Control of environment Flashcards
1
Q
Factors that affect light levels in a protected structure
A
Light transmission into greenhouse affected by:
- shape of structure
- thickness of glazing bars
- orientation of structure
- cladding material and its condition
- site factors
2
Q
How does the shape of a protected structure affect light levels?
A
- The proportion of light passing through glass is dependent on the angle between the glass and the light rays
- Most light will pass into a structure where the light strikes the glass at an angle of 90 degrees
- Angle of incidence is angle between ray incident on a surface and the line at right angles to the surface at that point (normal)
- greenhouse with sloped sides is more effective at receiving rays of light during winter months when angle of sun is close to horizon e.g. Mansard greenhouse
- High eaves will present the maximum amount of glass to the winter sun at the best angles for good light transmission e.g. mansard structure.
- A structure with stronger construction materials e.g. aluminium and steel is preferable, as less material required/narrower profiles, so less light blocked
3
Q
How does the orientation of a protected structure affect light levels?
A
- Orientation refers to position of the protective structure in relation to the compass points (N, E, S, W).
- Amount of natural light entering the structure is determined by the amount of glass presented to the sun. This maximises light
entry into the growing structure. - Preferred orientation for single-span greenhouse is East/West i.e. ridge line runs east to west.
- Presents the maximum amount of glass to the sun and maximises natural light entry in the winter months
- Fewer structural members causing shading within greenhouse
- Commercial Venlo structures with multiple spans best orientated north to south to reduce amount of shading needed.
4
Q
How does the cladding material and its condition of a protected structure affect light levels?
A
- Different cladding materials have different efficiencies of light transmission
- Glass 90% with no reduction as it ages
- Polythene film 85-90% when new but reduces with age
- Polycarbonate 80%
- Dirt or condensation on the glass will reduce the amount of light transmitted
5
Q
How does the site of a protected structure affect light levels?
A
- Not shaded by trees or existing buildings.
- Shelter from wind which will drive down temperatures within greenhouse
6
Q
Ways to maximise light transmission in a new greenhouse
A
- Shape - include high eaves and an angled roof
- Site - avoid shade and exposed locations
- Orientation - east to west to maximise light transmission
- Construction - strong steel/aluminium frame to allow low structure to glass ratio
- Keep cladding materials clean to maximise light transmission
7
Q
Methods of controlling temperature within a protected environment
A
- Heating (gas, oil, electricity)
- Heat distribution using circulating water and air
- Ventilation - passive / natural or active /forced (using fans)
- Damping down (evaporation)
- Shading
8
Q
Methods of heating a greenhouse
A
- Methods of heating - paraffin, oil, bottled or natural gas, electricity
- Electricity - need available electricity supply to site, expensive but less so than other sources, provides dry heat that will not raise relative humidity, easily controlled automatically by thermostat
- Paraffin - portable, expensive (more than electricity), no electric supply needed, gives of carbon dioxide (enrichment). Also produces water which raises RH which may lead to Botrytis, no precision control of temperature
- Gas - mains gas supply may not be available necessitating bottle gas. If combustion is poor may lead to production of toxic gases being released e.g. nitrogen dioxide. Moisture given off during combustion which will raise relative humidity.
- Large commercial greenhouses use hot water or steam in pipes: burn oil or gas outside greenhouse and heat conveyed to greenhouse by hot water conveyed through pipes and cooling water is returned to the boiler to be reheated. Can bet controlled by a thermostat and can be distributed in small bore pipes which take up minimal space.
- Reduce heating bills - improve insulation e.g. bubble polythene on coldest wall and one not used for light transmission (usually north wall), reduce external wind speed, maximise natural light transmission, subdivide protected structure and only heat one section of it, improve monitoring and accuracy control of heating system.
9
Q
Methods of cooling a greenhouse
A
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Forced draught/fan ventilation:
- Cool air enters glasshouse through either a large fan or ventilator at one side of
the glasshouse. Hot humid air is drawn across the glasshouse and expelled by force through an extractor fan on the other side. - This results in positive air movement across a crop, for efficient gaseous exchange, lowering RH and provides optimum ventilation
(reduces fungal diseases).
- Cool air enters glasshouse through either a large fan or ventilator at one side of
-
Natural ventilation:
- Cool, less humid air enters a glasshouse at low level through side ventilators, and as hot air rises within the glasshouse, leaves through the ridge ventilators/ roof vents
-
Damping down:
- Wet pathways, water evaporates reducing air temperature
-
Shading:
- Shade paint, blinds or shade netting on outside of structure.
- Lowers the amount of light entering greenhouse, lowering temperature increase
10
Q
Methods of controlling relative humidity within a protected environment
A
-
Increasing RH:
- Damping down - spraying water and benching in protected structure with water to assist in cooling process. As water evaporates it has cooling effect, removing the latent heat and lowering the temperature, increasing evaporation rate, raises humidity. This should be done in morning.
