Botany - (ebook) - Planning a Home Greenhouse

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COOPERATIVE EXTENSION SERVICE UNIVERSITY OF MARYLAND AT COLLEGE PARK

UNIVERSITY OF MARYLAND EASTERN SHORE

perature can fluctuate greatly; close supervision would

be required if a manual ventilation system were in use.

Therefore, unless close monitoring is possible, both

hobbyists and commercial operators should have auto-

mated systems with thermostats or other sensors.

Thermostats can be used to control individual units,

or a central controller with one temperature sensor can

be used. In either case, the sensor or sensors should be

shaded from the sun, located about plant height away

from the sidewalls, and have constant airflow over

them. An aspirated box is suggested; the box houses

each sensor and has a small fan that moves greenhouse

air through the box and over the sensor (Figure 5). The

box should be painted white so it will reflect solar heat

and allow accurate readings of the air temperature.

Watering Systems

A water supply is essential. Hand watering is accept-

able for most greenhouse crops if someone is available

when the task needs to be done; however, many hobby-

ists work away from home during the day. A variety of

automatic watering systems is available to help to do

the task over short periods of time. Bear in mind, the

small greenhouse is likely to have a variety of plant

materials, containers, and soil mixes that need different

amounts of water.

Time clocks or mechanical evaporation sensors can

be used to control automatic watering systems. Mist

sprays can be used to create humidity or to moisten

seedlings. Watering kits can be obtained to water plants

in flats, benches, or pots.

CO2and Light

Carbon dioxide (CO

) and light are essential for

plant growth. As the sun rises in the morning to pro-

vide light, the plants begin to produce food energy

(photosynthesis). The level of CO

drops in the green-

house as it is used by the plants. Ventilation replenishes

the CO

in the greenhouse. Because CO

and light

complement each other, electric lighting combined with

injection are used to increase yields of vegetable

and flowering crops. Bottled CO

, dry ice, and com-

bustion of sulfur-free fuels can be used as CO

sources.

Commercial greenhouses use such methods.

Alternative Growing Structures

A greenhouse is not always needed for growing

plants. Plants can be germinated in one’s home in a

warm place under fluorescent lamps. The lamps must

be close together and not far above the plants.

A cold frame or hotbed can be used outdoors to

continue the growth of young seedlings until the weath-

er allows planting in a garden. A hotbed is similar to the

cold frame, but it has a source of heat to maintain prop-

er temperatures.

For More Information

For more information on environmental control sys-

tems, refer to Extension Bulletin 351 Greenhouse

Heating, Circulation, and Ventilation Systems. For

further discussion of hotbeds and cold frames, see Fact

Sheet 633 Hotbeds and Cold Frames for Starting Annual

Plants, also available from your county Cooperative

Extension Service office.

What should a gardener consider when planning to

build a small hobby greenhouse? What materials should

be used to build it? Does it need heating and cooling?

Where can it be placed on the property? There are

many considerations, and careful planning is important

before a project is started.

Building a home greenhouse does not need to be

expensive or timeconsuming. It can be small and sim-

ple, with a minimum investment in materials and

equipment, or it can be a fully equipped, fancy, auto-

mated conservatory. The final choice of the type of

greenhouse will depend on the growing space desired,

home architecture, available sites, and costs. The green-

house must, however, provide the proper environment

for growing plants.

Regardless of the size and type of greenhouse you

choose, consider how much time you have to manage

the system. Do not be too ambitious; some new green-

house owners find they do not have as much time as

they thought. On the other hand, it is a misconception

that greenhouses require constant attention. The envi-

ronment can be maintained with minimal upkeep using

automatic controls, which operate the heating, ventila-

tion, watering, humidity, and artificial lighting systems

when no one is home. A hobbyist should invest in

automatic controls and start with easy-to-care-for plants.

Sometimes the hobby grows into a business, so give

some thought to the possibility of expanding your

greenhouse in the future.

Constructing the greenhouse yourself can make the

project more enjoyable and less expensive if you are

handy with tools. Prefabricated greenhouses can be pur-

chased, or they can be built of simple frames. However,

only qualified electricians and plumbers should install

the automatic systems.

Location

The greenhouse should be located where it gets max-

imum sunlight. The first choice of location is the south

or southeast side of a building or shade trees. Sunlight

all day is best, but morning sunlight on the east side is

sufficient for plants. An east side location captures the

most November to February sunlight. The next best

sites are southwest and west of major structures, where

plants receive sunlight later in the day. North of major

structures is the least desirable location and is good only

for plants that require little light. Morning sunlight is

most desirable because it allows the plant’s food produc-

tion process to begin early; thus, growth is maximized.

