Greenhouses and Season Extenders for Vegetable Gardening - Fairbanks, Alaska

1. Greenhouses and ❂ 9
Season Extenders
C
ertain crops grow just fine in Alaska’s short growing
season, even when direct seeded (planted outside in
the spring), while other crops benefit greatly from
season extension techniques and/or greenhouses. Cool
❂ Topics in this chapter
season crops that do well without any help include potatoes, ❂❂ Selecting a site
turnips, radishes, beets, lettuce and other greens, peas and ❂❂ Season extenders
carrots, which can all be direct seeded. Crops in the Brassi-
❂❂ Greenhouses
caceae family (broccoli, cabbage, cauliflour, kale, etc.), are
❂❂ Greenhouse benches and
also cold hardy, although seeds should be started indoors to
beds
ensure they reach maturity by the end of the growing sea-
son. Many warm season crops that grow well with a little ❂❂ Greenhouse utilities
help from season extension techniques include basil, snap ❂❂ Popular greenhouse crops
beans, strawberries, zucchini and other types of squash, to in Alaska
name a few. Tomatoes and cucumbers are ubiquitous green- ❂❂ Glazing and covers
house crops. Although season extension techniques require ❂❂ Watering systems
additional time and money, for crops such as strawberries ❂❂ Natural ventilation
it’s usually worth the additional effort and expense. In gen- ❂❂ Pollination
eral, more expensive techniques afford a greater degree of
control over and ability to use an outside heat or ventilation
source. With the additional costs associated with structures
such as a greenhouse, it makes sense to grow higher value
crops. Any season extension technique will require some
additional expense and labor and so two questions could be
asked: Is the crop valuable enough (to you or at the market)
to warrant the additional expense? Can the crop be grown By Pat Patterson, Extension Program Assistant,
successfully without additional help? Lane County, Oregon State University.
Edited by Heidi Rader, Extension Faculty,
Agriculture and Horticulture Agent, Cooperative
Extension Service, University of Alaska Fairbanks.
Adapted from Greenhouses for Home Gardeners:
Structures and Equipment, HGA-00337, University
of Alaska Cooperative Extension Service.

2. 170 • Greenhouses and Season Extenders—Chapter 9
the soil. A special type of polyethelyene
Selecting a site mulch called Infra Red Transmitting mulch
The most important factor in selecting (IRT-76®) was tested at the UAF Agricul-
a location for a cold frame, hoop house, ture and Forestry Experiment Station and
high tunnel or greenhouse is sunshine. was shown to warm the soil almost as much
Protection from strong winds is also very as clear plastic mulch, while also suppress-
important since they are more susceptible ing weeds. Equal, if not greater, yields were
to wind damage than many other types of achieved with clear plastic mulch, but only
construction. Other considerations include when herbicides were used.
factors important in site selection for any
type of building: level ground, well-drained Cold frames and hotbeds
soil and a location that does not collect Cold frames and hotbeds are inexpensive
water from the surrounding landscape. It structures for growing cool-weather crops
is often recommended that high tunnels or in early spring and fall. Recycled shower
greenhouses in northern climates should be doors or windows work particularly well
oriented with the long side running in an for a cold frame. They range from simple to
east-west direction to maximize sunlight, elaborate. Cold frames rely on the sun for
but for home gardeners with a limited num- their sole source of heat. Heat is collected
ber of site options, other location factors when the sun’s rays penetrate clear plastic,
often dictate the orientation. glass or fiberglass (Figure 1). Hotbeds are
heated by soil-heating cables, steam-car-
Season extenders rying pipes or fresh, strawy manure buried
beneath the plant rooting zone.
To get the most from a garden, extend the In early spring, a cold frame is useful for
growing season by sheltering plants from hardening off seedlings that were started
cold weather in both early spring and fall. indoors or in a greenhouse. Hardening off is
Row covers, cloches, plastic mulches, hoop important because seedlings can be set back
houses or a combination of these techniques seriously if they are moved directly from
can save your crops from unexpected frosts a protected location to a garden. A cold
in late spring or early fall and can allow you frame provides a place for gradual adjust-
to confidently transplant earlier than you
otherwise would.
