4 things to consider and common mistakes to avoid when building your light deprivation greenhouse.
1. Make gravity your friend instead of your enemy.
Since the black out tarp is only attached to the ridge of the greenhouse, there is absolutely nothing to hold it in place next to the greenhouse if you have built out of level in any way. I liken this to someone that has long straight hair. If they tilt their head, gravity will make the hair drop straight down, but that is not where the head is.
The best way to prevent this is to fully grade the site pad to be level throughout. Make the site a perfect rectangle by measuring the sides, ends and diagonals to make sure they match up. Do not for get the alignment of the greenhouse hoops and ground posts. A hoop out of place can cause headaches as described in #4 below.
That being said, being out of level between the two ends of the greenhouse is by far the most critical and nearly impossible to correct.
If you must build on a hillside, then site the greenhouse so that it is level from end to end and put one side on the uphill side and the other on the downhill side. Then still place your ridge at the center peak. While the rollup tarps will not be equal, they will both still function.
2. Do not sacrifice adequate ventilation.
Achieving full blackout is the idea behind light deprivation, but if you eliminate necessary ventilation both active and passive, you risk losing your crop to molds and disease.
The biggest disadvantage to pulling tarps or using a Golden Arm type device is that by fully covering the greenhouse, airflow is shut off. Luckily, several work-arounds have been created to help with this such as ducting placed under the tarp edges with inline fans. This may be all that is needed, or may be inadequate when extra ventilation may be needed.
The industry solution that has done the most to solve this airflow problem is the use of ‘breathable walls’ or ‘light traps’. When installed properly, these will prevent the passage of light with only minimal loss of airflow in the order of 5-10%.
Generally based upon the natural wind direction at the site, one end of the greenhouse is designated as the passive side where air will enter and the opposite side, designated as the active side were air a fan will exhaust air out of the greenhouse.
Generally, the square footage of the passive side should be twice that of the active side. The ventilation fan should be sized such that the total cubic feet of air in the greenhouse can be exhausted in one minute. That is the CFM of the fan should match the volume of air in the greenhouse. Here is a helpful website to assist in calculating the volume of your greenhouse: https://www.vcalc.com/wiki/KurtHeckman/Greenhouse+Calculator
The breathable walls are used on both the passive and active ends of the greenhouse. On the passive end, it is best to mount then high enough so that they will be able to passively exhaust hot air from the upper areas where the hot air will collect even when the active fans are not on. On the active side, wed the breathable walls with the fans. Generally they go on the inside so that the sun doesn’t degrade the plastic they’re made of. Since they will still slightly decrease the flow, they can be slightly oversized compared to the fan.
Automatic controls based upon day time and night time set point temps will turn your fan on as needed, especially when the blackout tarps are covering the greenhouse.
3. How to prevent light from entering under the greenhouse sides when the blackout tarps are down.
It hardly makes sense after doing everything to put on a blackout light dep system to have light streaming in under the sides. The trick here is to create a channel into which the rollup pipes can rest when down which makes it very difficult for light to pass regardless of the direction it is coming from. Most greenhouses will have two sets of roll ups on each side of the greenhouse. The first is for the short clear rollup pipes that allow cross airflow during the day, while the second is the rollup pipe for the blackout tarps. Each of these pipes is 1 3/8” and with the tarps on them about 1 ½ “ or more in diameter. Given this the minimum depth of the channel is 3” to 3 ½”. The width of channel should be about 2” to 2 ¼” prevent interference with the rollup pipes when they are coming in or out of the channel. If they are prevented from dropping in fully, light will get in. The base around the greenhouse can be made from pressure treated or redwood 2×6’s as can the channel outer egde. The channels can then be lined with black out tarp extending 8-11” above the channel and then fed through the channel and resting as a skirt on the ground about a foot. Gravel can be placed on this skirt to prevent light from coming under the wooden sides.
4. Auto dep rollup motors or Chain pullies. Which is right for you?
This is based upon your personal preferences and budget. Either one will work. You can save the cost of the rollup motors and the controller by going chain pull if you’re on a budget. But you will need to be there to raise and lower the tarps on schedule. The plus is that that can easily upgrade to motors at any point. The auto dep controller and motors will let you set the clock and the motors will do their job on schedule. The fine tuning is to set the motor stops so that the motors turn off before the tarps go to far down or too far up. This will require checking them from time to time. Recall mistake #1 about being level. If the green house is not level or if the hoops are not well lined up, the blackout tarp can develop wrinkles or creases from the wind and rollup to far of not drop down properly. So level, plumb, square and well aligned hoops/arches is critical to a well functioning auto dep system.