2. Learning objectives
2.2 State the benefits and limitations of using protected structures
for growing plants, (for example tomato, Lycopersicon
esculentum) compared with growing the same plants outdoors.
2.3 Describe the effect of the environmental factors listed in 2.1 on
plants in a protected environment.
3. Know the structural and cladding materials used for a range
of protected structures.
3.1 List and describe the characteristics of a range of materials
used for framework construction, including steel, aluminium,
wood, and plastics. State the benefits and limitations of EACH.
3.2 Describe the properties of different cladding materials which
can be used for structures, including glass; polyethylene film;
polycarbonate; acrylic sheets; shade netting; and horticultural
fleece. State the benefits and limitations of EACH.
3. Thanet Earth video –
answers to worksheet
The floor is covered in white plastic to maximise
light reflection inside the house which speeds
ripening.
In winter heat is provided and supplementary
lighting. The ventilators are not opened so carbon
dioxide is also supplemented.
The lower leaves are removed so ripening fruit is
not shaded
Nutrients are provided in the irrigation water (which
runs out of the rockwool blocks and is recycled).
The carbon dioxide is a by-product of the heating
plant
Pests are controlled by predatory insects.
4. Protected Growing -
tomatoes
Tomatoes – Lycopersicon esculentum
Natives of South America. Tender
perennials – but grown as annuals.
Raised from seed under glass – but
plants ready to plant can be
purchased at four sets of leaves
stage.
Cordon or vine types are grown
under glass and bush types outdoors.
5. Tomatoes in the greenhouse
Planted out into growbags or 20l containers.
Irrigation systems assist with regular even
watering
Supports must be provided for cordons
Side shoots are removed as they develop.
Tap flower trusses to encourage pollination.
Remove lower leaves to prevent shading of
fruit
Growing tip stopped once five trusses form.
6. Care of greenhouse
tomatoes
Water – must be constant and evenly
supplied.
Nutrients – supplied as liquid feed high in
potash.
Light – shade in high summer or fruit may
scorch. Otherwise clean glass and white
plastic beneath containers.
Temperature – avoid cold and extreme heat.
Control by ventilation, shading and damping
down.
7. Care of greenhouse
tomatoes 2
Ventilation and humidity – humidity encourages
fruit set and efficient use of water. Damping
down cools the greenhouse and increases
humidity.
However humid conditions (particularly if the air
is cool) encourages botrytis – so ventilation is
crucial.
Balancing act between controlling temperature
and maintaining humidity.
Carbon dioxide levels are also restored by
ventilation so open vents in sunny conditions.
8. Pests and diseases -
greenhouse
Whitefly – resistant to most pesticides.
Controlled by good hygiene, fatty acid
sprays and predatory insects.
Greenfly – control as for whitefly.
Blight – protected growing protects
from spores (good idea to close the
ventilators on warm humid nights).
TMV – buy virus free plants and don’t
smoke in the greenhouse.
9. Tomatoes - outside
Bush varieties best suited, no need to pinch
out side shoots – small varieties can be
grown in cloches. Protect fruit from contact
with soil.
Watering needs attention and feeding with
high potash feed is required.
Planted out in late May or June when the
nights start to warm up.
Very susceptible to blight – protective
spraying with copper based fungicide.
10. Comparison of outdoor with
greenhouse tomatoes
Season Shorter for outdoor tomatoes
Light Natural levels available – but cannot be
supplemented
Temperature Grow under cloches or use fleece at night –
but control is limited
Water Rain is available – but irrigation needed
Humidity Natural levels – maintain by windbreaks
Carbon dioxide Availability is not an issue
Pests etc Very vulnerable to blight.
11. Advantages and limitations
Indoor - advantages Indoor - limitations
Longer growing season Close control of humidity
needed
Protection from blight Expense of greenhouse
Protected from cold All irrigation must be provided
Outdoor - advantages Outdoor - limitations
Bush types – no need to pinch
out side shoots
Vulnerable to cold – cloches
etc only limited control
Less watering needed Badly affected by blight
Natural levels of nutrients Shorter growing season
12. Construction materials –
basic considerations
Thermal efficiency – important if structure is
used in early spring or winter.
Strength – strong frame materials mean thin
glazing bars and better light transmission
Maintenance – wood needs most, aluminium
or plastic least.
Life span
Cost – cedar is most expensive, galvanised
steel next.
Light transmission
13. Construction materials -
frames
Wood – soft wood or cedar. Requires regular
maintenance, cedar is expensive. Thick
glazing bars but good thermal efficiency.
Aluminium alloy – little maintenance. Strong
so thin glazing bars. Poor thermal efficiency.
Plastic. Short life span. Low maintenance.
Steel – usually galvanised, if not short life
span. Thin glazing bars but low thermal
efficiency and very heavy.
14. Glazing materials
Glass – horticultural or safety. Safety glass
is heavier and more expensive. Good light
transmission (95-97%) but poor thermal
efficiency.
Polycarbonate – insulates but has less light
transmission (85%). Double wall is more
expensive than ordinary glass. Shorter life
than glass.
Polyethylene sheeting – light transmission
89%, some insulation value, short lifespan.
15. Learning outcomes
2.2 State the benefits and limitations of using protected structures
for growing plants, (for example tomato, Lycopersicon
esculentum) compared with growing the same plants outdoors.
2.3 Describe the effect of the environmental factors listed in 2.1 on
plants in a protected environment.
3. Know the structural and cladding materials used for a range
of protected structures.
3.1 List and describe the characteristics of a range of materials
used for framework construction, including steel, aluminium,
wood, and plastics. State the benefits and limitations of EACH.
3.2 Describe the properties of different cladding materials which
can be used for structures, including glass; polyethylene film;
polycarbonate; acrylic sheets; shade netting; and horticultural
fleece. State the benefits and limitations of EACH.
