more difficult is preparing the plants for winter when limited control over day light and temperature is available. If your plants have been growing in the greenhouse under warm and constant temperatures, and are still growing happily, how can you make them set bud and prepare them for a second season in the growth chamber? Commercial growers use a procedure called black-out in mid or late summer. Day length is reduced to about 10 hours (in British Columbia) by shading the plants using black cloth, e.g. the cover goes on at 7:00 pm and is removed the following morning at 9:00 am. Usually this is accompanied by a lower fertilizer concentration (esp. Nitrogen), a reduction in moisture levels and -if possible- a slight drop in temperatures to cause mild (but not severe) stress. Blackout is not as easy to apply as it sounds, since good ventilation is desired. The treatment is applied for 7 to 10 days, and resulting bud formation should begin to be visible in 2-3 weeks. Though it is not a very natural procedure, firm plants and firm buds can be obtained this way.

Simply moving plants from 22°C (greenhouse) into cool temperatures (2°C) will not have the same effect: while the signal is clear, the biochemical reactions of the plant are slowed down at low temperatures. The only way to avoid this is to find a place where you have more control over temperature and day length, or are in sync with the seasons outdoors.

Growing season compaction

We reduced the length of the first growing season from 25 to 17 weeks by reducing phase A from 3 to 2 days, and phase B from 4 to 3 days. This resulted in only a slight change in average temperature. This not only saved some time, it also reduced the maximum size of the plants in the warmest treatments. As a result, competition in the second season was reduced below levels that would lead to widespread mortality. At the same time, it still took the plants through all the physiological stages and prepared them for the climates they would experience in the second season.