What are the most important greenhouse properties?

I am considering upgrading to a more advanced greenhouse, either glass or plastic. I intend looking at buying and moving into an existing facility. I can’t afford the time or cost to build a new set-up. What do you believe are the most important aspects of the greenhouse properties to consider, apart from glass versus plastic? 

Answer by RICK DONNAN

Greenhouse height
Over recent decades the height of greenhouses has been steadily rising. This has been shown to improve climate control by giving a bigger buffer zone above the crop. I would recommend that you go to at least four metres to the gutter, or preferably higher. Modern new glasshouses are typically seven or more metres high.

Any greenhouse, especially the lower ones, will benefit from the installation of horizontal airflow (HAF) fans. This relatively cheap addition can significantly improve uniformity of the air conditions above and in the crop.

Solar radiation
All parts of Australia have high light levels in summer.

Sunlight comes in three different forms, dependent upon the wavelength of the radiation. These are:

• Photosynthetically Active Radiation (PAR) is used by the plant for photosynthesis. It consists of about 45% of total solar radiation energy

• Infra-Red radiation (IR), which is heat energy and consists of about 55% of total solar radiation energy

• Ultra-Violet radiation (UV), which is a small proportion of solar radiation, but is not transmitted through most greenhouse coverings, other than low iron glass and some expensive specialised plastics.

The influence of the amount of PAR light upon yield is approximately as follows:

1% loss of PAR light = 1% loss of yield

This particularly applies at low light levels.

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 the leaves can’t transpire and evaporate enough water to keep the plant cool. In this case the leaf temperature can rise significantly. This has two major impacts:

• direct tissue damage due to the high plant temperature, and

• 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.

Reducing this heat load is critical so that plants are not stressed, which will result in yield reduction, or even crop loss in extremes. Getting a crop through summer is a challenge here, more than offsetting the benefits of higher levels of PAR.

Reducing heat radiation
There are two major ways to reduce heat radiation onto the crop, namely moveable shade screens, and ‘white-wash’ type products.

White-wash is applied to the outside of the greenhouse cover. Apart from standard white-wash there are sophisticated products, which transmit more of some wavelengths while reflecting others. The advantage of an outside layer is that it reduces the heat load entering the greenhouse. The difficulty is that it is either ON or OFF and can’t be used for short term control. It also reduces PAR light input, even when solar radiation is low.

Moveable screens typically have narrow strips of a reflective surface, separated by open mesh to allow air movement. Their big advantage is that they can be closed to reduce heat radiation onto the plants when solar radiation is too high. They can then be moved out of the way when radiation levels are low. Although the plants are protected from high direct heat radiation, there is still a need to vent the hot air above the screen. Depending upon the design, they may take up extra space in the roof, giving some permanent transmission loss.

Heating
Southern areas of Australia can get cold conditions in winter, especially during the night, hence there is a need for heating. One method of reducing heat loss and hence the cost of heating is to have a ‘double skin’. However, the downside to this is that the second skin also doubles the light transmission loss and hence lowers yields.

What can be done instead of a second skin is to install a ‘thermal screen’. This is a moveable solid screen, which can be pulled closed during the night to retain the heated air around the plants overnight and so reduce heat loss.

The simplest form of heating is direct fired heating, which requires gas using well-maintained burners and monitoring equipment. Inefficient combustion can result in the production of some dangerous carbon monoxide (CO) as well as carbon dioxide (CO2). Burning gas also produces water, which could possibly lead to problems with condensation on the plants.

In advanced greenhouses, the typical system used is ‘hydronic’ heating. This uses hot water from the boiler, and dry heats the air by flowing the water through steel pipes just above the greenhouse floor. These pipes also double as rails, which allow the use of harvesting and scissor-lift trolleys for crop management such as layering and deleafing, etc.

Heat buffer storage tank
A useful addition that is environmentally friendly and can soon pay for itself is a heat buffer storage tank.

When fuel such as gas is burnt it produces CO2, which can be used to increase photosynthesis. However, if fuel is burnt to provide heating overnight, the CO2 can’t be used because there is no sunlight. With a buffer tank, the gas is burnt in the boiler during the day when the CO2 produced can be piped into the greenhouse to increase yield. The heat is captured as hot water, which is stored in the buffer tank to be used when needed at night.

Climate control
Making full use of the benefits to be gained from a sophisticated greenhouse cannot be done by manual control. A major requirement is to have a reasonably good climate controller to optimise conditions. There is a similar need for a reasonably good irrigation controller. This can be independent, but is usually linked to the climate controller. Ω

PH&G November 2015 / Issue 161


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