by Gary Mount
Come to think of it, the very idea of a greenhouse is a contradiction. I find this to be true every year at this time as I prepare our greenhouse for the winter. The greenhouse must let in lots of light, yet conserve heat as much as possible, be sturdy enough to withstand high winds, yet be economical enough in construction, maintenance and operating cost so that a crop can be grown for a profit. The greenhouse is your basic engineering nightmare.
One of the ways we have been able to build and maintain a profitable greenhouse is by living in New Jersey. Not only are the winters relatively mild and the winter light intensity moderate, our friend Bill Roberts lives in New Jersey and works as an agricultural engineer at Rutgers University. He is a nationally recognized expert in greenhouse design and operation and through the Rutgers Cooperative Extension, his expertise is available to New Jersey farmers like me!
To build the best greenhouse for our use, Bill recommended a pipe frame house covered with clear polyethylene. The pipe frame is anchored in concrete and covered with two layers of “poly”, with a small fan inflating the space between the layers. The air space provides insulation and strength. A single layer or un-inflated double layer would flap back and forth in the wind and soon start to tear. After all, it is only six thousandths of an inch thick!
Built into the greenhouse are gas heaters and cooling fans. Too little or too much heat is critical. On a cold winter’s night, heater failure can lead to the loss of the crop in only a few hours. On a very sunny day in late spring, a failure of the exhaust fans can cause the temperature to soar – killing the crop in less than an hour. Finally there are the operation controls, which must control the ventilation louvers, fans and heaters all in one unit with very little room for error. Added to this is a device which will telephone a key employee or us if the temperature gets too high or too low. If this happens, a trip to the greenhouse is mandatory, the sooner the better.
This fall, we have to replace the plastic on the greenhouse. The sun’s ultraviolet rays weaken the plastic and cause it to become cloudy – that is less able to let in light. We usually have to do this every three years. This is an exacting job that must be done on a calm day. Everything must be fastened just right. A small hole can let the wind tear off the whole cover in short order.
This job will get done soon. The next crops- freesia and cyclamen – are already growing. The next time you see a day that is calm and at least 50 degrees, you will know what is happening at Terhune Orchards.