THE GLASSHOUSE ENVIRONMENT

Plants in the glasshouse are totally reliant on the skills of the grower. Everything has to be provided under an artificial environment – water, nutrients, warmth, light and air.

WATERING/IRRIGATION SYSTEMS

Water quality is important because hard water can cause difficulties to the grower who relies on overhead irrigation systems as the higher the levels of calcium and magnesium salts in the water the greater will be the spotting of the foliage as the water dries, and build-up of calcium deposits will regularly block the system. Calcium salts will lock up the trace elements in the water supply and cause deficiency symptoms. The pH of the water supply should ideally be in the range of 5.5 – 6.5.

There are three basic types of irrigation with many variations within those types.

1. Overhead irrigation systems are flexible allowing plant arrangement and spacing to be altered easily without affecting the performance of watering equipment. These type of layouts are generally quite inexpensive to install, but the disadvantage is that a large proportion of the water that is used is lost, depending on plant spacing. A great advantage is that such systems are ideal for crops that require high humidity levels.

2. Surface irrigation systems are often used to water specimen plants or individual pots on a bench. The types of equipment used are often nozzles, drip systems and trickle lines, in each case water is directed close to the plant so there is minimal wastage. The one major disadvantage of individual watering systems is the risk of blockages, which when they occur means that certain plants receive no water at all. This would never happen with an overhead system.

3. Sub-irrigation systems are usually installed on to carefully graded and sloped floors or permanent benches. Water is applied from below so that the foliage does not become wet and every pot on a bench receives adequate water without becoming waterlogged. Such systems operate on capillary action. Sub-irrigation systems use either sand or capillary matting which is sold in large rolls specially for this use. There is a variation to this method called the flooded bench which works on intermittently flooding the whole bench with water to a depth of two or three centimetres for a short period before being fully drained.

Glasshouse watering based on this system is often very successful and has few of the problems associated with the other systems.

Types of glasshouse shading

1. Automatically controlled shading screens supported over the crop internally.

2. Roller blinds fitted externally.

3. Special paint which can be sprayed over the glass.

Foliage pot plants should be grown under some form of shading in the summer when they are approaching saleable or marketable size. Good light levels are needed when the plant is smaller, but sudden change to the poorer light levels in shops and houses after sale can have devastating effects. The use of some shading prevents fading of leaves and acclimatises the plants for the lower light levels to come.

Of all the methods of shading used the woven sheets supported on wires are the most versatile as these can be linked to motorised systems and drawn over or back according to the changing weather conditions. This flexibility is not possible when any of the proprietary paints have been used, which on a sunny day work well, but in dull weather there can be resultant low light levels.

When a sudden hot sunshine problem arises, the plant has to produce vastly more water to help the leaves stay cool. If the roots are not active enough to do this, the leaf (and plant) may overheat, wilt and/or become scorched. Some leaf edges and leaf tips may scorch even without obvious wilting. Usually such plants are very close to the glass.

Temperature integrating iar

This is an inexpensive and easily made piece of equipment for checking temperatures in the heated glasshouse. It is used to measure the true mean night temperature. The response area of the thermometer is increased, the variations and continual fluctuations during the night period are overcome.

Air releases its moisture which condenses on the cooling unit from which it drips into a removable container.

These are quite cheap to run and could easily remove a gallon or two from the air overnight from a glasshouse of 100m2. They have great advantages for some growers of crops like geraniums for example in that during the bleak mid winter plants may be kept frost free and in very dry air – suffering little even if growing not at all. If they were as cold as in the normal high humidity of dank winter days then botrytis and rust would be much more likely to occur.

Humidity control

(1) De-humidifier cooling.

(2) Evaporative cooling.

Cooling the glasshouse can be achieved by damping down the crop and then relying on air movement to cause cooling by evaporation. This method is different from shading in the sense that the temperature being directly reduced is achieved without loss of light intensity. Evaporative cooling is often used on crops which benefit from maximum light intensity. When light levels are high, these crops generally require a high atmospheric humidity to maintain active growth as in the case of cucumbers, many foliage pot plants and bromeliales. Spraylines can be used for this function and often forms part of a misting system.

Shading

Small glasshouses need shading in order to reduce the scorch which can quickly happen from around early February when early damage may occur to the tender leaves in winter through to about September. Scorch occurs when the leaf heats up. Leaves normally stay cool because

Where heating and ventilation systems are set up to work automatically, each are controlled by a thermostat which is positioned in an ASPIRATED SCREEN which in turn is placed in the glasshouse. An aspirated screen is an insulated metal cylinder which encloses and protects the thermostat, and ensures that a constant stream of air is forced across the thermostat through the cylinder so that an accurate temperature reading can be obtained. The cylinder also protects the thermostat from SOLAR RADIATION which can seriously affect the performance of the thermostat.

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