1. Protected Cropping Propagation Structures Reasons for protected cropping: 1. To provide more suitable growing conditions – It enables off season production – Enables production of crops not suited to the local environment (climate) 2. Greater returns of capital

2. Cropping structures 1.Greenhouses 2.Walk in tunnels (a form of greenhouses), with no temp control, fans i.e simple structure 3.Low tunnels 4.Shade houses Factors affecting choice of structures a) degree of climate control required b) climate c) finance d) crop type

3. Shade houses Shade Houses Shade houses are usually constructed over a frame of timber or aluminium and are covered with shade cloth.

4. Plastic greenhouse Tunnel Walk in tunnel Glass greenhouse

5. Components of a green house 1. The frame Refers to the skeletal structure The principal consideration in selecting the frame is its load-bearing requirement The cost & availability, durability & suitability will also be factors Examples :Stainless steel, galvanized steel, aluminium, treated wooden poles. 2. Cladding material This refers to the covering material Examples: glass, plastic, fibreglass etc

6. Components of a green house 3. Cooling & ventilation system Greenhouses need exhaust fans to exchange inside and outside air and to equalize temperatures within the house. This will depend on structure chosen 4. Heating system Use of heaters, steam pipes, lights 5. Floor. Growers may choose from a range of floor types from bare ground to concrete to plastic, to gravel depending on the intensity of use and availability of capital.

7. Components of a green house contd 6. Installation of power and utility sources Electrical, generators, cabling etc 7. Benches Although plants can be gown on the ground, accurate temperature control is difficult to achieve. Benches, therefore, are recommended. There are a variety of bench styles available e.g those with a wooden frame and a wire mesh base.

8. Greenhouses Factors affecting location of a greenhouse Topography & soil type Shade/light Accessibility: Access to the greenhouse should be convenient for both people and utilities Expandability Utilities/water: locations of sources of heat, water, and electricity; Markets

9. Greenhouse Shapes Even-span greenhouse: self supporting with distance from ridge to eave equal in distance Ridge & Furrow: two or more even-span greenhouses connected at the eaves. Quonset: arch-shaped bows placed in a row. Appears tube shaped Gothic: The gothic frame construction is similar to that of the Quonset but it has a gothic shape). Wooden arches may be used and joined at the ridge. The gothic shape allows more headroom at the sidewall than does the Quonset. Sawtooth: greenhouse with a vent near the eave of the lowest side wall that allows for natural ventilation. Lean to

10. Uneven span Lean to Even span

11. Saw tooth Ridge and furrow

12. 1.Glass Greenhouses The most expensive Only type prior to 1950 Can last more than100 years

13. Advantages of glass greenhouses Durability: glass lasts up to a 100+ years Glass withstands chemical spray unlike plastic High transmittance 97% Less condensation problems due to more panels and openings compared to plastic which is continuous Very stable to uv compared to plastic Easier to dispose without environmental detriment i.e recycling

14. Disadvantages of glass greenhouses Fragile Has high heating costs due to more openings- a lot of heat is lost through them Glass is heavy hence it needs a lot of supporting structures, thus more costs.

15. 2. Film plastic greenhouses Made of polythene, polyester, polyvinyl chloride & polyvinyl flouride as cladding material The skeletal structure can vary, e.g wooden poles, steel poles Advantages Material is cheap no use of sophisticated materials Low cost of heating Very flexible i.e several designs/types used due to mouldabilty Easy to repair

16. 2. Disadvantages of Film plastic greenhouses Short lived- less durable than glass, burns easily Prone to u.v damage High dust retention thus reducing transparency Pollute the ground on disposal Loses infrared more rapidly compared to glass

17. Type of film plastics 1.Polythene Most common type in Zimbabwe Has condensation problems in winter A poor barrier to radiant heat Can have infrared blocking chemicals 2. Vinyl (polyvinyl chloride) Tend to have static electrical charge Accumulates dust

18. Type of film plastics 3.Polyester (mylar brand) Good durability Costs more than polythene No electrical charge Light transmittance is equal to glass Has high capacity to block radiant energy 4. Polyvinyl flouride (PVF) Has a life expectancy of more than 10yrs Transmission of PVF is 92%

19. Rigid Panel Greenhouses Polyvinyl chloride (PVC) Has since been overtaken by polythene Fibreglass- reinforced Plastic (RFP) Costs more than plastic but less than glass Comparison with glass: More resistant to hail or vandals Surface damaged by dust, abrasion and pollution Light transmittance equivalent to glass Easier to cool than glass Fewer structural material required compared to glass

20. Greenhouse environment a) Air temperature Varies in a vertical direction Inequality in the horizontal plane maybe caused by the ventilation system, layout of the heating system and the use of thermal screens b) Root zone temperature It tends to lag behind the environmental temperature in the greenhouse in general c) Humidity Transpiring plants contribute to the humidity in the greenhouse. Can be artificially raised in various ways

21. Greenhouse environment contd d ) Carbon dioxide Outside conc = 340vpm Inside greenhouses concentration maybe lower due to crop uptake Enrichment is sometimes necessary e) Light (diffused light) Diffused lighting : the light scatters into the growing area so that it covers a wider field, although with less intensity. The incoming light breaks up into different directions, therefore less "complete dark spots" in the growing area. Diffuse light increases plant energy production because the light bends around corners to reach the lower leaves, not just the upper canopy.

