Greenhouse Structures

GREENHOUSES

Structures covered with transparent material

Allows sunlight to enter for plant growth and maintenance

Artificially heated and cooled

Uses

Overwintering tender plants Starting seeds Starting cuttings Raising vegetables and flowers out

of season Growing specialty plants

Designed to local weather extremes

Minimum yearly temp. Max. wind speeds Max. snow loads Best light transmission

Environmental considerations for plants

Light Temperature Humidity

Headhouse

Office, storage, and work space from which greenhouses are located

Types of Greenhouses

Attached Freestanding

Attached Greenhouse

Connected to building Floral shop Garden center Office Home

Attached Greenhouses Advantages:

Less construction material needed Can be utilized to heat homes in

winter, cool in summer through heat and humidity change

Disadvantages Existing building can shade the

greenhouse, limiting light needed for plants

Ventilation and temperature are difficult to control.

Lean-To

Attached to building, usually on the south-facing side

Ridge to the roof is attached to the building side

Attached Even-Span

End wall is attached to a building Rafters are equal length

Attached Even-Span

Advantage More available space

DisadvantageMore expensive to build and heat

than a lean-To

Window mounted

Attached to windows as a pre-fabricated unit.

Disadvantage Limited space Plant environment

is difficult to control

Freestanding Greenhouse

Even Span Uneven- span Quonset Gothic Arch

Even-Span

Rafters of equal length

Usually clears spans w/ truss supports

ADVANTAGE: Allows more freedom of space/ movement and equipment

Uneven-Span

Rafters of unequal length Usually used for hillside placement

ADVANTAGES: maximizes sun light DISAVANTAGES: internal orientation of

greenhouse can make working in it difficult

Quonset

Curved roofs; military hut-style May or may not include sidewalls

Gothic Arch

Cathedral arch-forward style

Eliminates truss supports

Adds a double layer of polyethylene covering

Gothic Arch

Advantages Less construction material Less land needed for spacing Heat conservative due to less exposure Workers and equipment can move

through the buildings rather than between buildings

Gothic Arch

Disadvantages Creating different environmental

conditions for crops is more difficult Connected houses do not distribute

snowfall weight well and can collapse Require heating cables to melt snow to

prevent collapse Difficult to ventilate, requires fans to

circulate

Types of Connected Greenhouses

Gutter-connected – made up of even span greenhouses

Venlo or Dutch – twice as many ridges as gutter

connected Barrel Vault – quonset style with walls joined

Sawtooth – lean to style joined together

http://www.b2science.org/

Review

What is a greenhouse?

What are the four types of freestanding green houses?

Review

Three environmental considerations for Plants?

Objectives for the day

What are the structural components of a greenhouse?

What are five different ways of heating a greenhouse?

What are three ways of cooling a greenhouse?

What is a forcing structure?

Components of Greenhouse Structure

Frame Foundation Concrete Footings Sidewalls Covering

Frame

Pressure-treated wood

Aluminum Galvanized steel NOTE: glass

greenhouses require more structural support than polyethylene

Foundation

Continuous concrete bed upon which greenhouse is built

Water and electrical lines are placed before concrete is poured

Concrete Footings

Non-continuous supports located below the frost line (cannot be affected by frost heaves)

Take the place of foundations.

Spaced at intervals according to placement of support anchor posts

Sidewalls

Between the footings and supports

Greenhouse Coverings Glass Provides excellent light

transmission. Long-lasting. Double-strength, grade B weight

used for greenhouses except in cold areas, where thermopane (with a ¼ to ½ inch layer of trapped air between panes) is used.

Expensive installation and heating. Requires good structural support

of aluminum or steel. Sash bars Frame glass panes; held by glazing

compound. Bar caps Attached to outside of panes to

hold glass in place.

Greenhouse Coverings

Polycarbonate Lightweight Ease of installation. Good heat insulation. Good light transmission. Available in clear,

corrugated single layer panels.

Good replacement for corrugated fiberglass-reinforced plastic.

Disadvantage: Flammable

Greenhouse Coverings

Corrugated Fiberglass-Reinforced Plastic

Low in cost. Ease of installation. Tensile strength. Longer lifespan. Good light penetration. Can degrade over time

from ultraviolet damage, dust, and pollutants.

Flammable and may increase insurance costs.

Greenhouse Coverings

Polyethylene Inexpensive installation. Low heating cost. Short-lived. UV inhibitors increase its

lifespan. Produces condensation

which increases wet plant foliage and disease possibilities.

Light reduction compared to glass.

Greenhouse Coverings

Acrylic Lightweight Easy to install Good heat

insulation. Good light

transmission. Long lifespan. Flammable.

HEATING

• Steam-produced in a boiler-Circulated by pipes in sidewalls and overhead-Used mostly when several greenhouses are attached-this steam can also be used to sterilize media

Heating (cont.)

• Hot water-similar to steam but used in smaller houses

• Forced-Air-Local Units force hot air into houses-Can be moved through plastic overhead tubing

Heating

• Infrared Radiant-Individual unit heaters-Directly warms the plants in the greenhouse-Conserves energy but is generally cooler than other types of heat.

Heating Con’t

Solar Restricted to passive designs Crops require little additional heat

Unit Heaters

Fired by natural gas, propane, fuel oil, coal, or wood.

Electricity not recommended, due to reliability.

Determining size HVAC

Climate Area of exposed greenhouse surface Type of construction and materials = # of BTU’s need maintain inside

temp.

Cooling

Fan and Pad

Exhaust fan draws air through moist cellulose pad.

Air is cooled through evaporation

For low humidity conditions

Fog evaporative

Fog is generated within the greenhouse Heat is absorbed by the water droplets

and evaporated, cooling the greenhouse environment.

Used in propagation houses

Ventilation

Ventilation

Natural Fan Tube

Natural

Air exchange is fostered through open ridge and side venting, under thermostatic control.

Not advised for winter cooling due to possible plant injury

Fan-Tube

Fans bring in cool air which is mixed with warm air and distributed though holes in plastic tubing running the length of the greenhouse

May be used in conjunction with heating systems

Shading

Fabric Compound

Fabric

Black polyethylene pulled over the greenhouse to reduce light intensity and temperature.

Comes in a range of shade percentages.

Compound

Liquid compounds applied to glass or polyethylene covered greenhouses to reduce light intensity and temperature

Forcing Structures Used in Raising Plants

Lath House/ Shade House

Provides some shade and wind protection for plants

Allows moisture and air to permeate structure

Used as an outdoor setting for plants with some protection

Cont’d

Much smaller than a greenhouse For plant establishment,

acclimatization, or plants requiring shade

Weatherproofed wood or galvanized steel

Frame is three sided: roof, back, two sides

Con’t

Covering Wooden roof is slatted to permit

weather and light, but not full sunlight Plastic net fabric is commonly used, in

a range of shade percentages

Overwintering Structures

Cold Frame

Hotbed

Cold Frame

Unheated Structures Wind protection and

moderated temp. control Placed partially below

ground level Frame is wood W/ hinged

or sliding glass top opening or covering of plastic film

Backs are higher and tops slanted down for weather protection

Insulation can be used on tops of plants or containers

Hotbed

Similar to a cold frame in structure, but with and underlying heat source

Used to start seedlings or cutlings

Heat from pipe (steam or hot water), heat coils (electricity), or natural sources are used (manure)

Hotbeds have brick or concrete sides and back with a glass top slide or hinged covering

Review

What are the structural components of a greenhouse?

What are five different ways of heating a greenhouse?

What are three ways of cooling a greenhouse?

What is a forcing structure?