Greenhouse Management: A Guide to Operations and Technology provides detailed, step-by-step instructions, in layman's terms for ALL aspects commercial greenhouse plant production. The text is a complete reference on greenhouse operations and technologies, and the science of growing crops. Greenhouse Management systematically starts the reader off by providing an in-depth discussion of greenhouse structures and design, environmental control systems, heating/cooling, growing media, fertilization, carbon dioxide supplementation, irrigation, pest management, and the production of container-grown crops. Finally, a series of appendices provide numerous data relevant to greenhouse management and operations. The information in this easy-to-use guide is distilled from a variety of sources, including scientific literature, extension publications, and grower experience and has the added value of numerous citations to more in-depth discussion on many topics. The book is thoughtfully organized presenting a seamless flow of topics within chapters making it easy to find specific information that interests the reader. No one concerned with greenhouse management can afford to be without this book.
The greenhouse structure represents both the barrier to direct contact to the external environment and the containment of the internal environment to be controlled. The greenhouse has to protect plants against extreme temperatures, wind, snow, rain, hail, birds, and insects. The efficiency and productivity of a greenhouse operation is largely dependent on the type of growing structure used. Since there are many designs to select from, it is important to become familiar with the advantages and disadvantages of each. The following is a discussion of commercial greenhouses and their structural components.
Heating is a major concern to commercial greenhouse operations. This is due primarily to the costs involved in the purchase and operation of heating equipment as well as the potentially disastrous effects of a poorly designed system. Greenhouse heater requirements depend upon the amount of heat loss from the structure. Heat loss from a greenhouse usually occurs by all three modes of heat transfer: conduction, convection and radiation. Usually many types of heat exchange occur simultaneously. The heat demand for a greenhouse is normally calculated by combining all three losses as a coefficient in a heat loss equation. Suitable energy sources include natural gas, LP gas, fuel oil, wood and electricity. The cost and availability of these sources will vary somewhat from one area to another. Convenience, investment and operating costs are all further considerations. Savings in labor could justify a more expensive heating system with automatic controls.
Greenhouses structures by design gather light and to trap the considerable heat contained in sunshine. Greenhouses are so efficient at retaining relatively low levels of solar energy, that without specialized ventilation and cooling equipment, high greenhouse temperatures severely impact plant growth. Ventilation is an air exchange process that replaces the warm moist air inside the greenhouse with cooler, often less moist, outside air. Ventilation can be provided by natural or forced air systems. Natural ventilation is driven by two mechanisms, namely the pressure field induced by the wind around the greenhouse and the buoyancy force induced by the warmer and more humid air in the greenhouse. Forced ventilation is accomplished by fans that are capable to move large quantities of air at relatively low pressure drop. Proper ventilation not only cools the greenhouse, but also reduces the humidity level inside the greenhouse and replenishes the carbon dioxide that plants consume during daylight hours in the process of photosynthesis. At the same time, ventilation can reduce the concentration of pollutant gases and during winter, in cases where the heating unit is installed in the greenhouse, keep the combustion of the fuel at high efficiency since the lack of adequate oxygen results in incomplete combustion and carbon monoxide buildup. Techniques for cooling greenhouses can be organized into several categories, including pad and fan evaporative cooling systems, mist and fog systems, and shade control. Pad and fan systems draw outside air through a wet porous pad causing water to evaporate. This results in lowering the vapor pressure deficit, raising the relative humidity, and decreasing the air temperature. Mist and fog systems operate in much the same way except that they add moisture directly to the greenhouse environment where it then evaporates. In both cases, ventilation is required to exhaust the humidified air and exchange it with drier air so that evaporative cooling can continue. Shading does not actually cool but rather reduces the amount of solar radiation reaching the plant. It reflects or absorbs incoming solar radiation before it reaches the crop.
