Year-Round Gardening With a Greenhouse

By Conrad Link and David Ross.

The greenhouse is a specialized structure designed for growing plants year-round. A clear or translucent cover permits sunlight to enter, which heats the greenhouse during the day. When excessive sun heating occurs, ventilation is needed. During cold nights and much of cold days, a heating system is required to maintain the desired temperature.

After the initial investment in land and the greenhouse structure, the main expense will be for heat and labor. If the owner and family are the labor force, then heat becomes the biggest expense. Other costs will include soil and growing media, fertilizer, pesticides, pots, seeds, and bulbs. The part-time greenhouse operator must develop a market for his products and skill with attention to details that result in quality plants.

Crops to be grown will be influenced by where and to whom they will be sold. Marketing includes selling wholesale to flower shops and garden centers or selling retail directly to the consumer.

A greenhouse should be on a site that takes advantage of full sun, provides good water drainage and utilizes windbreaks. Electricity and a good water supply are needed. A separate building should be used to store equipment and supplies, to provide work space, and perhaps to house the heating system.

Size of the greenhouse should be well planned. If the hobby or business endeavor proves successful, the greenhouse will soon be too small. Plan the size, location, and layout to permit future expansion.

Larger greenhouses are more efficient and more economical to operate as they cost less per square foot and the environment can be maintained more uniformly. Heating and ventilation systems are the most expensive items needed for a greenhouse. Their costs per unit area are less in larger greenhouses.

Structural Options

Many styles of greenhouse frames exist; select one that is pleasing and practical for you. The frame may be wood, steel, or aluminum. The cover can be glass, plastic sheet, or fiberglass, each available in different sizes and qualities.

A popular low cost greenhouse is the pipe frame or curved roof style. The foundation is a series of pipes driven into the ground to support the curved roof members. Roof members may be made of steel or aluminum pipe or may be a curved truss.

The cover is a single or double layer of greenhouse-quality, ultraviolet-inhibited 6-mil (0.006 inch) thick or heavier plastic sheet. Plastic sheet is good for one or two years, depending on the material quality and weather. An air-inflated double layer of plastic film can reduce heating costs by 30 percent.

For a more permanent cover, use a clear greenhouse grade fiberglass. Fiberglass is available in several grades having service lives of a few years to perhaps 20.

Plans are available through your county or state Cooperative Extension Service for wood frame greenhouses which can be covered by plastic film or fiberglass. Wood in contact with the soil should be pressure-treated or painted with a wood preservative, Copper naphthenate is a safe preservative near Plants; creosote and pentachlorophenol are harmful to plants.

A good quality greenhouse can be built with a good foundation and rigid frame, or an inexpensive greenhouse can be built with a temporary frame to give seasonal plant protection.

A glass greenhouse is the third possibility. Glass and aluminum or steel combine to make a long lasting, beautiful greenhouse. The glass, rigid aluminum or steel frame, and a sturdy foundation make the initial investment high. However, annual maintenance is much less.

While the glass greenhouse is a showplace, the beginner Will find the less expensive, temporary plastic or fiberglass greenhouse well suited as a first structure.

Two additional structural options are the hotbed and cold-frame. These are low-walled frames with cover to give plants Protection during cool, windy spring weather. A hotbed has a heat source in the soil. A 3 x 6 foot coldframe or hotbed can be used to advantage for starting vegetable or flowering plants.

Heating Systems

The greenhouse can be heated with steam, hot water, or hot air. The system can be fired by any of the conventional fuels. The heating system should be fully automatic and as free from maintenance labor as possible.

A thermostat is used to control heater operation. The fan on a hot air heater should be wired to run continuously to maintain uniform air temperature throughout the greenhouse.

Two smaller heating units instead of one large one provides some insurance in case a heating unit fails. A small standby electrical generator is good to have for a power failure. Heating units must be vented to the outside if there are combustion gases. Provide an air inlet near the heating unit so oxygen is available for combustion.

Heater size is determined by the following equation: heater size (BTU/hr) = (total surface area in square feet) times (night temperature difference between inside and outside, F) times (a heat, loss factor).

