For this exam paper, you will need to be able to draw and describe a range of protected structures, together with an explanation of why they are used.
I’m not going to draw all the structures here – frankly, my attempts at drawing using software packages on a laptop are embarrassing. I believe that for the exam an outline drawing with labels showing any important points or features will be enough.
The uses of the structures could be:
- Greenhouse – used in plant propagation for cuttings and seedlings, also for protected growing of tomatoes and fruit. It is large enough to walk in to, and has glazed sides and roof. It may have beds within it for growing directly in the ground, or it may have staging for pots. Apart from the glass areas, it can have some brick walls from the ground to about a metre high, and bars holding the glass made of wood, aluminium or steel. You need to be able to describe/draw different greenhouse shapes, so some basic diagrams are shown. On the left is a Venlo style greenhouse, in the middle is a Widespan which is the same as a Venlo, but with fewer pillars inside. The structure on the right is a Mansard or curvilinear style, which has panels of glass at different angles to allow sunlight through at all times of the day.
- Polytunnel – this could be one which is big enough to walk in to and used to grow salad crops or tomatoes. A low polytunnel is a bit like a cloche and is used to protect small areas of beds. They have ribs made of metal and a white polythene sheet stretched over the ribs.
- Cold frame – this is used to harden off young plants. The lid can be propped open to differing amounts to regulate the amount of exposure the plants get. These are generally made of wood with a glazed section in the roof, but can also be made of brick with a wooden-framed lid.
- Cloche – a bell-shaped protective cover for an individual plant. It can be made of glass or clear plastic.
- Conservatory – generally used to display tender plants and houseplants. It is attached to the side of a house and has a glass roof. It may have totally glass walls, or can be brick up to about a metre from the ground.
Why grow indoors?
Growing plants under protection means that you have greater control over the environment the plants are in and can supplement things if needed.
For tomatoes (Solanum lycopersicum), the benefits and limitations of growing them in a protected environment (in the UK) are:
- Higher yields and higher quality
- Fruits for a longer period
- Can grow a larger range of different varieties
- It is more expensive than growing outdoors
- It requires more effort and you have to provide all of the water needed, which could be significant
- different pests and diseases build up indoors
Atmospheric controls
Temperature
It could be very cold in your protected structure, in which case you could provide heating for your plants. These could be electric heaters (efficient, but you will need a power supply), gas (but you need to ensure that the ventilation is good) or paraffin (cheap, but you cannot control the temperature). You could also have a hot water system, with the heated water circulating through pipes, but this would be expensive to set up and is not really an amateur level set up.
It could also be very hot in your protected structure in summer, in which case you could open windows to allow natural ventilation, or use fans to force the air to move out and draw fresh air in. You could also “damp down” the structure, which is where you splash water on the floor and it takes some heat out as it evaporates. You could also provide shade by using shading paint on the glass, or shading blinds.
The diagrams are supposed to show the ventilation/airflow in a greenhouse. The picture on the left has 2 vents in the roof, so the air comes in on the left, flows around the greenhouse and exits in the right vent – this is known as the wind effect. The picture on the right has vents at the bottom of the greenhouse and at the top. The air is drawn in at the bottom and exits the greenhouse at the top – this is known as the chimney effect.
Relative Humidity
At any given temperature, there is a maximum amount of water vapour which can be held in the air. The Relative Humidity is a measure of the amount of water vapour in the air as a percentage of the maximum it could be at that temperature. Warm air can hold more water vapour than cold air. Plants use the diffusion of water vapour at their stomata to draw up more water from the roots via the transpiration stream. If the Relative Humidity is high, water might not be able to be lost from the leaves to the surrounding air, meaning that the plant cannot take up more water and nutrients from the soil. You can try to improve this by improving ventilation or damping down.
Light
Good light levels are needed for growth, although different amounts are needed by different types of plant. You can provide supplementary lighting if there is not enough natural light, using metal halide bulbs. Alternatively, you may need to provide hours of darkness to induce some plants to flower, when you could use blinds. For example, Euphorbia pulcherrima (poinsettia) needs 13 to 16 hours of darkness in order to flower.
Atmospheric gases
Since Carbon Dioxide is needed for photosynthesis, a shortage could limit growth. On an amateur scale, this shouldn’t be a problem, but commercial growers can sometimes pump in Carbon Dioxide in to their protected structures.
Irrigation
Watering can be by hand using a watering can with a rose, which is good because you can see and decide how much water each plant receives. You could also have a capillary matting system, where water is drawn up from the capillary mat by the plants which are in pots resting on the mat. This system is useful since the plants take up the right amount of water and you can have lots of different plants all together, however, it can spread diseases really quickly. A drip system could also be used where a hose with holes in it drips water into the pots. This system is good, but does not allow much flexibility.
Site and orientation
You want to find a good site for your protected structure – it should not be in the shadow of a building or tree, nor between buildings which could cause a wind-tunnelling effect. You probably might not want it at the bottom of a slope as it could get really cold there. In terms of orientation, the longer side should be East-West to get the best light all day.
Biological and cultural controls
Protected environments can mean a build-up of pests and diseases. You can use a biological control to control pest, such as Encarsia formosa for Glasshouse whitefly (see R2103 for other examples). In terms of cultural control, you could disinfect all of your pots and fumigate the greenhouse using fungicidal smoke regularly.
Biological and cultural controls have the advantage over chemical controls as they are specific and there is no chance of the operator becoming injured. Also, some pests are becoming resistant to chemical control.
Gardening studies 