Greenhouse Control – use of blackout cloth for day-length reduction  

Whilst some plants will only flower when the daily hours of light exceed a certain minimum others will only flower when the day-length is below a particular maximum.  To get these plants to flower at times of the year when day length is naturally long, blackout cloth may be used to create darkness from early evening until night-time and then again in the morning from just before sun-rise until sufficient hours or darkness have been reached.  One aspect of using blackout cloth that must be addressed in the design of the greenhouse is the fact that because the cloth is of necessity a closed weave cloth and it also needs to reach right to the edges or the roof and also drop down at least two or three walls, there is a difficulty in providing adequate ventilation by normal methods.  This can lead to an excessive build-up of humidity.  A further difficulty is that many plants require uninterrupted darkness to initiate flowering and in these cases periodic purging to reduce humidity or temperature cannot be used.  One solution is to use exhaust fans in the uncovered wall of the greenhouse and then provide special air inlets in the opposite wall via cowlings or bent tubes whose inside walls have been painted matt black to prevent light entering.   

Normally the control system such as the Autogrow AutoVent 3 would be set to close the blackout cloth early evening until a time when night has fallen when the controller may withdraw the cloth if required for humidity and or temperature control.  Then again early in the morning the blackout will be closed again and will remain closed until the required dark period has been achieved.  These controllers will also control humidity and/or temperature by switching fans on/off or by purging using the roof vents.  

Note that the blackout scheme should achieve a light level below about 0.4 umol/m2/sec.  Note however, that moonlight will never approach this level and so it is quite safe to open the blackout during moonlit nights.  

In order to prevent flowing of low day-length plants during winter, lighting systems of relatively low intensity may be used once or twice during the night to interrupt the otherwise long nights.  Frequently, florescent lights are used for this purpose.  Again, controllers such as the AutoVent 3 will perform this function with ease.