RHS Level 2 Certificate

Week 12 – Understanding the control of the environment in protected structures

Learning objectives4.1 Describe the factors that affect light levels in protected

structures, including shape of structure; site factors; orientation; type and condition of cladding materials.

4.2 Describe how the temperature can be maintained in structures, including heating by gas, oil or electricity; heat distribution using circulating water and air; cooling by forced or natural ventilation; evaporation; and shading.

4.3 Describe methods of changing the relative humidity (RH) in a protected environment, including the effects of ‘damping down’, ventilation and temperature changes.

4.4 Describe manual and automated methods of irrigation, including the use of watering cans, hose pipes, capillary systems and ‘drip’ systems

4.5 Describe how light levels can be manipulated, by the use of supplementary lighting and shading, including blinds and shading paints.

Factors affecting light levels

Site – needs an open site with no overhanging trees or shading buildings

Orientation – north/south gives even light distribution; east/west best for winter light

Glazing materials – compromise between light transmission and cost and insulation values.

Shape of building Use of shading or supplementary lighting

Light levels – shape of building

Light will pass through a transparent material if it hits the surface at 90°. Otherwise it will be reflected at a corresponding angle to the angle of incidence.

No material however is perfectly transparent so some light will always be reflected from impurities etc within the material.

The angle of the light changes with the season and the latitude.

So the angle of the roof and sides of the greenhouse can make a radical difference to the amount of light that passes through the glazing.

Light Levels – shape of building

With a traditional straight sided greenhouse with a roof at about 30° light from a low sun (winter and spring) passes through the sides but is mainly reflected from the roof.

Dutch Light greenhouses have sloping slides which increase the light transmission but have the same issue with the roof

Light levels – shape of building

With this Mansard greenhouse each pane of glass is at a slightly different angle. Whatever the time of year some part of the glass is at 90° to the light. This is very expensive to build. Poly-tunnels have a similar response to light.

Controlling light – blinds and shading

Reducing light intensity within the greenhouse will protect plants from scorching and may reduce temperature. Black out shading can also be used to create artificial ‘night’.

Materials used include blinds (external or internal), shading paint and shade netting.

External shading is most effective at reducing temperature.

Factors to be considered include cost, life span, work involved and ease of adjustment.

Controlling light – supplementary lighting

The light source chosen must include PAR Commonly used are High Pressure Sodium

and compact fluorescent lights. HPS generates heat and is more expensive

to run but gives more light than compact fluorescent lights.

Artificial lighting can be used to add to existing day light in winter and/or to extend the ‘day’ to mimic light conditions at other times of year.

Controlling temperature - heating

Possible sources of heat – electricity, paraffin or gas or use stored heat from the sun.

Electricity is cheaper than gas or paraffin and more controllable but expensive to install.

Gas and paraffin are less controllable and produce condensation but are cheaper than electricity.

Solar thermal can buffer temperatures in a cool greenhouse but a very elaborate set up would be needed for any greater level of heating.

Insulation is vital to avoid wasting heat.

Controlling temperature - cooling

Use of external shading – blinds or shade netting most effective, shading paint has some effect

Evaporation of water – ‘damping down’. Energy used to turn the liquid to gas is taken from the heat in the air or in the greenhouse floor etc.

Ventilation – natural or forced. Hot air moves upwards and draws cool after it. Top and bottom vents must be open for natural ventilation.

Water - irrigation

Methods – by hand using a can or hose; by capillary watering systems; by drip or spray watering systems.

Aim is to maintain water content of the growing media at the ideal level.

Drip or spray systems can be automated and easily adjusted to conditions.

Capillary systems can look after themselves while you go away for a few days.

Hand watering allows precise control and plant monitoring.

Water – Relative Humidity

A measure of the amount of water vapour in the air relative to the maximum it could hold; this varies according to the temperature.

Ideal levels vary according to the plant and its maturity.

Relative Humidity can be increased by misting or damping down and reduced by ventilation.

Question sheet answers

1. Shade the outside of the greenhouse, damp down in the morning or early afternoon, open the side and top ventilators in the morning and close at night.

2. Blinds allow easy adjustment on cloudy days and provide good reduction of temperature on hot days.

3. It removes the need for daily watering and increases humidity in hot weather

4. Benefits: precise amounts applied, plants are checked daily. Limitations: hard work in large areas and needs someone to do it.

5. Because light is reflected back through the glass by the inside surface of the water droplets.

Learning outcomes

4.1 Describe the factors that affect light levels in protected structures, including shape of structure; site factors; orientation; type and condition of cladding materials.

4.2 Describe how the temperature can be maintained in structures, including heating by gas, oil or electricity; heat distribution using circulating water and air; cooling by forced or natural ventilation; evaporation; and shading.

4.3 Describe methods of changing the relative humidity (RH) in a protected environment, including the effects of ‘damping down’, ventilation and temperature changes.

4.4 Describe manual and automated methods of irrigation, including the use of watering cans, hose pipes, capillary systems and ‘drip’ systems

4.5 Describe how light levels can be manipulated, by the use of supplementary lighting and shading, including blinds and shading paints.