Controlling environment in protected structures Flashcards
Humidity and plant processes
Affects rate of transpiration (low RH can lead to wilt affecting photosynthesis)
Can also affect uptake of minerals (high RH results in reduced transpiration pull and therefore reduced uptake in minerals)
Susceptibility to P&D (High RH = fungal, Low RH = spider mite)
Increasing humidity decreases temp
Propagating plants require high RH so leaves do not lose water before roots form
Light and plant growth
Low levels result in etiolated growth, spindly plants and reduced flowering. can also make plants more susceptible to p & d
High light levels can cause scorching
Carbon dioxide and plant growth
Low levels can reduce growth
Irrigation and plant growth
Excessive - root rot, reduced uptake of minerals, oedema
Inadequate - wilt (effects light interception), reduced uptake of minerals,
Erratic - fruit splitting
Temperature and plant processes
Affects photosynthesis
Higher temperatures can reduce (humidity)
Optimum temp required for germination
Low air temp can result in weak growth
Also inhibit bud initiation and flowering - poor fruit setting
Factors affecting light levels
Orientation - placing the structure so that the ridge runs east to west will maximise light transmission due to the sun’s course. East to West is good to maximise light throughout the year. If only growing crops in summer, a North to South orientation will give an equal amount of sun to each side and help to reduce overheating on the hottest days.
Positioning within garden - no trees or high hedges should obstruct winter light
Shape of construction - angle of incidence - Mansard preferred to traditional
Condition of cladding - new, scratched, dirty, screened
Lighting - how to supplement
Supplementary lighting includes
High pressure sodium lamps - wide spectrum for growing and flowering
Metal halide lamps - high pressure mercury lam - expensive but good spectrum for vegetative growth
Temperature - heating systems
Every 5 degree increase in temp, trebles the cost of heating
Piped heating systems - gas, oil and solid fuels are burnt in a boiler and heat conveyed by hot water or steam through pipes - similar to modern central heating systems
Free standing heaters - natural or propane gas heaters can be thermostatically controlled. CO2 and water are by products giving rise to condensation at low temperatures. Bottled gas is more expensive than natural gas
Paraffin heaters - paraffin heaters can give off high levels of sulphur and CO, which is damaging to humans. High grade has low sulphur content
Electric tubular heaters - economical if thermostatically controlled and used for maintaining frost-free conditions
Bench heating - can be heated with soil warming cables (usually laid under sand). Thermostat must be fitted.
Placing of thermometers and use
Half way along the length of the house sheltered from direct sunlight and away from a heat source or draughts (1m above plants)
Heat conservation
Insulation - plastic bubble wrap attached to sides of the glashouse
Thermal screens - sheet of metallic material drawn across the crop at night reducing loss of heat closest to crop
Cooling by shading
Shading paints - commonly plastic pain in early summer
Blinds and shading fitted to the outside (reduces temps more compare to fitting on inside)
Blinds and shading hung inside - should be light colour to reflect light back of of the glass before it is converted to heat.
Dark blinds and shading inside the structure will not keep temp down, but are used to eliminate glare of sunlight (eg around propagating plants)
Cooling by ventilation - also reducing humidity
Cooling by damping down - also increasing humidity
Optimum - area covered by ventilators should equal 1/6 of floor area.
Cold air enters through doors and side vents and hot air rises and exits through the ridge vents - creates air currents.
Forced ventilation
Fans draw air ensuring efficient gas exchange, lowers humidity
Damping down surfaces in hot dry conditions will reduce air temperatures by leading to an increase in the rate of evaporation
Temperature and relative humidity
Hot air can hold more water vapour than cold air. As air cools water vapour that cannot be held within the colder air forms water droplets.
As temperature rises, the amount of water vapour within the air mass compared with the potential volume that can be held becomes less and relative humidity falls
Controlling humidity
Lowering: space out plants
Passive ventilation
Forced ventilation
Increase temp
Dehumidifier
Raising: crowd plants
Closing vents
cover containers
damp down
fog/mist
Turn heater off
Irrigation methods
Watering can
hose
drip hoses - relies on delivery of water as a drip to the base of the base of the plant. Water passes through a series of pipes which become smaller as they get nearer to the plant. May have a timer
hydroponics
sprinkler systems
automated versions
capillary matting/sand linked to a header tank or topped up by hand