The heat loss factor is 0.7 for air-separated double plastic sheet and 1.2 for single layer glass, fiberglass, or plastic sheet. These figures should be increased by adding 0.3 for hobby (small) greenhouses or for windy locations.

For further details read the bulletin, Hobby Greenhouses and Other Gardening Structures, available from NRAES, Riley-Robb, Cornell University, Ithaca, NY 14853 for $2.

Ventilation, Shading

Ventilation is essential for producing good quality plants. The temperature must not get too high, and a supply of carbon dioxide must be maintained. Ventilation can be provided by natural convection, using side and roof vents, or by mechanical means using exhaust fans and inlet louvers. Thermostats and electrical motors are used to automate ventilation.

The ventilation system must be able to change the air once each minute in a large greenhouse, and to change it one and a half times each minute in the hobby (small) greenhouse.

Winter ventilation requirements are about one-quarter air change per minute. Two fans, with one having two speeds, are often used; the low speed of one fan is enough for winter. Motorized intake louvers are placed on the opposite wall.

The volume of a greenhouse is length times width times average height and is given in cubic feet. The fan rating will be in cubic feet per minute (cfm).

Shading materials such as saran cloth, movable lath strip covering, lime and water, and dilute white latex paint are used to reduce light intensities and to cut the solar heat load in summer. Light reduction is necessary for those plants which grow best in low light.

Many plant functions are controlled by the length of day. Some plants such as petunia, China aster, or tuberous begonia naturally flower in the long days of summer (long-day plants) and others such as chrysanthemum or poinsettia flower in the short days of fall or winter (short-day plants).

Other plants such as carnation, rose, lilies, and everblooming begonia flower regardless of the day length (day-neutral plants).

A greenhouse operator must protect short-day plants with a light tight cover to induce flowering when days are long. Artificial light is used on long-day plants to induce blooming in winter months.

Temperature Control

A well designed heating and ventilating system allows the greenhouse operator to maintain the most efficient and economical temperature for plant growth. Greenhouse night temperatures are generally maintained at 50 to 70 F, depending on the kind of plant. The temperature is permitted to rise 10 to 15 degrees during the day before ventilation is started.

The effect of temperature on growth varies with plants. Seedlings of many crops are started at a warm temperature and then grown at lower temperatures. This is true of annual vegetable and flower plants germinated and started at 70 to 75 F, grown at 65 , and finished at 55 .

Water that is safe for drinking is appropriate to use in a greenhouse. Water from ponds and wells is fine, providing it doesn't contain excess amounts of salt.

When plants are watered, apply a sufficient amount to moisten the entire volume of soil plus some that will drain through. This drainage helps prevent buildup of salts from the water or fertilizer used. Frequency of watering is determined by size of the plant, temperature, and the growing medium's ability to hold water.

Water is applied manually with a sprinkling can or hose. Spray nozzles or porous plastic tubing are used for watering cut flower crops. Trickle tubes may be placed into individual Pots or plants. Such watering systems may be made automatic using a time clock switch that is set to water at designated times, or by using devices that operate on dryness of the soil.

Capillary watering of pot plants is possible by placing them on a bed of sand kept continually moist. Recently a carpet-like mat of natural or synthetic fibers has been used in place of the sand.

Soils, Growing Media

Plants may be grown in many types of soils, soil mixtures, or mixtures of organic matter and inert materials without soil. The growing media mixture must be uniform in texture, hold sufficient water and drain well, be porous and well aerated, and pest-free. It need not have any available nutrients as these are supplied in fertilizing.

Growing media ranges from fertile top soil with no additions, to a variety of mixtures that may include sharp sand, peat, perlite, bark and wood chips, sludge, or composted leaves. When using soil, select a sandy loam or loam, preferably one containing organic matter. Be careful of soils containing herbicides as they may damage your plants.

Sterilize soils and growing media before use to reduce the problem of soil insects, diseases, and weed seed. Steaming is most effective but certain chemicals may do the job. Growing mixtures are prepared by the greenhouse operator or bought already prepared. Commercial mixtures are often more economical because they are sterilized, ready to use, and may even contain some fertilizer.

Proper application of fertilizers is another part of growing under greenhouse conditions. Have soil tests made of mixtures with a high proportion of soil. Mixtures without soil generally contain little available fertilizer nutrients.