The goal of all environmental control systems for greenhouses is to enhance the growth of the plant, and provide a mature crop in timely fashion, with desirable quality as demanded by the market of the producer. The pressures of labor availability and costs, energy costs, and market demands increasingly make efficiency and automation key components for success and profitability. Environment control technology affects all of these critical areas, and many others, so understanding controls and implementing their use is more important than ever. Precise control of the greenhouse environment is critical in achieving the best and most efficient growing environment and efficiency. Temperature and relative humidity (and/or vapor pressure deficit) need be monitored in every greenhouse. Carbon dioxide levels should be monitored since it is an important greenhouse climate variable, which enhances the growth of the plants. Light levels should be checked at least periodically in the greenhouse to make sure covering materials are performing adequately, but ideally light levels need to be checked on a regular basis in order to know the optimal temperature regime for the crop. The electrical conductivity and pH of both the feed and drain solutions should be monitored in every hydroponic system.
Energy costs are the third highest cost for most greenhouse growers, behind labor and plant materials according to a U.S. Department of Agriculture, 2009 Census of Horticultural Specialties. If not designed and maintained properly greenhouses can use a lot of energy, therefore, energy conservation in greenhouses can make a large contribution to an operation’s profitability. Fortunately for greenhouse operators, there are several opportunities during a new build to reduce future energy costs related to the operation of the business. For those who plan ahead, incorporating energy-saving measures into the design of a new greenhouse or addition can reduce costs dramatically and offer paybacks within a reasonable time period, depending on the heating and cooling options installed. For those who are retrofitting, there are several choices to help provide needed reduction in energy-related expenditures, too. As a greenhouse operator, consider the following factors in making energy conservation decisions.
Light is an essential factor in growing plants in greenhouses. The rate of growth and length of time a plant remains active is dependent on the amount of light it receives. Light energy is used in photosynthesis, the plant’s most basic metabolic process. When determining the effect of light on plant growth there are three areas to consider: quality, intensity, daily duration, and daily light integral. Light quality describes the wavelengths (colors) of light. Intensity is important because rate of photosynthesis and growth increase up to an optimum intensity level and then decrease at higher levels. Light duration is important because the ratio of light to darkness each day controls processes such as flowering and form of growth in many plants. Daily light integral (DLI) is the amount of photosynthetically active radiation (PAR) received each day as a function of light intensity and duration (day). In commercial greenhouses, several strategies can be used to help properly manage light levels throughout the day and seasonally. Some of the primary reasons why greenhouses manipulate light levels include temperature and irrigation management, photoperiod control, minimizing crop stress, and optimizing photosynthesis. Some of the lighting technologies include, incandescent bulbs, halogen incandescent bulbs, fluorescent lamps, compact fluorescent lamps, high-intensity discharge lamps, and light-emitting diodes.
In a greenhouse filled with plants, carbon dioxide concentration will closely follow ambient outside concentrations during the day as long as ventilation is needed. Carbon dioxide concentrations rise during the dark period because plants are not using carbon dioxide for photosynthesis and respiration. In the greenhouse, there may be times when carbon dioxide levels fall below outdoor levels and limit plant growth. Carbon dioxide levels can fall quite low in airtight greenhouses when vents are kept closed for extended periods during the winter months. With little air exchange to the outside on cold bright days, photosynthetic rates can be high and deplete indoor carbon dioxide levels below outdoor ambient levels, thus limiting photosynthetic rates. In a tightly closed greenhouse with plants, the carbon dioxide concentration can drop to 150 to 200 ppm; normal ambient level of carbon dioxide is approximately 350 ppm. This concentration is at or close to the carbon dioxide compensation point where plant growth is adversely affected. An option in counteracting potential carbon dioxide drawdown in greenhouses is to supply supplemental carbon dioxide. Many studies have shown that carbon dioxide concentrations (e.g., 800 to 1,000 ppm) well above ambient levels can benefit plant growth. Typically, a three- to four-fold increase in carbon dioxide concentration yields a 10 to 25 percent increase in plant growth. Supplemental carbon dioxide is often referred to as “CO2 fertilization” or “CO2 enrichment.”