Deciduous trees, such as maple and oak, can effec-

tively shade the greenhouse from the intense late after-

noon summer sun; however, they should not shade the

greenhouse in the morning. Deciduous trees also allow

maximum exposure to the winter sun because they shed

their leaves in the fall. Evergreen trees that have foliage

year round should not be located where they will shade

the greenhouse because they will block the less intense

winter sun. You should aim to maximize winter sun

exposure, particularly if the greenhouse is used all year.

Remember that the sun is lower in the southern sky in

winter causing long shadows to be cast by buildings and

evergreen trees (Figure 1).

Good drainage is another requirement for the site.

When necessary, build the greenhouse above the sur-

Planning a Home Greenhouse

Fact Sheet 645

David S. Ross

Extension agricultural engineer

Department of Agricultural Engineering

P94/R96

Figure 5. Thermostats in the middle of the greenhouse

in a shaded, white, and aspirated box

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30,1914, in cooperation with the U.S. Department of Agriculture, University of Maryland at College Park,

and local governments. Thomas A. Fretz, Director of Cooperative Extension Service, University of Maryland at College Park.

The University of Maryland is equal opportunity. The University’s policies, programs, and activities are in conformance with pertinent Federal and State laws and regulations on nondis-

crimination regarding race, color, religion, age, national origin, sex, and disability. Inquiries regarding compliance with Title VI of the Civil Rights Act of 1964, as amended; Title IX of the

Educational Amendments; Section 504 of the Rehabilitation Act of 1973; and the Americans With Disabilities Act of 1990; or related legal requirements should be directed to the Director

of Personnel/Human Relations, Office of the Dean, College of Agriculture and Natural Resources, Symons Hall, College Park, MD 20742.

rounding ground so rainwater and irrigation water will

drain away. Other site considerations include the light

requirements of the plants to be grown; locations of

sources of heat, water, and electricity; and shelter from

winter wind. Access to the greenhouse should be conve-

nient for both people and utilities. A workplace for pot-

ting plants and a storage area for supplies should be

nearby.

Types of Greenhouses

A home greenhouse can be attached to a house or

garage, or it can be a freestanding structure. The chosen

site and personal preference can dictate the choices to

be considered. An attached greenhouse can be a half

greenhouse, a full-size structure, or an extended win-

dow structure. There are advantages and disadvantages

to each type.

Attached Greenhouses

Lean-to. A lean-to greenhouse is a half greenhouse,

split along the peak of the roof, or ridge line (Figure 2).

Lean-to’s are useful where space is limited to a width of

approximately 7 to 12 feet, and they are the least expen-

sive structures. The ridge of the lean-to is attached to a

building using one side and an existing doorway, if

available. Lean-to’s are close to available electricity,

water, and heat. The disadvantages include some limita-

tions on space, sunlight, ventilation, and temperature

control. The height of the supporting wall limits the

potential size of the lean-to. The wider the lean-to, the

higher the supporting wall must be. Temperature con-

trol is more difficult because the wall that the green-

house is built on may collect the sun’s heat while the

translucent cover of the greenhouse may lose heat

rapidly. The lean-to should face the best direction for

adequate sun exposure. Finally, consider the location of

windows and doors on the supporting structure and

that snow, ice, or heavy rain might slide off the roof of

the house onto the structure.

Even-span. An even-span is a full-size structure that

has one gable end attached to another building (Fig-

ure 2). It is usually the largest and most costly option,

but it provides more usable space and can be length-

ened. The even-span has a better shape than a lean-to

for air circulation to maintain uniform temperatures

during the winter heating season. An even-span can

accommodate two to three benches for growing crops.

Window-mounted. A window-mounted green-

house can be attached on the south or east side of a

house. This glass enclosure gives space for conveniently

growing a few plants at relatively low cost (Figure 2).

The special window extends outward from the house a

foot or so and can contain two or three shelves.

Freestanding Structures

Freestanding greenhouses are separate structures;

they can be set apart from other buildings to get more

sun and can be made as large or small as desired

(Figure 2). A separate heating system is needed, and

electricity and water must be installed.

The lowest cost per square foot of growing space is

generally available in a freestanding or even-span green-

house that is 17 to 18 feet wide. It can house a central

bench, two side benches, and two walkways. The ratio

of cost to the usable growing space is good.

When deciding on the type of structure, be sure to

plan for adequate bench space, storage space, and room

for future expansion. Large greenhouses are easier to

and a two-stage thermostat are needed to control the

operation.

A two-speed motor on low speed delivers about 70

percent of its full capacity. If the two fans have the same

capacity rating, then the low-speed fan supplies about

35 percent of the combined total. This rate of ventila-

tion is reasonable for the winter. In spring, the fan oper-

ates on high speed. In summer, both fans operate on

high speed.

