Greenhouse Plant Covers
A great variety of supplies are available for adapting greenhouses to particular needs. There are a number of different options in regards to the category of plant covers for greenhouses. Generally speaking, this category can be divided into plant blankets, insect barriers, and shade net. Plant blankets are probably the most useful tool available for produce growers after greenhouses, hydroponic production, a source of heat, and automatic controls.
Plant blankets offer a flexible and efficient way of improving the environment within a greenhouse. These plant covers are designed to maintain the heat and radiation within a greenhouse during cold periods such as overnight or winter cold snaps. At the same time, they can shield against the radiation associated with heat build-up within a greenhouse during hot spells, such as the midday heat of a hot summer day.
This two-way capacity makes these blankets appropriate for a great variety of uses during the year. They can be moved around in order make adjustments and have optimum daytime, nighttime, or seasonal conditions. Plant blankets can reduce daytime temperatures by up to 10˚C (18˚ F) and maintain temperatures overnight by up to 5° C (9° F).
When crops heat up, plant blankets trap the hot air closest to the crop, reduce the volume of air space that will be heated, and reflect the radiation back to the crop. This can reduce heating costs significantly, possibly by as much as a half.
Under extreme and hot conditions, plant blankets are also valuable for reducing the radiation entering by day. This reduces the heat load in the greenhouse and helps to maintain a respectable amount of humidity around the plants, and to reduce water evaporation and plant stress.
Plant covers need to be carefully managed to assure that plants continue active growth.
Shade net, another type of plant cover, provides some of the same functions that blankets do. Under extreme and hot conditions, shading reduces the amount of radiation that penetrates into the greenhouse. This in turn reduces the heat load within the greenhouse and helps to maintain moisture around the plants and reduces stress for the crop. The use of shade net to maintain a fresher ambience in a greenhouse is most effective when the outside of the greenhouse is covered with the net.
When shade net is used, light colors should be used, since they reflect solar radiation uniformly and do not absorb as much heat as darker fabrics. A whole range of shade net is available providing anywhere from 30% to almost total shading.
Whitewashing is another option for shading that can be used to reduce the amount of radiation that enters a greenhouse. It can be a mixture of water and lime or diluted paint that is applied to the outside of the greenhouse. It reduces the amount of solar energy that enters the greenhouse and consequently lowers the temperature. The disadvantage of this type of sun screen is that it is applied seasonally, at the beginning of summer, for instance, and then cannot be easily changed to adjust conditions from day to day or within the season. This will be the cause of reduced light levels even when lower temperature is not needed. The final effect can be quite significant reductions in light levels and, consequently, in crop yield. There are whitewash products available which become transparent when they become wet to allow more light to pass through during times of rain. During periods of hot cloudy weather these products can be wetted down in order to increase the transmission of light.
Insect barriers (anti aphid mesh)
Insect barriers such as anti-aphid mesh are used to exclude flying pests and those carried by the wind from the greenhouse.
The main disadvantage of insect barriers is that they restrict air flow. This can have a significant impact on the ventilation capacity of a structure. The size of the opening in the fabric is the determining factor as to whether or not a particular pest is kept from entering. However, the finer the fabric, that is the smaller the opening, the less air flow there will be in the greenhouse.
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