Plastic mulches
Plastic mulches are useful for warming
the soil and suppressing weeds in Alaska.
They should be placed as tightly as possible
on the soil; plants such as zucchini, corn,
peppers and pumpkins can be grown in the
plastic. Clear polyethylene mulch warms up
the soil, but also acts as a perfect environ-
ment for weeds to grow. Black polyethelyne
mulch blocks the sunlight required for weed
growth, but is not very useful for warming Figure 1.—Cold frame.

3. Chapter 9—Greenhouses and Season Extenders • 171
Figure 3.—Solar pod.
Figure 2.—Dutch light. New cold frame designs include passive
solar energy storage. For example, black
ment to outdoor weather. You also can start containers filled with water absorb heat dur-
cool-weather crops right in the cold frame ing the day and release it at night. The solar
to transplant later to the garden, or you can pod is a design that provides this type of
grow them to maturity in the frame. In the heat storage (Figure 3). Other designs have
summer, cold frames are useful for growing a very high back and a steep glass slope and
warm season crops. Annuals can be started are well insulated. They may include mov-
in a frame weeks before they can be started able insulation that is folded up in the day
in the open. You can replace the soil in a and down at night or during extremely cold
portion of the cold frame bed with a me- weather.
dium suitable for rooting cuttings. Frames can be built from many materials;
The ideal location for a cold frame is a wood and cinder block are most common. If
southern or southeastern exposure with a you use wood, choose a species that resists
slight slope to ensure good drainage and decay or one of the new plastic woods.
maximum light exposure. A sheltered spot Never use creosote- or pentachlorophenol-
with a wall to the north provides protection treated wood, because these substances
against winds. Sinking the frame into the are harmful to plants and humans. Kits are
ground a bit also provides earth insulation. available, some of which contain automatic
To simplify use of the frame, consider ventilation equipment.
a walkway to the front, adequate space There is no standard size for a cold
behind the frame for removing the sash and frame. Size depends on the amount of avail-
perhaps a counterbalance to make it easier able space, desired crops, and size of the
to raise and lower the sash. Some gardeners available covering. Do not make the frame
make their cold frame lightweight enough too wide for weeding and plant care; 4 to
to be moved from one section of the garden 5 feet usually is the maximum width. The
to another. Another possibility is the Dutch sash should be sloped to the south to allow
light (Figure 2), which is a large, but porta- maximum exposure to sun.
ble, greenhouse-like structure that is moved A simple method of insulation is to place
around the garden. burlap sacks filled with leaves over the

4. 172 • Greenhouses and Season Extenders—Chapter 9
frame at night to protect against freezing. Cloches and row covers
Another method is to stack bales of straw
against the frame. Cloches originally were bell-shaped
Ventilation is most critical on sunny days glass jars set over delicate plants to protect
when temperatures rise above 55°F. Raise them from the elements. The definition has
the sash to prevent heat buildup inside the expanded, however, to include many types
frame. Lower it early enough to conserve of portable structures that shelter plants
heat for the night. An automatic nonelectric from drying winds and cold air. The idea
lifter is a good investment. is to provide a greenhouse-like atmosphere
A cold frame can be converted to a hot- for seeds and small plants in order to get an
bed. For a manure-heated bed: early start on the season or to extend the fall
1. Dig out an area 8 or 9 inches deep garden as long as possible.
(deeper if you need to add gravel for Cloches are set over individual plants or
increased drainage). made into tunnels for whole rows. They trap
2. Add an 8-inch layer of strawy horse solar radiation and moisture evaporating
manure. from the soil and plants. Cloches generally
3. Cover with 6 inches of good soil. are lightweight, portable and reusable. The
best designs can be closed completely at
For an electric-heated bed: night to prevent frost damage and opened
1. Dig out an area 8 or 9 inches deep. or completely removed during the day for
2. Lay down thermostatically controlled good air circulation. Make sure cloches
electric cable in 6- to 8-inch loops, are anchored or are heavy enough that they
evenly spacing the cable but never al- don’t blow away.
lowing it to cross. The hotcap and the ubiquitous cut-off
3. Cover with 2 inches of sand or soil. plastic jug are simple forms (Figure 4).
4. Lay out hardware cloth to protect the More elaborate ones include fiberglass
cable. tunnels, special plastic cloches, row cov-
5. Cover with 4 to 6 inches of good soil. ers with slits to allow aeration and panes
of glass connected by specially designed
hinges to form a tent (Figure 5).