The most important environmental parameters that need to be controlled for optimal greenhouse climate are temperature, light, relative humidity, and carbon dioxide (CO2). Temperature is the most important single parameter in greenhouse controls as temperature has a significant role in plant growth and development. The optimal temperature depends on the plant species grown and desired level of photosynthetic activity. Different crop species have different optimum growing temperatures and these optimum temperatures can be different for the root and the shoot environment and for the different growth stages during the life of the crop. Temperature can also affect plant quality. A plant may flower at a fast rate at high temperatures, but it may have reduced quality compared to a plant grown cooler. Grown for longer periods of time under cooler temperatures, plants generally have thicker stems, greater branching, more flowers and larger flowers. Typical greenhouse temperatures vary between 50 to 68 degrees F (10 to 20° C). Too high temperature reduces plant growth, eventually resulting in plant wilting and death whereas too low temperature limits plant growth.
The production of greenhouse crops involves a number of cultural inputs. Among these, perhaps the most important is the growing media or substrate for growing plants. Growing media are made of engineered materials designed to provide ideal physical and chemical characteristics for the root environment. In greenhouse agriculture, it is important that the substrate has good structural characteristics so it can hold up to frequent irrigation, root growth, temperature change, pH and EC over the life of the crop. At the same time, it has to serve as a reservoir for water and nutrients and be able to allow air exchange between the root system and the aerial environment. It is important that the substrate has low cation exchange Capacity (CEC) so there is better control over fertilizer management and pH of the substrate. Growing media for greenhouses contain a variety of soilless substrates such as peat moss, vermiculite, perlite, shredded coconut husks (coir), or composted materials plus starter nutrients and a wetting agent. Recipes are specially formulated for propagation, specific crops or general use. Field soils are generally unsatisfactory for the production of plants in containers. This is primarily because soils do not provide the aeration, drainage and water holding capacity required. They also need to be pasteurized or fumigated to prevent the development of diseases and germination of weed seeds.
A dependable supply of high quality irrigation water is a vital component of any greenhouse growing operation. Characteristics of irrigation water that define its quality vary with the source of the water. There are regional differences in water characteristics, based mainly on geology and climate. There may also be great differences in the quality of water available on a local level depending on whether the source is from above ground (rivers and ponds) or from groundwater aquifers with varying geology, and whether the water has been chemically treated. There are a number of factors, which determine water quality. Among the most important are alkalinity, pH and soluble salts. But there are several other factors to consider, such as whether hard water salts such as calcium and magnesium or heavy metals that can clog irrigation systems or individual toxic ions are present. Poor quality water can be responsible for slow growth, poor aesthetic quality of the crop and, in some cases, can result in the gradual death of the plants. Water quality factors outside the acceptable range do not necessarily mean that the water source cannot be used. There are several water treatment options available to greenhouse operations that can correct many water quality problems. However, water treatment systems differ in installation costs, operating costs, mode of action, space requirements, water volume treated, and worker safety. No single system is best for all greenhouses. A combination of both chemical and physical treatment methods is often needed. Before investing in any treatment system, however, it may be advisable to investigate the possibility of switching to an alternate water source, or mixing water sources, if it is an economical alternative for solving a water quality problem. Water usage is affected by many variables. Most important of these is the level of solar radiation within the greenhouse. The main sources for irrigation water are groundwater from wells, surface water, drainage ponds, rain, and municipal water.
Greenhouse crops are irrigated by means of applying water to the media surface through drip tubes or tapes, by hand using a hose, overhead sprinklers and booms or by applying water through the bottom of the container through sub-irrigation such as ebb-and-flood systems, or by using a combination of these delivery systems. Although there are many different types of greenhouse watering systems the most commonly used system is micro-irrigation or more commonly called drip irrigation. Irrigation scheduling is important in greenhouse crop production. Under-irrigation generally results in yield loss and low produce quality. Conversely, over irrigation increases the crop's susceptibility to diseases, energy cost for pumping, water loss, and environmental pollution due to nutrient leaching. The need for increased control over water availability and water quality while meeting environmental objectives has led many greenhouse production systems to adopt recycling irrigation runoff as a solution. In certain regions, the main concern may be a lack of quality water, while in other regions it may be protection of water supplies.
