Your question implies that the main function of applying whitewash to a greenhouse cover is to prevent the transmission of UV (ultra violet) light into the greenhouse. This is not correct. The main function of whitewash is to reduce the heat load upon the crop in the greenhouse by reflecting a significant proportion of the incoming Infra Red (heat) portion of solar radiation. This is aimed at periods of high solar radiation, that is, around summer.
Sunlight comes in three different forms, dependent upon the wavelength of the radiation. These are:
1. Photosynthetically Active Radiation (PAR) is used by the plant for photosynthesis. It consists of about 45% of total solar radiation energy.
2. Infra-Red radiation (IR), which is heat energy and consists of about 55% of total solar radiation energy.
3. Ultra-Violet radiation (UV), which is a small proportion (under 1%) of solar radiation, but is not transmitted through most greenhouse coverings, other than low iron glass and some expensive specialised plastics.
The main reason for using low iron glass, which is more expensive, is to improve PAR light transmission. Typical standard greenhouse glass transmits about 88% light. Low iron glass transmits about 92%. Because 1% gain in PAR light typically gives a 1% increase in yield, this is a cost/benefit analysis exercise and many new glasshouses use low iron glass.
There are also claims that having UV transmitted into the glasshouse helps suppress diseases, especially fungal/mildew types. Proving this is a challenge, because of the number of influences involved, such as disease pressure, most of which are very difficult to quantify.
Staff safety is obviously an issue. In a normal greenhouse with no significant UV transmission, staff getting sunburn is not an issue. In a glasshouse with low iron glass, action needs to be taken to avoid sunburn.
High levels of Infra Red (heat) radiation
High solar radiation levels give high PAR light for photosynthesis and hence growth. However, it also brings in heat radiation, which is the main driver of transpiration. If the level of radiation is too high then plants can’t transpire and evaporate enough water through the stomata in the underside of their leaves to keep cool. In this case the leaf temperature can rise significantly. This has two major impacts:
1. direct tissue damage due to the high plant temperature, and
2. to protect the plant from drying out, the leaves will reduce the openings of their stomata to reduce water loss, giving a further temperature rise. Also, photosynthesis (growth) will be lower because of the reduced gas exchange of CO2 in and O2 out for the photosynthesis reaction. The high heat input also raises the temperature of the greenhouse air.
Reducing this heat load is critical so that plants are not stressed, because this will result in yield reduction, or even crop loss in extremes. Getting a crop through summer is a challenge, especially in hot climates like Australia, more than offsetting the benefits of higher levels of PAR.
Whitewash is applied to the outside of the greenhouse cover. This reflects some of the total radiation hence reducing what is transmitted into the greenhouse. In turn, this reduces the problems mentioned about by greatly reducing the thermal load on the plants. It also reduces the PAR input, but in sunny conditions PAR levels are quite high.
Apart from standard whitewash there are sophisticated products which transmit more of some wavelengths, especially PAR, while reflecting more of others, especially IR. There are also specialised products to make removal easier and more effective.
The advantage of an outside layer compared to the alternative of moveable internal screening is that it reduces the heat load actually entering the greenhouse. The difficulty is that it is either on or off and can’t be used for short term control, unlike moveable screens.
Applying and removing the whitewash can be awkward and can be a safety issue. Larger operations often use helicopters to do the job.
Whitewash also reduces PAR input in dull conditions, leading to yield reduction that day. So, guessing when to apply, and especially when to remove, whitewash can be a tricky management decision. I have visited a greenhouse when they were enduring a heatwave only two weeks after removing the protective whitewash.
For interest, the solar radiation leaving the sun contains about 5% UV and is unchanged when it reaches Earth’s outer atmosphere. However, when it reaches the ozone layer, which is located about 20 to 30 kilometres above the Earth in the stratosphere, over 80% of the UV is absorbed by the ozone. The UV absorbed is mainly the dangerous (to humans) high frequency/short wavelength end of the spectrum.
Methyl bromide was recognised as a powerful destructor of ozone, which could lead to an increase in the size of the hole in the ozone layer and hence an increase in dangerous UV levels. This led to the gradual world-wide cessation of the use of methyl bromide, especially as a soil sterilant (showing that international co-operation against a perceived environmental threat is possible).
Methyl bromide and commercial hydroponics
Methyl bromide actually played a part in the advancement of hydroponics on a commercial scale.
Holland has always been the leader in glasshouse technology and growing. Their main growing areas are mostly below sea level and have a high water table. In the late 1970s, the methyl bromide used for sterilising the glasshouse soil was detected in the ground water. Consequently, its use was progressively banned and growers returned to the previous sterilisation method of steaming. However, this was after the 1970s oil crisis and the price of oil to produce steam was much higher, and steam proved to be less effective than methyl bromide.
The end result was that many growers decided to get out of the soil and into hydroponics. By this time the use of rockwool had progressed and dripper fed rockwool slabs became the preferred technique. By the late 1980s, most Dutch glasshouse vegetable and cut flower crops were grown hydroponically, mostly in rockwool. Ω
PH&G February 2016 / Issue 164