You can quickly assemble an excellent
Quonset-type cloche by covering a 4- to
5-foot piece of concrete reinforcing wire
with row cover or plastic. The heavy-duty
wire comes in 5- and 7-foot lengths. Al-
ternatively, willows or PVC pipe can also
be used to build this hoop structure. If you
have a raised bed garden, the hoop house
can be permanent. If you are planting
directly in the ground, rebar can be used to
anchor PVC pipe.
Spun-bonded polypropylene is also a
Figure 4.—Cloches made from a cutoff plastic soda bottle and a popular row cover. It permits passage of air
milk jug. and water, but is substantial enough to trap

5. Chapter 9—Greenhouses and Season Extenders • 173
Figure 5.—Season extenders and plant protection methods include plastic and glass.
heat, shield young plants from wind and in- work on a rainy day, are inexpensive and
sects and provide a modified atmosphere for simple to build, offer increased reliabil-
early vegetable crops. Lay these row covers ity, can decrease disease, weeds, and pest
loosely over a planted row in early spring. incidence, and increase yield and quality of
As the plants grow, they raise the light- warm season crops. However, in contrast to
weight cover. You can leave these covers on greenhouses, these light-weight structures
most vegetable plants until warmer weather. can be easily damaged by snow or wind, so
These covers also protect turnips from root they must be well anchored. They can also
maggots. Spun-bonded polypropylene fab- be more labor intensive because they often
rics come in different weights, with heavier lack automated ventilation or heating sys-
weights affording greater protection from tems. An advantage for commercial growers
frost but allowing less light through. These is that since high tunnels are not considered
fabrics generally offer between 3 and 5 de- permanent structures, they are not taxable.
grees of protection from frost. They should Hoop houses can easily be built from
be anchored with soil, stones or wood PVC pipe, white spruce poles or willows.
(stakes are not ideal as they create a hole in An easy, inexpensive way to make a hoop
the cloth). With care, this cloth can be used house is by converting carport structures.
year after year. Hoop house or high tunnel kits, usually
made from bent metal pipe (galvanized
Hoop houses or high tunnels steel), can also be ordered. Special high
The next step on the continuum of sea- tunnel kits come with a fancy wire called
son extension techniques is a hoop house, “wiggle wire.” This wire is designed to hold
which you can walk through, or a high tun- the visqueen on without tearing holes in it.
nel, which you can drive a tractor through. A double-walled high tunnel, with a fan to
Crops are almost always planted directly in maintain air space in between the layers,
the ground in these structures. Hoop houses is an inexpensive way to insulate a hoop
or high tunnels can be a pleasant place to house; however, this decreases light pen-
etration to some degree.

6. 174 • Greenhouses and Season Extenders—Chapter 9
Greenhouses Freestanding greenhouses
Greenhouses, although generally more Freestanding greenhouses isolate green-
expensive than other season extension house activity from the home and offer
techniques, provide the greatest degree more flexibility in greenhouse size, shape
of control. Because they are permanent, and configuration. They can be constructed
greenhouses can more easily be insulated in the best location on the property for light
and heated, if desired. Automatic ventilation interception, drainage and wind protection.
systems can also provide increased reli- Freestanding greenhouses are also easier to
ability with relatively less labor. In Alaska, expand than attached greenhouses.
greenhouses are used to start seeds in the Access to freestanding greenhouses may
spring, grow crops such as tomatoes and be more difficult, especially if they are used
cucumbers and provide the perfect environ- in the winter. They require separate utility
ment for beautiful hanging baskets. Some services or lines from the house for water,
growers use greenhouses year round to electricity and, for some greenhouses, heat.
