Lesson 4 - understand the control of the environment in protected structures

Question 1

List the factors that affect light levels in protected structures

Answer 1

Shape of the structure
Site factors
Orientation
Type and condition of cladding material

Question 2

Describe how the shape of the structure affects light levels

Answer 2

Angle of incidence - if it is less the 40 deg, most of the light is transmitted through the glass, if it is more, mos the light is reflected causing light loss.
Dutch light greenhouses are more effective at receiving light in winter months than standard greenhouse.

Question 3

Describe how site factors affect light levels

Answer 3

Exposed/sheltered
Latitude

Question 4

Describe how orientation affects light levels

Answer 4

In autumn to spring, most light comes from a southerly direction.
Preferred orientation for a single span greenhouse is eat to west (door in eastern or western end)
Commercial venlo greenhouses should run North to South.

Question 5

Describe how the type and condition of cladding materials affect light levels

Answer 5

New, scratched, dirty, screened/painted with shading compound all affect how much light transmits. Glass allows the most light through. Condensation reduces light transmission. Old polythene and polycarbonate plastic discolours as it ages and reduces light transmission.

Question 6

Describe how the temperature can be maintained in structures

Answer 6

Heating by gas, oil or electricity
Heat distribution using circulating water and air
Cooling by forced or natural ventilation
Heating and Ventilating systems can be triggered by thermostats.
Evaporation - use of damping down in summer to reduce temperature.
Shading
Shield thermometers from direct sunlight to get an accurate reading.

Question 7

How cost effective are different types of heaters for greenhouses?

Answer 7

Electrical heaters operate at close to 100% efficiency but the cost of power is high at the moment. However it is effective for frost protection for seedling in spring.
Parafin heaters are cheaper to fuel, but need refuelling often. They also create water when burning which is not good for the plants in winter.
Commercial greenhouses use hot water or steam pipes to heat the air.
Soil warming mats and cables are a cheap way of heating small spaces.

Question 8

How can you change the relative humidity in a protected environment

Answer 8

Damping down
Ventilation
Temperature changes

Question 9

What is meant by damping down

Answer 9

Wetting the surfaces of the heated structure with cold water.

Question 10

Describe patterns of ventilation in a structure which will help adjust the relative humidity

Answer 10

The use of a forced draught or fan ventilation and natural ventilation systems

Question 11

Describe how change in temperature affect relative humidity which in turn affect water and nutrient uptake

Answer 11

Most plants have a preferred relative humidity level. The higher the temperature, the higher the relative humidity can be. The higher the humidity, the slower the transpiration rate. If the relative humidity is too high or low, transpiration and nutrient uptake will stop completely.

Question 12

Describe manual and automated methods of irrigation

Answer 12

Watering cans
Hose pipes
Capillary Bench - deep bench lined with heavy plastic sheeting and then filled with 5-8cm of washed sand. The sand is kept moist through the use of a tank of water that continually resupplies the water. Pots placed on top must have holes at the bottom and that the compost in the pot comes into contact with the sand.
Capillary Mat - Synthetic material that is placed over a heavy plastic sheet. The moist mat is covered with thin black polythene sheeting which has fine perforations. Mat is kept continually moist.
Drip systems - a series of pipes and finer tubes and nozzles supply water to the surface of each container/ They can be turned on and off or run constantly on a trickle feed.

Question 13

What are the benefits and limitations of overhead watering

Answer 13

Water on the leaves can cause fungal infections. Easy to overwater the plants. However, compost can dry out completely within waterings.

Question 14

What are the benefits and limitations of watering from below

Answer 14

Foliage remains dry, reducing the risk of fungal diseases. The risk of overwatering is greatly reduced. However, the plants need monitoring to check they are not drying out and are receiving the correct amount of water.

Question 15

What is the most effective way of watering container grown crops

Answer 15

Overhead irrigation systems are often used by commercial greenhouses as they are flexible and allow plant arrangement and spacing to be altered easily without affecting performance of the watering equiptment. However, a lot of water can be lost.

Question 16

What is the most effective way of watering crops grown in the border soil

Answer 16

A soak hose is effective.

Question 17

Describe how light levels can be manipulated by the use of supplementary lighting and shading

Answer 17

The use of removable blinds and white shading paints to avoid overheating in protected environments.
Supplementary lighting to extend the growing season
The use of artificial light/dark periods to prepare poinsettia crops

Question 18

State what is meant by cultural controls for pests and diseases

Answer 18

Modifying the environment to reduce pests and diseases. This includes having good garden hygiene, cleaning the greenhouse at least once a year, cleaning pots and trays between uses, isolate new plants, monitor plants for pests and diseases.

Question 19

State what is meant by biological controls for pests and diseases

Answer 19

The introduction of a natural predator to the greenhouse to combat the pest.

Question 20

Describe the use of cultural controls to limit 2 pests or diseases

Answer 20

Cleaning the greenhouse, in particular the frame, to reduce the red spire mite. Cleaning pots and trays before use with disinfectant to prevent damping off.

Question 21

Give 2 examples go how biological controls are used to limit 2 pests of diseases (latin names for biological controls are not required)

Answer 21

Predatory mite can control the glasshouse red spider mite. Ladybirds can be introduced to control aphids.

Question 22

State the advantages. that cultural and biological control have over chemical control methods

Answer 22

Chemical controls may harm beneficial insects. Chemical controls expose the user to possible hazardous chemicals. Chemical controls often are difficult to dispose of in an environmentally friendly way. Spillage of chemical controls, or them getting into water systems, is an environmental hazard.

Question 23

what is passive ventilation

Answer 23

Opening vents to allow air to move

Question 24

what is active ventilation

Answer 24

Use of exhaust or extraction fan to move air across the house. Vents are placed in the sides of the greenhouse structure and into one is placed an extraction fan that pulls air out of the greenhouse, while a vent opposite to this fan is opened to permit air to enter the greenhouse.

Question 25

How can you maintain the temperature of a heated structure cheaply?

Answer 25

Insulate the coldest wall of the greenhouse, generally the north wall. Bubble polythene is good for this. Horticultural fleece can be placed over plants at night time.
Reduce air leakage by repairing any breaks in the cladding and checking door and window fittings.
Create shelter for the greenhouse, such as a windbreak on the north end of the greenhouse. Paraweb or living plants an be used.