The term “micro-irrigation” describes a family of irrigation systems used for greenhouses that deliver water and nutrients in precise amounts and at controlled frequencies directly to the plant’s root zone. With micro-irrigation systems, an extensive network of pipe is used to distribute water to emitters that discharge it in droplets, small streams, or through mini-sprayers. Micro-irrigation pipeline systems are generally described as branching systems. Various branches are given names such as main, submain, and lateral. Choosing the right size main, submain, and lateral pipe to match the flow rates from the water source is important. Basic components can include a pump and power unit, a backflow prevention device if chemicals are used with water, a filter, a water distribution system, and some devices for controlling the volume of water and pressure in the system. Depending upon the source and quality of well water, the grower may need a water filter system on the water supply. Removing particulate matter will extend the life of pumps, valves, pressure regulators, and injectors, and is important in reducing emitter clogging.
Plant nutrition refers to the chemical elements taken in by plants that are essential to their growth and development. Fertilization is the term used when these elements are supplied to the environment around the plants. Maintaining adequate nutrition is among the most critical aspects of producing greenhouse crops. At present most growers utilize a liquid feed program either by fertigation or foliar application as their primary means of supplying plant nutrients. This program may also be supplemented with granular or slow release fertilizers added to the growing medium. The frequency of fertilizer applications also influences plant growth. In some cases it is important to supply nutrients at peak periods of vegetative or reproductive growth. However, it is generally accepted that a constant feed (soluble fertilizer at each irrigation) is the best system to optimize plant growth. A constant feed program may also be modified so that nutrients are applied at every other irrigation. This approach may be necessary under conditions of high soluble salts. The balance of plant nutrients is important in producing vigorous, efficient plants. In some cases when nutrients are out of balance severe deficiencies or toxicities may occur. Therefore it is important to consider both the source and amount of fertilizer used. Fertilizer refers to any compound that contains one or more chemical elements, organic or inorganic, natural or synthetic that supply the nutrient need of the plants. There are various types of fertilizers available for use in greenhouses, each of which are suited to different situations and serve different purposes. With the increased cost of fertilizers and concerns about the adverse environmental impacts, there is great interest in fine-tuning fertilizer management. The goal is to match application source, rate, timing and method to meet the plant needs and achieve optimum levels of fertilizer use efficiency.
Maintaining adequate nutrition is among the most critical aspects of producing greenhouse crops. At present most growers utilize a liquid feed program as their primary means of supplying plant nutrients. This program may also be supplemented with granular or slow release fertilizers added to the growing medium. The frequency of fertilizer applications also influences plant growth. In some cases it is important to supply nutrients at peak periods of vegetative or reproductive growth. However, it is generally accepted that a constant feed (soluble fertilizer at each irrigation) is the best system to optimize plant growth. The balance of plant nutrients is important in producing vigorous, efficient plants. In some cases when nutrients are out of balance severe deficiencies or toxicities may occur. Therefore it is important to consider both the source and amount of fertilizer used.
Fertigation consists of applying simultaneously water and fertilizers through the irrigation system, supplying the nutrients required by the greenhouse crops. Using a fertigation system, a grower can apply the nutrients exactly and uniformly only to the wetted root volume, where the active roots are concentrated. This remarkably increases the efficiency in the application of the fertilizer, which allows reducing the amount of applied fertilizer. Nutrient characteristics such as solubility and mobility are important and irrigation water quality factors such as pH, mineral content, salinity, and nutrient solubility must be considered. The macronutrients nitrogen, potassium, phosphorus, and magnesium are the most common nutrients applied by fertigation, but micronutrients such as boron, zinc, iron, calcium manganese, and copper can also be applied through the irrigation system. In addition, to fertilizers other chemicals can be injected through the irrigation system, including chlorine, acid, herbicides, nematicides, and fungicides.