grow crops hydroponically using geother- A freestanding house usually has higher
mal heat. Greenhouses are broadly catego- heating costs than the attached house. It has
rized as attached or free-standing. more exposed surface, leading to greater
heat loss. The freestanding greenhouse also
Attached greenhouses has a higher initial construction cost than
the attached greenhouse since it requires an
An advantage of attached greenhouses is
extra wall and service line installation.
that they usually have easy access, which
makes it more convenient to attend to
A-frame
plants. They can create an extension of your
indoor living space, and they may provide A-frames have sloping sides all the way
solar heating and add an insulation layer to to the foundation, which makes them the
the house. They usually have lower heating best style for passive snow removal and
costs and lower initial construction costs limiting snow buildup (Figure 6). Like hoop
(because there is one less wall to build) houses, they are relatively inexpensive and
and it is easier to tap into the home water, simple to build. Frames are often made
electrical and heating systems. Size limita- from two-by-four lumber and are generally
tions and location restrictions are potential covered with a single layer of polyethylene
disadvantages of attached greenhouses. film. People sometimes attach a second
Both are determined by the availability of layer of film to the inside if they use the
suitable exterior wall space. Compared to a house during cooler weather. The biggest
freestanding greenhouse, site location may drawback of the A-frame style is that it
not be the sunniest space in the yard. Anoth- limits both house width (and therefore size)
er disadvantage is that greenhouse moisture, and headspace within the structure.
summer heat, dirt, insects and chemical It is more awkward to work in an A-
odors may enter the home. frame house than in other styles of green-
houses, and flats are often grown on the
ground due to the lack of head space. Soil
temperatures may remain cold in a green-
house and lag well behind air temperatures,

7. Chapter 9—Greenhouses and Season Extenders • 175
gable style pro-
vides considerably
more headspace
than an A-frame or
Gothic arch and has
much more room
for growing hang-
ing plants above
Figure 6.—A-frame and slant-leg greenhouses. benches. Gable
greenhouses can be
covered with poly-
so if the roots of plants placed directly on ethylene film, but generally a rigid type of
the ground are cool, growth may be restrict- material is used such as polycarbonate, glass,
ed from reduced water and nutrient uptake acrylic or fiberglass.
even though air temperatures are conducive
to good plant growth. One way to limit this Slant-leg
problem is to put flats and pots on a layer of The slant-leg style is similar to the gable
insulation placed over the soil. style, except that sidewalls are slanted
slightly outward (Figure 6). This makes
Gothic arch it wider than the vertical-leg gable style,
Gothic arch greenhouses have an arched providing additional floor space. Like the
roofline and walls that form a continuous gable style, it has a flat, sloping roof that
shape. Snow slides off them better than from sheds snow well, depending on the pitch.
a hoop house, but not as well as from an The slanting sidewalls require a reinforced
A-frame. They have more headspace than an frame, with the result that slant-leg houses
A-frame, but not as much as a hoop house of are very sturdy and suited for locations with
similar height. Frames are constructed from heavy snowfall and thunderstorms.
both pipe and wood. Roof supports must be
flexible enough to make the arch curvature Pit
(glued plywood strips are one type of ma- Pit greenhouses are built partially below
terial used). Gothic arch greenhouses are ground. They are most commonly built
frequently covered with polyethylene film, where topography is suitable, such as the
but they can also be covered with flexible side of a hill with good sun exposure. Pit
fiberglass and polycarbonate panels. greenhouses can have lower heating and
cooling costs, since the earth acts as insula-
Gable tion to keep them warmer in the winter and
Gable-style greenhouses have sloping, flat cooler in the summer. They are relatively
roofs connected to vertical sidewalls. The expensive to construct, because reinforced
angle determines how well snow slides off, walls are required in the pit and both interi-
as well as the total height of the structure. or and exterior drainage systems are usually
The well-defined roofline is adaptable to ef- needed to keep the pit dry.