Refer to the earlier example of a small greenhouse. A

16-foot wide by 24-foot long house would need an esti-

mated ft

per minute (cubic feet per minute; CFM)

total capacity; that is, 16 ×24 ×12 ft

per minute. For

use all year, select two fans to deliver 2,300 ft

per

minute each, one fan to have two speeds so that the

low-speed rating is about 1,600 ft

per minute and the

high speed is 2,300 ft

per minute. Adding the second

fan, the third ventilation rate is the sum of both fans on

high speed, or 4,600 ft

per minute.

Some glass greenhouses are sold with a manual ridge

vent, even when a mechanical system is specified. The

manual system can be a backup system, but it does not

take the place of a motorized louver. Do not take short-

cuts in developing an automatic control system.

Cooling

Air movement by ventilation alone may not be ade-

quate in the middle of the summer; the air temperature

may need to be lowered with evaporative cooling. Also,

the light intensity may be too great for the plants.

During the summer, evaporative cooling, shade cloth,

or paint may be necessary. Shade materials include roll-

up screens of wood or aluminum, vinyl netting, and paint.

Small package evaporative coolers have a fan and

evaporative pad in one box to evaporate water, which

cools air and increases humidity. Heat is removed from

the air to change water from liquid to a vapor. Moist,

cooler air enters the greenhouse while heated air passes

out through roof vents or exhaust louvers. The evapora-

tive cooler works best when the humidity of the outside

air is low. The system can be used without water evapo-

ration to provide the ventilation of the greenhouse. Size

the evaporative cooler capacity at 1.0 to 1.5 times the

volume of the greenhouse. An alternative system, used

in commercial greenhouses, places the pads on the air

inlets at one end of the greenhouse and uses the exhaust

fans at the other end of the greenhouse to pull the air

through the house.

Controllers/Automation

Automatic control is essential to maintain a reason-

able environment in the greenhouse. On a winter day

with varying amounts of sunlight and clouds, the tem-

Small fans with a cubic-foot-per-minute (ft

/min)

air-moving capacity equal to one quarter of the air vol-

ume of the greenhouse are sufficient. For small green-

houses (less than 60 feet long), place the fans in diago-

nally opposite corners but out from the ends and sides.

The goal is to develop a circular (oval) pattern of air

movement. Operate the fans continuously during the

winter. Turn these fans off during the summer when the

greenhouse will need to be ventilated.

The fan in a forced-air heating system can some-

times be used to provide continuous air circulation.

The fan must be wired to an on/off switch so it can run

continuously, separate from the thermostatically con-

trolled burner.

Ventilation

Ventilation is the exchange of inside air for outside

air to control temperature, remove moisture, or replen-

ish carbon dioxide (CO

). Several ventilation systems

can be used. Be careful when mixing parts of two

systems.

Natural ventilation uses roof vents on the ridge line

with side inlet vents (louvers). Warm air rises on con-

vective currents to escape through the top, drawing cool

air in through the sides.

Mechanical ventilation uses an exhaust fan to move

air out one end of the greenhouse while outside air

enters the other end through motorized inlet louvers.

Exhaust fans should be sized to exchange the total vol-

ume of air in the greenhouse each minute.

The total volume of air in a medium to large green-

house can be estimated by multiplying the floor area

times 8.0 (the average height of a greenhouse). A small

greenhouse (less than 5,000 ft

in air volume) should

have an exhaust-fan capacity estimated by multiplying

the floor area by 12.

The capacity of the exhaust fan should be selected at

one-eighth of an inch static water pressure.The static

pressure rating accounts for air resistance through the

louvers, fans, and greenhouse and is usually shown in

the fan selection chart.

Ventilation requirements vary with the weather and

season. One must decide how much the greenhouse will

be used. In summer, 1 to 1

air volume changes per

minute are needed. Small greenhouses need the larger

amount. In winter, 20 to 30 percent of one air volume

exchange per minute is sufficient for mixing in cool air

without chilling the plants.

One single-speed fan cannot meet this criteria. Two

single-speed fans are better. A combination of a single-

speed fan and a two-speed fan allows three ventilation

rates that best satisfy year round needs. A single-stage

2 7

Figure 1. Select location carefully. Note where the shade line occurs in both the winter and summer.

Winter sun

Summer sun

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UNIVERSITYOFMARYLANDEASTERNSHOREUNIVERSITYOFMARYLANDCOLLEGEPARKCOOPERATIVEEXTENSIONSERVICEUNIVERSITYOFMARYLANDATCOLLEGEPARKUNIVERSITYOFMARYLANDEASTERNSHOREperaturecanfluctuategreatly;closesupervisionwouldberequiredifamanualventilationsystemwereinuse.Therefore,unlessclosemonitoringispossible,bothhobb...

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