We have received many questions regarding the construction of greenhouses for tobacco transplants from wood. Plans and blueprints are available to build both rigid-frame and post-truss greenhouses from wood. These plans are contained in the Plans Catalog from the Biosystems and Agricultural Engineering Department's building plans booklets.
There are a number of things for a farmer to consider before building a greenhouse from wood:
1. The primary reason that many give for wooden greenhouses is that they are cheaper. Although pressure treated wood greenhouses are slightly cheaper than commercial quonset packages, the cost difference is small. Plus, this cost difference is probably offset by all the labor necessary to fabricate all the pieces of the greenhouse plus the cost of equipment for this fabrication.
2. The rigid-frame and post-truss greenhouse designs are appropriate for tobacco houses. However, the trusses in the post-truss design create significant shadow in the greenhouse and reduce the amount of light that the plants will receive. Spring light intensities are high enough to overcome the shadow in these houses for tobacco plant production but these greenhouses decrease light too much for other winter growing conditions.
3. It is imperative that the salt type preservatives like Osmose and others be used for the pressure treated wood used to build the greenhouses. Volatile chemicals from any other type of wood preservative will damage or kill the plants inside the greenhouse.
4. Polyethylene greenhouse film is more difficult to attach to a wooden frame greenhouse. It would be quite important to use metal locking devices to secure the plastic on all edges. Additionally the sharp edges of wooden members and the chance for slivers of wood to tear the plastic may greatly reduce the life of the plastic. The greenhouse peak or ridge tends to separate the greenhouse roof into two sections so both sections need to be inflated separately.
5. A pre-fabricated metal quonset greenhouse arrives at the farm with all the appropriate equipment that has been sized for the specific greenhouse. Often, builders of a wooden greenhouse, simply find salvage fans and furnaces for their greenhouse. This often leads to undersized heating and ventilation systems. It is nearly impossible to determine the air moving capacity (cfm - cubic feet per minute) of a salvage fan. Thus the appropriate ventilation standards are rarely met.
6. Foundations and footings must be completed properly. For both snow and wind conditions, the loads imposed upon the structure must be carried to the ground by the foundation and footings. They must resist uplift, overturning, and downward acting loads. The downward load includes not only snow but also the dead weight of the structure itself.