ficient roof-ventilation systems. Frames are
constructed from both metal and wood. The

8. 176 • Greenhouses and Season Extenders—Chapter 9
type of bench is commonly used for plant
Greenhouse benches and propagation, where cables can be buried
beds for bottom heat and mist nozzles installed
The layout and design of greenhouse above.
benches determine the amount of usable Plants can be grown in solid bottom
greenhouse space and the ease of working benches to maturity, depending on their
within that space. Experiment on paper with size and rooting depth. They are frequently
different bench sizes and different patterns used commercially for growing cut flow-
of benches and aisles to help you decide ers. In addition to efficient bench layout, the
on the best overall design. Benches must creative use of racks, shelves and overhead
be strong enough to support the weight of hanging plants can take advantage of all
plants and soil (or growth media), as well potential growing spaces in a greenhouse.
as the weight of the water in a fully watered Growing conditions vary, both vertically
pot. They should be a comfortable working and horizontally, within even a small green-
height (usually about 30 to 36 inches) and house, so effectively maximizing grow-
an arm’s reach in width. When a bench is ing space requires recognition of different
accessible from both sides it can be twice as greenhouse microclimates and knowing
wide. which plants will do well in each of them.
Benches can be constructed from a va-
riety of materials, including wood, gal- Ground beds
vanized pipe, concrete block and plastic. Tall plants, such as trellised tomatoes,
Wood, because of the moist atmosphere cucumbers and corn are frequently grown in
in greenhouses, should be pressure treated ground beds in greenhouses and hoop hous-
with a preservative that is nontoxic to es. Beds are often raised and about 3 feet
plants. Bed construction should facilitate wide; the soil must be 8 to 12 inches deep.
good airflow through plants, including Ground beds may be loosely mounded or
air circulation from below the bench top. enclosed within a wooden frame. Plant cul-
Greenhouse bench tops are made from ture is similar to raised-bed gardening out
slatted wood, wire mesh or expanded of doors.
metal sheets for this reason. Slatted wood
is relatively inexpensive and easy to work
with but doesn’t permit as much airflow as Greenhouse utilities
wire mesh or expanded metal. Wire mesh is All greenhouses require a water supply,
much less expensive than expanded metal most need an electrical supply and some
sheets, but over time it begins to sag and its require a fuel supply for heat. Home green-
useful life is much shorter than expanded houses are normally connected to utility
metal. Greenhouse bench materials should sources within the home. Before greenhouse
be light-colored (or painted white or alu- construction, make sure the home capacity
minum) to reflect light and maximize light is adequate for the added connections and
interception by plants. peak-use rates. Plan the installation and
Plants can also be grown directly in solid connections from the very beginning of the
bottom benches with sides that are filled greenhouse construction process, paying at-
with 4–6 inches of growing medium. This tention to where underground lines must be
run to freestanding greenhouses.

9. Chapter 9—Greenhouses and Season Extenders • 177
Ventilation venting the heater to the outside is necessary
to remove potentially harmful gases.
Ventilation systems promote airflow
through a greenhouse and help control both
temperature and humidity. The importance Popular greenhouse crops in
of managing temperature and humidity in Alaska
greenhouses is discussed in more detail in
the UAF Cooperative Extension Service
publication, Controlling the Greenhouse
Tomatoes
Environment, HGA-00136. Because tomato pollination and fruit set
depend on a greater degree of temperature
Heating control than many crops, it’s best to grow
Many home greenhouses are operated them in a greenhouse. Optimal tempera-
seasonally and shut down during the colder tures for tomatoes range from a minimum
parts of the year. They do not require a per- of 60°F during the night to no more than
manent heating system, but many gardeners 80°F during the day. Humidity levels of 70
provide supplemental heat during spring or percent are optimal for tomatoes.