Plant growth regulators (PGRs) are chemicals that modify the natural hormonal activity that controls plant growth and development. Most of the PGRs used in the greenhouse function as “growth retardants.” These PGRs reduce plant height by inhibiting the production of gibberellins, which are the hormones responsible for cell elongation. Their effect is primarily on stem, petiole and peduncle elongation. Leaf expansion may be reduced, resulting in smaller, thicker leaves with darker green color leading to reduced water requirements due to lower transpiration rates. Achieving the best plant response to PGRs requires integrating both the art and the science of growing. PGRs are not a substitute for good crop management practices and accurate environmental control. PGRs are commonly applied as sprays.
Plant propagation is the process of creating new plants. There are two types of propagation: sexual and asexual. Sexual reproduction is the union of the pollen and egg, drawing from the genes of two parents to create a new, third individual. Sexual propagation involves the floral parts of a plant. Asexual” propagation techniques, or “cloning”, include reproduction through stem and leaf cuttings, division, grafting, budding and layering. Asexual propagation also includes plants that are reproduced through bulbs, corms, rhizomes, offsets, and runners. Propagation is often done in protected culture environments such as greenhouses.
The warm, humid conditions and abundant food in a greenhouse provide an excellent, stable environment for pest development. Often, the natural enemies that serve to keep pests under control outside are not present in the greenhouse. For these reasons, pest situations often develop in this indoor environment more rapidly and with greater severity than outdoors. Successful control of insect and mite pests on greenhouse crops depends on several tactics. These include regular scouting or monitoring for problems, identifying pests and their life stages, keeping good records of pest management practices, using exclusion techniques, practicing good sanitation, testing soil or plants for nutrients, using biological controls when possible, and using selective pesticides, properly timed and applied. Some greenhouse insects can transmit diseases to the crops which are often more serious than the feeding injury that the insect causes. These insect “vectors” include some aphids, leafhoppers, thrips and whiteflies. In these instances, the diseases must be managed through early insect control.
Diseases are a major concern for greenhouse growers and can be a key limitation to profitable crop production. Disease management in greenhouses is critical because of the limited air circulation encouraged by tight spacing and constant irrigation. Furthermore, high humidity provides optimal conditions for reproduction of many fungal and bacterial pathogens. When disease management is neglected, pathogen populations build up and continue to increase as long as there is susceptible plant tissue available for infection and disease development. Successful crop production requires that crop diseases be managed so that the effects of diseases on the plants are minimized. The management of crop diseases is directed at preventing the establishment of diseases and minimizing the development and spread of any diseases that become established in the crop. The presence of diseases in greenhouses is a fact of crop production and growers must use all available options and strategies to avoid serious pest and disease problems.
Greenhouse environments provide a variety of benefits for plant production; however, many greenhouses favor pest development as well. The warm, humid conditions and abundant food are ideal for pest build up. Natural enemies that serve to keep some pests under control in the field are absent in the greenhouse. For these reasons, pest problems often develop more rapidly and are more severe in enclosed systems, which require the use of pesticides. Pesticides include any substances used either to directly control pest populations or to prevent or reduce damage to the crops in the greenhouse. There are many pesticides available for use in greenhouses, all of which control specific pest and disease problems in growing crops. Although many pesticides are designed to kill pests, some may only inhibit their growth, or simply attract or repel them. The most commonly applied pesticides are herbicides (to kill weeds), fungicides (to control fungi), and insecticides (to kill insects). Furthermore, pesticides can be grouped in a number of different ways based on their active ingredients and how they work such as synthetic pesticides, organic pesticides, inorganic pesticides and biorational pesticides.
When developing an integrated pest management strategy, it is important to know the target pests, their economic threshold levels, and multiple control strategies that can minimize pesticide inputs in the greenhouse. However, in many cases, pesticides need to be applied as a last resort to reduce pest infestations and optimize crop production. The objective in applying pesticides in greenhouses is to deliver an effective, uniform dose to a target area in a safe and timely manner. Inaccurate pesticide application is expensive and can result in wasted pesticide, marginal pest control, compromised worker safety, and possibly excessive pesticide carryover contributing to water contamination and/or vine damage. Many types of application equipment are available to apply pesticides. Some types can be used in a wide range of situations whereas others are highly specialized and are only used for a few specific pesticides.