Light is the source of energy for plants. Light energy combined with carbon dioxide and water is required for the process of photosynthesis. Therefore, it is important to ensure the maximum light intensity possible is provided to achieve optimum plant growth, particularly during lower light times of the year. The design of the growing structure and the orientation as well as natural light levels and shading determine the light intensity. Other factors effecting light include the frame itself, if timber is used for the frame it must be painted white to reflect the light, the covering material, glass transmits up to 89% of light, polyethylene 84% and fiberglass starts high but quickly diminishes as the material hazes from the UV rays. The covering material accumulates dust and grime, which reduce the light intensity by up to 20%. Therefore it is essential, that after the hottest part of the summer, the glass is cleaned. Plant density, growing system – multi leveled or single level and training method also determine the amount of light available for each plant and should be taken into consideration for each season. High light levels can be just as damaging as low radiation levels and often some form of shading is needed in summer for even high light crops, depending on the local climate.
Greenhouses usually require cooling during the summer months. Most locations experience temperatures that are detrimental to plants during summer. Temperatures inside the greenhouse are often 7 - 15C higher than outside. Adverse effects on plants from excessive heat include reduction of flower size, flower and fruitlet drop, lack of fruit set, lack of pollen viability, delays in flowering, smaller fruit size, loss in yield, poor product quality, reduced fruit shelf life and firmness and a number of other disorders.
Most plants prefer the air temperature between 15–24°C, but always check the optimum range for the crop being grown as this is essential for maximum yield and growth rates. Temperature optimums range from 16 C for lettuce to 28 C for melons. Most crops need a cooler night temperature for good flowering and fruit set, but the day/night difference varies between crops and manipulation of this can be used to compensate for other growth factors such as low light.
Summer cooling requires large volumes of air be brought into the greenhouse and pass through the entire plant zone. One complete air change per minute is the recommended rate for greenhouse cooling. Cooling will be dependant on outside temperatures and in tropical climates; other forms of cooling may be required to reduce temperatures to acceptable levels.
Root zone Temperature
Just as in soil, hydroponic growing substrates typically run temperature conditions a few degrees lower than that of the air. However, hydroponic crops are grown in limited volumes of media which has the ability to warm faster than soil and this should be taken into consideration in warm climates where root overheating can cause growth problems. In solution culture systems such as NFT and Aeroponics the nutrient can over heat rapidly during the day and may need some form of cooling. Conversely in winter, solution or root zone heating with a warmed nutrient solution can boost growth, particularly when air temperatures are being run on the cool side. Cool season crops such as lettuce benefit from solution warming in winter, but in tropical climates, solution cooling can mean economic crops of lettuce can be produced at air temperatures much higher than ideal. Root zone temperatures should always be monitored and adjusted for hydroponic crop production.
Relative Humidity and Vapour pressure deficit
The amount of moisture held in the air of the growing environment has major effects on many aspects of crop development and disease prevention. Relative humidity (RH) levels are a measure of the amount of moisture currently held in the greenhouse air and are influenced by factors such as the amount of water vapour given off by the plants during transpiration, any misting, fogging or damping down used for temperature control as well as ambient humidity levels in the air brought into the growing area. High RH slows the rate of water loss via transpiration from the plants, thus slowing the transport of water and calcium from the roots to developing plant parts. High humidity also increases the risk of fungal and bacterial pathogen infection, particularly where condensation may wet foliage. Low RH increases the rate of transpiration and foliage desiccation can occur, plants may also wilt during the warmest part of the day.
Greenhouse sensors often measure both RH and the Vapour pressure deficient (VPD) which is more meaningful from a plant growth perspective than RH. VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when saturated. VPD is better than just looking at RH as it takes into account the effect of temperature on the water holding capacity of the air, rather than giving just a relative measure of the water content of the air. VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation. Higher VPD values means that the air has a higher capacity to hold more water and this stimulates plant transpiration from the leaves. Lower VPD means the air is at or near saturation, so the air cannot take more moisture from the leaf in this high humidity condition. Higher VPD increases the transpiration demand and assists with prevention of conditions such as blossom end rot, fruit cracking and tip burn. VPD in crops such as tomatoes are best run below 0.062 psi (0.43 kPa) for good plant growth, however disease infection is most damaging below 0.030 psi, so greenhouse climates are best adjusted to keep above 0.030 to prevent disease outbreaks.