fall cold spells with portable gas and elec-
tric heaters. Small, portable heaters work
Cucumbers
well to protect plants on unexpectedly cold Cucumbers are sensitive to temperature,
nights, but gas heaters that are not vented humidity, light, carbon dioxide, moisture
to the outside can create problems if they and fertilizer salts. Optimal temperatures
are not maintained and burning cleanly. for cucumbers range from a minimum of
Incomplete combustion can create ethylene 65°F and 70° to 75°F. Humidity levels of 70
gas, which is a plant hormone that causes percent are optimal.
abnormal, distorted growth in excessive For greater detail on growing these
concentrations. Tomatoes and fuchsias are popular crops in greenhouses, refer to UAF
very sensitive to ethylene. Cooperative Extension Service publications
Several types of permanent heating units Cucumber Production in Greenhouses,
can be installed in greenhouses that are HGA-00434, and Greenhouse Tomato Pro-
operated during the colder parts of the year. duction, HGA-00435.
The choice of a heater often depends on
local availability and the cost of different fu-
els. A heater must be properly sized so that it
Glazing and covers
has adequate capacity to heat the greenhouse Glazing is the light transmitting material
on the coldest night of operation. Required used to cover season extenders or green-
heater output is determined by calculating houses. Many high-quality glazing mate-
potential heat loss, which depends on the rials are available, but they vary in their
surface area of the greenhouse, the insulat- strengths and weaknesses. Important prop-
ing value of the glazing, wind conditions erties to consider when selecting a glazing
and the temperature difference between are light transmission, heat retention, dura-
inside and outside air. As discussed above, bility, longevity, maintenance requirements
and cost.

10. 178 • Greenhouses and Season Extenders—Chapter 9
Plastic film Polycarbonate
Plastic film glazing is lightweight, very Polycarbonate is a rigid plastic glazing,
flexible and the least expensive type of but the sheets have enough flexibility to
greenhouse covering. A single layer has bend around gently curved surfaces such
high light transmission, but also high heat as the Gothic-arch greenhouse. It has high
loss in cold weather. Many hoop houses light transmission, low maintenance, is
have a double layer of plastic with an very strong yet lighter than glass and is a
insulating layer of air between them. This fire retardant. Polycarbonate is available
reduces light transmission by about 10 in single- and double-layer sheets. As with
percent, but reduces heat loss by 35 per- other glazing materials, the double sheets
cent. Use only greenhouse-grade plastics. lose some light transmission but reduce heat
They have additives that slow deterioration loss to a greater extent. Double sheets also
from ultraviolet light, are stronger and have have greater longevity than single sheets.
infrared inhibitors that reduce heat loss. Polycarbonate sheets must be installed to
Greenhouse-grade plastics will last 4 to 10 allow for some contraction and expansion
years, compared to other plastics that will of the material with temperature changes.
only last one growing season. Some materi-
als also have anti-drip surfaces that reduce Acrylic
droplet formation from water condensing on Acrylic is a rigid plastic glazing that is
the interior of the plastic film. clearer than polycarbonate, although it may
slowly yellow with age. It is available only
Glass as a double-layered sheet that lacks flex-
The introduction of synthetic glazing ibility and must be installed on flat sur-
materials has greatly reduced the number faces. Light transmission and heat loss are
of glass-covered greenhouses, but in many equivalent to double-layer polycarbonate.
ways glass is still the standard of compari- Acrylic has the longest life expectancy of
son. It has high light transmission and good any glazing except glass, but compared to
durability. Glass has the greatest longevity polycarbonate it attracts more dust and dirt,
of any glazing material (25+ years). Use scratches more readily and is more flam-
low iron glass for the highest light trans- mable.
mission and tempered glass for the greatest
strength. Double-pane glass reduces the Fiberglass reinforced plastic
relatively high heat loss of glass, although Fiberglass for greenhouse glazing is a
some loss in light transmission also results. rigid plastic that will bend around gently
Glass must be installed and maintained to curved surfaces. It has high light transmis-
prevent air and water leakage between the sion, but comes only in single sheets (flat
panes. Older greenhouses had overlapping or corrugated); it also has a high heat loss.
panes with a bed of glazing compound and Fiberglass reinforced plastic is lower in cost
sealant. Modern glass greenhouses use than other rigid plastic glazing, but it does
rubber or vinyl gasket material below the require more maintenance since it must be
edge-to-edge glass and bar caps over the top recoated every few years.
of the seam.

