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Horticulture 1000
HT1102 - Plant management 2 - Controlled environments
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8

Controlled environments

Various practices are used to change the environment inside a protective structure.

Temperature

Heating and cooling are both very important.

Heating

The heating system in a glasshouse needs to be able to raise the temperature to the required level and still be cost effective.

A heater in an orchid houseA heater in an orchid house.

Temperature can be increased in a number of ways:

  • glass or plastic traps heat inside greenhouses. Heat from the sun is trapped inside the structure
  • heating is used to raise the temperature to the required level. Electric fan heaters can be used. These are often connected to a thermostat, at a set temperature. When the temperature falls below this temperature the heating system is triggered to turn on
  • gas heaters can also be used. They have the extra benefit of giving off carbon dioxide, which increases the rate of photosynthesis
  • heated benches, heat pads or boards, or electric cables in the growing medium. They provide heat to plants from below and can be useful when propagating plants because it will help cuttings form roots more easily.
 
A hot bedA hot bed.

A hot bed is a frame with a heat supply at its base.

Cooling

In a protective structure the very warm conditions can raise the transpiration levels, which can increase the spread of fungal diseases. To avoid this, the inside of a protected environment is cooled.

Manual vent where the air entersManual vent where the air enters. A fanA fan. A solar screened greenhouseA solar screened greenhouse.

Here are some methods of cooling down the inside of a greenhouse:

  • vents: these can be opened on the side and top of greenhouses, or the doors at the front and back can be opened to create a draught
  • electric fans: these are controlled with thermostats, which switch on and off. Vents and fans will stop the humidity rising inside the house
  • hosing the floor down keeps the inside cool as the water evaporates. This is called damping  down
  • shade cloth can be put on the inside of the protected environment to provide low light or lower the temperature
  • whitewashing the outside of the structure with solar screen paint lowers the temperature.
 

Light

Fluorescent tubes will provide high intensity lightFluorescent tubes will provide high intensity light.

The amount of light that gets into the structure depends on the type of external covering that has been used. Artificial light increases the rate of photosynthesis and helps with vegetative and flower development.

Shade cloth and whitewash decrease light levels.

 

Watering systems

Where possible the grower should not let water get on the leaves of the crop as this can build up humidity around the leaves, increasing the chance of fungal diseases. Watering should be done in the morning. Any water that falls on the leaves, flowers or fruit will dry before the temperature drops in the evening.

Types of watering systems

The type of system to use will depend on the structure, crop being grown and the availability of labour. A system that is automated will be more costly but will have a low labour requirement.

Mist units
A mist unitA mist unit.

Mist units are used when propagating plants from vegetative material. They put a fine spray of water over the plant material as needed to keep it humid.

 
Trickle irrigation
Drippers used on orchidsDrippers used on orchids.

When each plant has its own container, trickle irrigation is used. Attached to the main water pipe are several tiny pipes called whiskers or drippers. Each whisker takes water to a container.

Advantages of trickle irrigation are:

  • only the roots are watered and the leaves don't get wet
  • less water is used
  • exactly the right nutrients can be added depending on the crop needs.
 
Capillary irrigation
Capillary irrigationCapillary irrigation.

A thick layer of felt-like material is put on the bench and water from a whisker trickles onto the fabric. The fabric gets saturated.

Plants sit on the fabric and soak up water when needed. Excess water will drain away and can be collected and recycled. This is useful for the growing of some houseplants.

 

Humidity

The humidity and temperature could be lowered in this begonia house by opening vents and using fansThe humidity and temperature could be lowered in this begonia house by opening vents and using fans.

The humidity is the amount of moisture in the air. In a controlled environment the humidity needs to be high enough so the rate of water loss from plants (transpiration) is not greater than the rate of water uptake.

Misting or damping down will raise the humidity. When the humidity gets too high there is a risk of fungal diseases and a build-up of condensation on glass or cladding. This can drip down and damage crops. All structures need to be kept clean.

 

Carbon dioxide levels

Increasing carbon dioxide levels will increase tomato plant growthIncreasing carbon dioxide levels will increase tomato plant growth.

Increasing the carbon dioxide levels above normal amounts can increase the level of photosynthesis and can increase plant growth. To increase levels of carbon dioxide the vents can be closed and then gas burnt which releases carbon dioxide. Gas heaters also put carbon dioxide in to the environment.

The amount of extra carbon dioxide to supply will depend on the crop type, stage of growth and the temperature and light intensity.

 

 

                                                   

Summary of conditions necessary to maintain healthy plant growth
Light
         
  • Light is needed for photosynthesis.
  •      
  • High light increases transpiration.
  •      
  • Shade houses can provide filtered light and this decreases transpiration.
  •    
Water
         
  • Water is needed for photosynthesis.
  •      
  • It is released during respiration and transpiration.
  •      
  • Extra water can be given to plants using irrigation systems.
  •    
Temperature
         
  • All plants need the right temperature to grow.
  •      
  • High temperatures increase the rate of transpiration.
  •      
  • Providing water can decrease temperature around plants.
  •      
  • Shade can decrease temperature.
  •      
  • Temperature around plants can be increased with the use of structures such as cloches or shelterbelts.
  •      
  • Some plants need a period of winter chilling to encourage bud burst.
  •    
Humidity
         
  • High humidity decreases the rate of transpiration.
  •      
  • Humidity can be increased with irrigation where necessary.
  •      
  • High winds and high temperatures can lower humidity and increase transpiration.
  •      
  • Fungal diseases prefer high humidity.
  •    
Oxygen
         
  • Oxygen is needed for respiration.
  •      
  • It is released during photosynthesis.
  •        
Carbon dioxide
         
  • Carbon dioxide is needed for photosynthesis.
  •      
  • It is released during respiration.
  •      
  • The amount of carbon dioxide around plants can be increased in a glasshouse situation using various methods of carbon-dioxide enrichment.
  •    

Complete the teacher marked Activity in your workbook

Key points   Key points

  • Within a structure most aspects of the environment can be changed to suit a particular crop.
  • Heating can be improved with suitable cladding and improved with a heating system suited to the crop and the structure.
  • Structures are cooled with the use of vents, fans, shadecloth and damping down.
  • Shade is supplied either with shadecloth or by painting the cladding.
  • Light is essential for photosynthesis. The amount of light received in a protected environment will depend on the type of cladding used and the availability of other light sources such as fluorescent lights.
  • The type of watering system chosen depends on the type of structure used, the crop grown and the availability of labour. Humidity is monitored and adjusted either with more water for lower humidity or by opening vents during times of high humidity.
  • Introducing fresh air into the controlled environment will give a new supply of carbon dioxide to a crop.
  • All structures need to be kept clean.
 

What's next?

  1. Revise irrigation and drainage. The information on these plant management practices can be found on HT1033 Soils, Irrigation/Mulches
  2. Complete the self-assessment form and cover sheet.
  3. Return your workbook and diary to your teacher.
  4. Continue with HT1103 Plant management 3.
Make a cloche