When the air in the greenhouse becomes fully saturated with moisture this is called the `dew point’ or `saturation vapour pressure’ and this is directly related to temperature. When the dew point is reached, free water forms on the plants and greenhouse structures in the form of condensation which needs to be avoided as it is a major disease risk. At saturation VPD, plants stop transpiring and physiological disorders start occur. The size of the vapour pressure deficit tells a grower how close to saturation and condensation the greenhouse environment is.
Controlling greenhouse humidity
Controlling the RH or VPD inside a greenhouse is relatively simple – moist air needs to be regularly vented out as the transpiration of a mature crop will bring RH levels up rapidly. Drier air can be rapidly brought in from outside to lower RH levels on a continual basis. If the outside air is very humid, lowering RH inside the greenhouse can be more difficult. Low humidity levels can be rectified with use of techniques such as damping down of the greenhouse floor, evaporation pans or pads (usually installed in front of air inlets which also have a cooling effect on the air), or automatic misting or fogging.
While misting or fogging systems are commonly used for plant propagation from cuttings and other material, in hydroponic greenhouses they are used to control humidity automatically usually via a number of sensors positioned in the crop canopy and greenhouse which feed data back to a central computer programme. Misting or fogging is then automatically controlled as required to increase humidity levels to within the optimum range. Most hydroponic greenhouses use misting or fogging primarily for temperature reduction during summer, rather than to increase humidity, which in a large, healthy crop tends to run on the high, rather than low side.
CO2 and O2
Carbon dioxide (CO2) and oxygen (O2) are vital gaseous nutrients required for crop growth and deficiencies in either of these will result in yield reductions. CO2 in the atmosphere is around 360ppm, however enrichment up to 1000 – 1200ppm in the greenhouse will give yield increases and decrease the time to harvest of many crops. CO2 enrichment of the greenhouse may be used to simply replace the CO2 taken up by the plants, thus preventing CO2 deficiencies (enriching up to ambient levels inside a relatively closed greenhouse) or boosted to levels of over 1000ppm depending on factors such as the requirement for venting, where high CO2 levels can be lost and the cost of enrichment. CO2 deficiency is common in winter greenhouses, usually in the few hours after dawn, when the vents remain closed to retain heat, but the crop, begins active photosynthesis, thus rapidly lowing the CO2 contained in the limited greenhouse environment. Growers should be aware of the need to draw in fresh CO2 supplies, even if this means a loss in greenhouse heat.
Oxygen is required for the process of respiration and while plant leaves have access to more than sufficient O2, plant roots may become deficient in many growing situations. While hydroponics does offer the opportunity for better oxygenation of the root system than soil based systems, root suffocation due to a lack of O2 is common in many densely grown hydroponic crops. Oxygen is only slightly soluble in water or nutrient solution with maximum rates of only 12 – 13 ppm of O2 held at around 10 C, this can be rapidly taken up by an active root system to the point were suffocation and root cell death starts to occur. Hydroponic growing media needs to be porus to allow oxygen penetration and methods used to enrich nutrient solutions with as much dissolved oxygen as possible.
Water is another vital input into all hydroponic systems and its quality must be assessed before use. Water for hydroponic production needs to be low mineral, relatively low sodium, free of disease pathogen spores and organic matter and any other contaminates which may affect plant growth. Many water supplies, both natural and town water supplies are not suitable for hydroponics. Well water may contain disease pathogens and unacceptable levels of certain minerals such as trace elements and sodium. Town water supplies are treated with chemicals to bring this up to drinking standard (such as chlorine), many of which may not be suitable for crop production. Water analysis should be carried out on all potential water supplies for hydroponics and the results assessed accurately for use in hydroponics (as this differs from the standard drinking water interpretation many labs perform). Sodium is the most common problem in water sources as is present in just about all water sources. Sodium sensitive plants such as lettuce should not be grown with a water source that contains any more than 40 ppm sodium, whereas more salt tolerant crops can be produced with higher sodium levels.