11. Chapter 9—Greenhouses and Season Extenders • 179
a faucet and a hose. The proportioner has a
Watering systems tube that is placed in a bucket of fertilizer
Season extenders and greenhouses concentrate, and when water moves through
Hand watering the proportioner it siphons some concen-
Most home greenhouses are watered by trate and mixes it with the water flowing
hand with a hose and suitable nozzle. One to the irrigation hose. Fertilizer injectors
advantage of hand watering is that is gives pump fertilizer concentrate into the irriga-
the gardener time to look at every plant tion stream. They are more accurate than
and evaluate its condition. It is also easy to proportioners and useful as the numbers of
adjust watering to be able to apply differ- plants, and thus volumes of water, increase.
ent amounts of water to different plants. They can also be plumbed into automated
However, as greenhouse size and number irrigation systems. When using fertigation,
of plants increase, hand watering becomes a backflow preventer must be used to keep
time-consuming compared with other meth- fertilizers from flowing backwards into wa-
ods. ter sources such as wells and ponds.
Drip systems Overhead sprinklers
Drip irrigation systems or soaker hoses Sprinkler systems are relatively simple
are efficient for watering when using season to install and convenient to operate. One
extenders. In greenhouses, drip systems can drawback is that excess water must be
be individual capillary tubes for each pot or applied; this makes up for water deflected
drip tape for ground beds. Drip irrigation by plant leaves and water that falls on the
is very efficient in water use and keeps the non-growing areas. Also, wet foliage and
non-plant areas dry so that you can work higher humidity levels increase the potential
around plants while they are being watered. for plant disease. Overhead sprinklers are
Drip systems require close observation to sometimes used in high tunnels, but more
be sure they are working correctly and need frequently in greenhouses.
maintenance to keep the water flowing from Greenhouses only
emitters. Drip systems are relatively easy to In addition to the previously mentioned
assemble, but they do take more time to set watering systems, the following techniques
up than most of the other watering systems. work well for greenhouses.
This is especially true when individual drip
emitters are installed to a large number of Capillary mats
pots. Capillary mats water plants from below.
They are water-absorbing materials placed
Fertigation on solid bench tops with a few drainage
Healthy plant nutrition is frequently holes. Pots or flats are put on top of the mat,
maintained in greenhouses by using soluble the mat is watered and water is wicked up
fertilizers that are delivered to plants in from the mat into and through the growing
their irrigation water. A number of devices medium. Periodic top watering is recom-
are available to help accomplish this. The mended to provide leaching and prevent
simplest and probably the most useful and potential salt buildup.
economical for a small home grower is a
proportioner that can be attached between

12. 180 • Greenhouses and Season Extenders—Chapter 9
Ebb and flow benches exchange. Ridge vents along the peak of the
Ebb and flow benches are similar to greenhouse are the most efficient in remov-
capillary mats in that plants are watered ing air, and a general rule of thumb is that
from below. They differ in how the water is the total area of the vent openings (ridge
delivered. The bench top of an ebb and flow plus side) should equal 20–30 percent of
bench is periodically flooded with a pool of greenhouse floor area. Solar-powered levers
flowing water, the water is drained and the that sense the temperature can be invalu-
cycle is repeated at regular intervals that able in cutting down the stress of manually
are appropriate for the water needs of the ventilating a hoop house or greenhouse.
plants. Greenhouses only
Although natural ventilation is effective,
Hydroponics
it sometimes benefits from the assistance
Water is the central part of the entire
of a small fan to speed the flow of stagnant
growing system for plants grown hydropon-
air. The air also moves passively along
ically. The water supply is built into other
pathways of least resistance, and a properly
aspects of plant culture. There are a variety
placed fan can introduce some turbulence
of hydroponic systems, ranging from plants
that reduces channeling and results in more
anchored by rock in a trough of water to
uniform overall airflow. Sometimes such
plants grown in Styrofoam trays that float in
fans are placed in the ceiling and blow verti-
a pool of water. In all types of hydroponic
cally downward across the bulk flow of air.
