Greenhouse Design and Installation
The process to build a greenhouse is no different than any other capital project. Planning, funding, and ordering are very likely the most difficult steps. The actual building project is relatively simple and it can be exciting. The completion of any project has rewards of satisfaction and future profit. Where do I start?
The first step is to try to evaluate how much growing space is going to be needed and how the structure is going to fit on your property. Quite often I will ask customers how large a greenhouse they want and they don’t have any idea. The best way to plan a greenhouse is to know how much you wish to produce. Figure out how many pounds of tomatoes you would like to grow or how many trays of seedlings you need and that will dictate greenhouse size. Well maybe how much money you want to spend has a little something to do with the final answer. The other factor is how much land do you have and what is the topography. It is very important to have enough room around the greenhouse for easy access with vehicles and equipment, snow removal, water drainage, and ventilation. Oh yes, and what about expansion. You will see very few greenhouses standing alone. They seem to like company!
Site selection is a very important decision. The greenhouse wants to be convenient for access, utilities and water supply. Most installations want to be made on level or nearly level ground. The shape of the greenhouse might be dependent on how the site is configured. Is the site square, long and narrow, a side hill that needs to be terraced? All of these factors will play in the final decision. Once the best site is selected then it is time to begin preparation. If the greenhouse is going to be used for in the ground growing it should be prepared like any field situation. If it is to be used for bench crops, a sand or gravel base should be added. Make sure the greenhouse is at least a foot above the grade of the land around it and that all of the water that will shed from the roof in a rainstorm or from melting snow has an easy drainage path. Also remember that most greenhouses require ground posts that are installed into the ground from two to four feet deep. Do you have ledge, boulders, coarse gravel or any other obstruction that could hamper installation? These need to be removed or holes bored with a rock drill. The site needs to be open so that ventilation systems will be effective. Both passive and mechanical systems require free air movement. A distance of at least ten to twenty feet around the perimeter is necessary.
The choice of structure is only limited by the above restrictions. There are many choices of companies that produce frames. They range from simple round cold frames to state of the art computer controlled systems. How do you decide? Budget is a very important aspect as well as the length of growing season. Short growing seasons make it difficult to justify the expense of a sophisticated system. If you will be using the structure for most of the year then considerations for fuel efficiency, light dispersion, processing the crop and volume of production will be more important. Growers in snow regions should consider a gothic shape house for better snow shedding. Tall side walls are important for natural ventilation and for snow accumulation after it slides off the structure. Many growers start with a small simple system and then plan on expansion later to adjust for changes in demand of the crops to be grown. A small simple system for many is a structure of 1000-2000 square feet covered with polyethylene. If you have a demand for 1000 pounds of tomatoes a week or you need 50,000 seedlings ready all at once then that is going to help you decide what to build.
The next steps after you have chosen a site and the proper structure is installation. The free -standing gothic frames are very easy to build. Time invested can be quite extensive however. A 2000 square foot structure will require about 75 man-hours to complete. This includes setting up the metal frame, installing hip and baseboards, building end walls, and covering the frame with polyethylene. Most manufacturers provide instruction manuals with the frame and provide support if needed. The most important step is to make certain that the layout is square and uniformly pitched or level. If this is done carefully the frame should install easily. Some growers choose to hire experienced crews to do the installation. The cost for this is approximately fifty cents to one dollar per square foot. The more complicated systems may justify hiring a professional crew.
A few points about installation that need to be pondered include the type of lumber to be used, door sizes, end wall materials, and ground cover. The choice of lumber is dependent on philosophy and budget. The most longevity for baseboard is achieved from pressure treated wood. If an organic crop is to be grown this may not be possible. Spruce, hemlock, cedar, and pine all also choices. The rate of decay varies with these woods and failure of the baseboard must be considered. The hip board should be inspected at each polyethylene change to make sure it is sound for the next several years. End walls are normally made with a treated sill and untreated wall studs. Wall coverings are made from plywood, boards, polyethylene, structured sheets, or some combination. Doors should be large enough to provide ventilation and easy access. The ground cover choices include fabric, concrete, sand, gravel, loam, and asphalt. Individual situations will determine choice.
Mechanicals are divided into two categories. Heating and cooling need to be addressed for any application. Early spring production will require night heat and then almost immediately morning cooling and on some days a continual change from heat to cooling for moisture control. Unheated spring crops need ventilation for cooling and moisture control. Winter crops may need heat and cooling at times depending on the weather, but almost constant control of moisture is critical. Low light levels tend to soften plant tissues and leaf diseases could develop. Heating is obtained using several methods. Fuel types are propane/natural gas, fuel oil/kerosene, wood, and electricity. Transfer of the heat from the source is achieved either by hot air or hot water systems. Fuel type is a personal choice based on preference and available supply. Fuel oil is about one-third less expensive than propane if the price per gallon is the same. Wood is economical if supply is local. Actual wood fuel cost is difficult to calculate as many operations harvest and process on site. Consistency of all night heating is difficult with wood unless the furnace is supplemented with another type of fuel or one of the new wood fired boilers/furnaces is used. Electricity does not seem to be viable on a large scale, however for periodic supplemental heat it is an option. Hot air distribution is simpler and less expensive than hot water, but hot water may be used to heat the soil or benches. Cooling choices are mechanical, passive, or a combination of both. Mechanical includes electrically powered fans and intakes, evaporative cooling pads, and automated passive systems. Passive systems include manually operated roll-up sides, ridge vents, and end vents. The ridge- vents in combination with roll-up side’s work very well, but is quite expensive to install. Most unheated spring to fall applications are sufficiently cooled with rollup sides. Winter and early spring crops may need both mechanical and passive systems to adequately control the environment. A fan system would cost about $.50 per square foot to install. A complete ridge-vent system would be about $1.00 per square foot. Rollup sides are about $.15 per square foot installed. Good quality mechanical controls will save fuel and create a more consistent environment. Estimates for heating system size can be calculated at 100btu per hour per square foot of greenhouse area. Exhaust fan capacity is estimated by multiplying greenhouse area in square feet by eight. Horizontal airflow fan systems are the best choice to circulate heated air and to maintain dry foliage to reduce mold outbreaks.
The choices for covering a greenhouse are numerous. The most common cover is polyethylene film either as a single layer or double layer with inflation between the layers. The newer films are treated to reduce heat loss and to diffuse transmitted light to increase adsorption by the plants. Each layer of polyethylene increases heat retention but also reduces light levels so a balance must be reached. Glass is still the best glazing for light transmission, however, cost of installation is very high. The other choice is a structured sheet made of polycarbonate or acrylic. These materials are longer lasting than polyethylene and less expensive than glass. The cost for a double layer polyethylene cover is about 20 cents per square foot. Structured sheets are about $1.00 per square foot and glass is about $3.00 per square foot. The choice of covering is determined by each individual preference.
The decision on what size, type, and season to operate a greenhouse will only become apparent after several years of trial. There is no question that in most climates protective growing systems repay the cost of installation in a very short time. In many instances one season is all that is needed.
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