In hydroponic production, all plant nutrients are supplied via the nutrient solution and should not be applied in solid form in the growing media. Selecting media with a low CEC (Cation exchange capacity) is important for hydroponics where nutrition is supplied directly and regularly via nutrient ions dissolved in water. This allows very accurate and concise control over plant nutrition which is one of the major benefits of hydroponic production. Formulation of hydroponic nutrient solutions can be a complex process, however it is one growers must come to terms with as relying on premixed nutrients can lead to nutrient imbalances and yield reductions. Each crop in each different growing situation will have specific nutrient requirements, making customised nutrient formations which a regularly monitored and adjusted, important for commercial production. Hydroponic plants need relatively large amounts of the macro elements, N, P, K, Ca, S, Mg and smaller amounts of the trace elements Fe, Mn, Zn, Cu, B and Mo. Many crops benefit from the addition of `beneficial nutrients’ such as Ni, Si, and Se which have been shown to boost growth in many plant species. Plant nutrients are derived from high soluble fertiliser salts of which `greenhouse grade’ or `analytical grade for trace elements’ should be selected. Commonly used salts include calcium nitrate, potassium nitrate, monopotassium phosphate, magnesium sulphate, iron chelate, zinc sulphate, manganese sulphate, boric acid, copper sulphate and sodium molybdate, although others exist which may be used in smaller quantities.
Control of the Environment
There is now available on the market computer controlled equipment to manage the greenhouse environment. The computers are capable of delivering a 15 to 25% saving in costs and reduce labour considerably. They are able to control temperature, humidity, light intensity, CO2 injection application of black cloth shade, light reduction as needed, ventilation fans and irrigation. Some of the advantages are:
Heating and Ventilation Systems
- Computer controlled environments can control the temperature to within one-tenth of a degree where manual control is at best within 2-3 degrees. They also do the job gently, which puts less load on the equipment rather than the abrupt changes from manual operation.
- It works 24 hours a day, 7 days a week. It will deliver the most cost-efficient control of your heating/ventilating system every minute of the day and night.
- Most computer systems have inbuilt alarms which notifies the grower of factors such as electricity or pump failures, over heating or other problems which could damage the crop.
Heat must be supplied at the same rate with which it is lost in order to maintain the desired temperature. Heat is lost by conduction, infiltration and radiation. Heat is conducted through the covering material in conduction loss. Infiltration is heat lost as the warm air escapes through cracks in the structure while radiation loss is where heat is radiated from the warm interior through the covering to colder objects outside. The majority of heat loss in greenhouse structures is through the vents which must be opened periodically to lower humidity and bring in fresh supplies of CO2 for photosynthesis
Solar heating systems have become popular as the price decreases and the efficiency increases.
There are tables that accurately calculate the heat requirements for greenhouses.
Ventilation is required to remove used air and maintain air circulation. This constant circulation reduces the likelihood of a fungal disease outbreak. Cool air is introduced by evaporative cooling, where fresh air is cooled and pumped into the greenhouse and the hot air is sucked to one end and dispelled. Computer controlled systems feed in optimum amounts of heat or ventilation if appropriate. Good systems try to avoid using heat – striving for the most cost efficient way to control that rise in humidity. If vents and an air-circulating fan will do it, they will not use heat. However, the grower has the optimal control as he can adjust the computer to respond to his preference.
Other methods of reducing the light intensity and heat build up include thermal screens, which are a dark colour sleeve that fits onto the roof panels or over the outside of the greenhouse structure. They reduce the amount of radiation light entering the greenhouse. A good computer environmental control system will extend the thermal sheets anytime during the day when the light exceeds a preset level set by the grower.
Some species, such as many ornamental and cut flower crops benefit from short periods of complete darkness or blackout. These blackout periods produce shorter, more compact plants that develop uniform flowering. The older the plant the more readily it will produce flowers. The number of flowers and flower buds on Begonias will be at their peak when day length is reduced to 10 hours and periods of blackout are implemented.
Shading includes the use of roll down blinds, either wooden or plastic slats. If the blinds are fitted on the outside the temperature inside is reduced, however, usually the ventilators cannot be opened. If the blinds are fitted on the inside the internal temperature is not reduced, but the light intensity is. Blinds can be automated to open and close on preset temperatures.
Another method is double sheets suspended from the internal roof trusses length ways. These mechanically controlled systems roll the double layers into very small diameter rolls that minimise the loss of light. The upper layer is a white polyester fabric that reduces light by 45% while the lower polyethylene level permits 90% of the light to pass to the crop.
Another method that is not as practical is to apply a shading paint or a product called Lightening Crystals, which can be sprayed on the roof and which is removable.