culture, water must be well aerated and
free of disease and provide a steady supply
of nutrients to plants. Hydroponic systems Pollination
require more monitoring time, more atten-
When covering
tion to their operation and more mixing and
crops that flower
adjustment of nutrient solutions, thereby
and fruit, it’s impor-
adding to the system complexity.
tant to consider how
they will be pollinated. For
Natural ventilation crops such as strawberries,
Season extenders and greenhouses opening and closing row covers
Natural ventilation generally provides with clothes pins or another mechanism is
adequate air exchange for season extenders one way to provide access to pollinators.
and small greenhouses. Natural ventilation This is also easily done with hoop houses or
depends on differences in temperature in- high tunnels. Planting flowers on the ends
side and outside the greenhouse. Ventilation of hoop houses or high tunnels or even in-
effectiveness increases as the temperature side can be a way to draw in pollinators. For
difference increases. Warmer, more humid more information on encouraging beneficial
greenhouse air rises out of vents in the top insects in your garden and season extend-
of the greenhouse and is replaced by cooler, ers or greenhouses, see UAF Cooperative
dryer air from outside that enters through Extension Service publication, Beneficial
lower side vents. Insects and Spiders of Alaska, PMC-10075.
Natural ventilation requires adequate Pollination for tomatoes in greenhouses
vent area to provide enough airflow and air is often simulated by shaking the plant or

13. Chapter 9—Greenhouses and Season Extenders • 181
by buzzing them with an electric buzzer UAF School of Natural Resources and
intended for such use. Seedless cucumbers
Agricultural Sciences
do not require pollination although seeded
varieties do. In a greenhouse, female flow- Growing Fresh Vegetables; Midnight Sun-
ers should be manually pollinated with a light and the Earth’s Warmth, SNRAS/
male flower. AFES Misc. Pub. No. MP 09-10.
Matheke, G.E.M., P.S. Holloway, and P.J.
Wagner. April 1991. IRT-76® Polyeth-
For more information ylene mulch film and growth of sweet
corn in Fairbanks, Alaska. Number 25.
UAF Cooperative Extension Research Progress Report.
publications
Other publications
Growing Everbearing Strawberries as An-
nuals in Alaska, HGA-00235. Bartok, J. 2000. Greenhouses for Home-
Beneficial Insects and Spiders of Alaska, owners and Gardeners, NRAES-00137.
PMC-10075. Ithaca, New York: Cornell University.
Controlling the Greenhouse Environment, Coleman, E. 1999. Four-Season Harvest:
HGA-00336. Organic Vegetables from Your Home
Cucumber Production in Greenhouses, Garden All Year Long. Chelsea Green
HGA-00434. Publishing Company.
Fluorescent Lights for Plant Growth, HGA- Ortho’s All About Greenhouses. 2001. Des
00432. Moines, Iowa: Meredith Books.
Greenhouse Tomato Production, HGA- Mohyuddin. M., Darby, D., Chang, J., and
00435. Betty Vladicka. 2004. Hobby Green-
Greenhouses for Home Gardeners: Struc- houses in Alberta. Alberta Agriculture,
tures and Equipment, HGA-00337. Food and Rural Development.
Plastic Mulch and Row Covers for Vegeta-
ble Production in Alaska. FGV-00647. Websites
Plastic Mulch and Row Covers for Veg- Greenhouse Plans
etable Production in Alaska (DVD), eHouseplans: www.ehouseplans.com/
HGA-01268. project_plans/greenhouseplans.html8
The Attached Solar Greenhouse, EEM- High Tunnels
01259. High Tunnels.org and www.hightunnels.
org
Planning and Building a Greenhouse
National Sustainable Agriculture Infor-
mation Service: http://attra.ncat.org

14. 182 • Greenhouses and Season Extenders—Chapter 9