- Overhead irrigation, misting or fog systems, lowering the air temperature
-
Decreasing RH:
- Increasing air temperature e.g. through heating
-
Increasing ventilation lowers RH and reduces fungal diseases
- natural ventilation (vents) - cool air enters the glasshouse at low level via louvers, ventilators or an open door and leaves the structure through the roof ventilators after being heated by the radiation in the glasshouse.
- forced draught / fan ventilation - position extractor fans either side of glasshouse. Extractor fans used to draw air across the greenhouse, lowering the temperature. Cool air enters the glasshouse via louvers or ventilators in the side of the glasshouse and the hot humid air is forced out by the use of extractor fans situated on the other side of the structure. Positive air movement across crop for efficient gaseous exchange.
- Dehumidifier (commercial use)
11
Q
Relative humidity
A
-
Relative humidity (RH), often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.
- Warmer air is capable of holding more water vapour than cooler air → as temperature increases RH decreases unless additional moisture is added to the air
- RH has a direct impact on the rate at which plants transpire. Generally, higher humidity will slow down transpiration and lower humidity will speed it up. But if relative humidity is too low, or too high, nutrient transport will slow down or stop completely.
- If evaporation from plants were to stop, the plant would stop transporting nutrient-rich water, and the roots wouldn’t take any more in. This would stop plant development, leading to slower growth, or no growth at all.
- In order for plants to transpire, the air surrounding them must be able to take in more water vapor. If the air has already reached the dew point, as in 100% saturation, there is no where for the water vapor to go. In this case, water will simply cease to evaporate, halting the xylem pull and stopping nutrient transport in the plant.
- See https://drygair.com/blog/how-humidity-control-improves-nutrient-transport-in-plants/#:~:text=Relative%20humidity%20%E2%80%93%20humidity%20has%20a,slow%20down%20or%20stop%20completely.
12
Q
Methods of controlling irrigation within a protected environment
A
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Manual watering
- watering can: with fine rose, used to direct water below or above leaf canopy. Can use mains water or water butt water, but must be hygienic. Benefit: can use for a foliar feed. Can adjust water droplet size delivery - rose facing upwards for fine spray, downwards for larger droplet size. Inexpensive to purchase, readily available and portable.
- hose with lance
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Automatic
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Capillary systems: Usually for container grown plants. Plant takes up how much water it needs, from below by capillary action,
determined by how wet or dry the root ball is. System also raises humidity around plants (microclimate) as bench remains wet.- Capillary bench is deep bench lined with plastic and filled with 5-8cm sand which is kept constantly moist. Compost must be in contact with sand.
- Capillary matt is synthetic material which is kept constantly wet placed over heavy plastic sheet. Containers sit on top of mat, but must have wick to ensure contact between compost and mat
- Benefits: can cope with a range of plants in different sized pots. Foliage remains dry reducing risk of fungal problems, risk of over-watering greatly reduced.
- ‘drip’ systems: water is delivered to the plant via small bore tubes whether it is growing in the border soil or container. Drip targeted to base of stem, so plant is able to use all the water provided. Minimum water lost to evaporation. All plants have same amount of water at same time. Can easily be semi-automated, and suitable for plant batches at same development stage.
- Soaker hoses used for border grown plants
- Overhead irrigation systems for commercial greenhouses. Ideal for crops that require high humidity levels but large proportion of water lost.
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Capillary systems: Usually for container grown plants. Plant takes up how much water it needs, from below by capillary action,
13
Q
Methods of reducing light levels within a protected environment
A
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Shading - application of shading to the exterior of the protective structure during the summer months. Shading can also be erected within the protective structure in the form of shading materials, to cut out the damaging effect of direct sunlight e.g. leaf scorch and overheating.
- Shade paint: applied to outside of glasshouse during early Spring as sun becomes stronger. Plastic based and sprayed or painted on with a brush. Wiped or washed off in autumn.
- Shade netting: suspended across the eaves base of the structure in early Spring, to avoid leaf scorch. Easily removed in Autumn when light levels reduced.
- Shade blinds: easy to open or close as needed. May be installed outside of greenhouse (exposed to elements and may become soiled or damaged) or inside greenhouse (less effective preventing heat build-up but protected from most damage).
14
Q
Methods of increasing light levels within a protected environment
A
-
Supplementary lighting:
- Additional artificial lighting used to supplement natural daylight within the structure.
- Artificial lights suspended above the growing crop, and activated according to the day length and natural light intensity.
- A range of lights can be used e.g. high pressure sodium lamps.
- Used to extend growing season by extending the growing day during the winter months when day length is shortened, as well as additional lighting during daylight hours when there’s insufficient light to achieve optimum growth.
- An early main season tomato crop may require supplementary lighting between potting and planting early in the season when natural light is poor.
15
Q
Use of artificial light/dark periods to prepare poinsettia crops.
A
- Poinsettias (Euphorbia pulcherrima) flowering and bract colouring initiated by short winter day length
- need total darkness, for 14 hours each day, starting about eight weeks before they are needed for display.
- During the day, the plants need bright light, along with the other routine care.
- The bracts will start to turn color in about four weeks
- Need a humid environment during this time