22. The sun as a source of energy

24. The greenhouse effect A naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapour, and methane, are able to change the energy balance of the planet by absorbing longwave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18° Celsius, rather than the present 15° Celsius.

25. Energy balance in the greenhouse Ultimate source of energy is the sun Part of it is transmitted back/reflected Part is absorbed by gases, clouds Part of it enters the greenhouse Some is absorbed by the soil or reflected by soil depending on type Infrared radiation is blocked from going out by glass or plastic Has a longer wavelength and raises temp in the greenhouse (the greenhouse effect)

28. Energy loss in the greenhouse 1. Convection/conduction Relatively warm air rises to the relatively cool roof and leaves the greenhouse by conduction through the roof material 2. Radiation Heat radiation from relatively warm objects (heat pipes, soil, crops etc) to the colder cover where it is absorbed further to the relatively cold sky

29. Energy loss in the greenhouse 3.Transpiration/Condensation Applies to transpiring crop Heat is required for transpiration i.e warm water vapour on reaching colder parts of greenhouse causes condensation upwards or on any surface to release heat to compensate on the condensation

30. Control of crop growth by influencing greenhouse environment 1. Light Light used by crops: under spring conditions- (temperate region): 1% more light, 1% more product Optimum light per single leaf = 80w/m 2 For horticultural purposes, we are usually interested in measuring three properties of light: – Irradiance - the amount of total light energy falling on a surface – Spectral quality – the wavelength composition and relative intensity – Duration - length of the daily lighting period

31. Control of crop growth by influencing greenhouse environment Combinations of these three characteristics determine the amount of instantaneous and cumulative energy that is available for photosynthesis Shading at high irradiance: a movable reflection screen should be used. Young plants maybe shaded without growth reduction at lower irradiance than mature plants

32. 2. Temperature a)Artificially- raised through the following heat sources: i)Boilers or heaters ii)Steam or hot water piped to various location All these should be thermostatically controlled Localized heat system: warm forced air can be used b) Naturally: raised through the greenhouse effect

33. Cooling the greenhouse Convection cooling or passive ventilation Active or forced air ventilation Evaporative cooling- use of high pressure mist (requires good air circulation) Evaporative cooling pads consist of corrugated sheets of cellulose glued in an opposite sequence, which generates air passage inside the pads. These sheets have very high ability in absorbing water and are chemically treated to prevent disintegration. Used together with suction fans to reduce temperature Use of whitewash spray: reflects heat

34. 3.Humidity Raised by: Lowering the temperature Reducing ventilation Wetting plants/walk ways and other surfaces Use of wet cotton material placed on the floor Lowered by: Increasing temperature- use heaters, warm forced air Increased ventilation- open panels, windows

35. 4. Carbon dioxide General crop response The lower the concentration in the greenhouse, the greater the effect of extra For most crops, an increase from 100 to 200vpm will give 40% more growth At high irradiance & high ventilation rates, CO 2 enrichment maybe beneficial Control of concentration: factors to consider include cost of enrichment vs profits

36. Carbon dioxide & transpiration At low light intensity, high CO 2 may reduce transpiration On dull days a max of 500vpm CO 2 is recommended No extra growth expected from higher CO 2 concentration under these condition Very high concentrations harm crops (>3000vpm)

37. Uses of Shade houses Plant propagation & for plants that thrive in shade e.g: ferns, clivia, orchids etc. Production Container grown plants Seedling production Helping new plants establish themselves Keeping plants for sale

38. Uses of Shade houses Advantages Cheap to construct Reduce the adverse effects of temperature (both high & low) during the day & night Increased RH Decrease in wind velocity and heavy rains

39. Other growing structures 1. Hotbeds Large wooden box or frame with sloping, tight fitting lid made of window sash Should be placed in a sunny but well drained area Size 0.9 x 1.8m Bottom heat is usually provided Used in nursery production

40. Other growing structures contd 2. Cold frames Standard glass with a hot bed sash Size 0.9 x 1.8m Used in nursery production to condition rooted cuttings or young seedlings before planting Look up advantages & disadvantages

41. Hotbed Cold frame