The conversion of solar energy into heat within a hoop house is the mechanism that enables warm season crops to be established earlier and grown later, compared to their field-grown counterparts. Favorable growing conditions made possible by heat hasten plant growth and fruit maturity. Properly managed hoop house heat can generate additional dollars for the grower.
Too much of a good thing is not always good, though. The same mechanism that creates a favorable hoop house environment during the early spring and late fall can create an unfavorable condition for both plants and people during the summer. Heat induced stress adversely affects plants in a myriad of ways, including reduced yield, lower fruit quality and increased susceptibility to insects and disease.
In Oklahoma, growers should avoid scheduling summer months for hoop house production, since this offers no competitive advantage over field production.
Despite our best efforts to beat the heat, expect harvest of late winter/early spring planted hoop house crops to extend into July. Heat induced stress common during July can also occur in June and even during May as evidenced by the 100 degree F-plus temperatures recorded in one of our fully vented houses in 1998. Heat induced stress is also a problem experienced during August when warm season crops are planted for late fall harvest.
The ultimate solution to this problem is to remove the poly-film cover from the house. Due to time and labor constraints involved with the repeated application and removal of the covering, we don't recommend this option and use it only as a last resort.
At the Noble Research Institute Horticulture Center, our efforts to reduce hoop house heat load have centered around two strategies: increase ventilation and reduce sunlight intensity.
Quonset-shaped structures have an inherent flaw in their design. The portion of the structure above the roll-up side vents traps heat. Raising the height of the side vents will reduce the depth of the heat pocket but cannot eliminate it. Installing vents at the top of the end walls is helpful but lack of size limits their effectiveness.
Draping shade fabric over the hoop house to reduce light intensity is a common practice among growers. At the Noble Research Institute, we've used shade fabric with mixed results. While the fabric is effective in lowering air temperature a few degrees, it also reduces the amount of light available for photosynthesis. Determining the amount of shade to apply and timing of fabric application and removal is difficult at best. Consequently, we've opted to use a different technology.
Several years ago we began installing "Kool-Lite Plus" brand poly-film on our hoop houses. This film has the unique ability to block more solar infrared radiation than conventional films. By blocking out more of this nonessential radiation, cooler house temperatures (up to 12 degrees F) can be maintained during peak afternoon and evening hours. We've been extremely pleased with the performance of this film. Expect to pay up to 75 percent more for Kool-Lite film. In my opinion, the additional cost can be justified considering the costs associated with the use of shade fabric, not to mention the difference in crop quality.
As mentioned earlier, Quonset shaped structures are efficient designs for trapping heat, a desirable trait for spring and fall but not for summer.
Beginning this year, we will be evaluating a different type of hoop house structure. This design utilizes straight walls, one four feet higher than the other. The low side is fitted with one roll-up vent while the high side contains two. The upper vent is what provides this design with superior ventilation characteristics. Variants of the "triple side vent" house are utilized extensively in the greenhouse ornamental industry.
Construction of a 17-foot by 68-foot triple side vent house was recently completed at the Noble Research Institute Horticulture Center just in time for planting. A cut flower trial was started in this house at the beginning of March. A second 25-foot by 68-foot triple side vent house will be constructed this summer.
Additional steel and labor are required to construct a triple side vent house. Expect to spend up to twice as much per square foot on a triple side vent house compared to a Quonset shaped structure.
It remains to be seen if this additional cost can be justified. We plan to thoroughly evaluate the performance of these new houses over the next few years. If they perform as well as some growers claim, the additional cost might be justified. We'll keep you apprised of our findings.