A greenhouse is more than just a protective bubble for pampered plants—it serves to coax the best out of your hydroponic system and creates a pleasant, relaxing and productive space. Having a tropical oasis to take refuge in during a cold, snowy winter might seem ideal, but a good deal of planning and research needs to be carried out first to make sure the right design for the local climate has been selected. A greenhouse can be a beautifully lit, fantastically green, comfortable and airy space, or it can turn into every plant’s worst nightmare—baking hot in summer, frozen solid in winter and dripping with condensation all year round. The difference is all in the design.
Before the 1950s, glasshouses were pretty much seen as small production units for the wealthy—who could afford to have personal gardeners growing out-of-season fruits and vegetables for their tables. The development of relatively inexpensive plastic film materials prompted a rapid expansion in many different types of greenhouse structures—growers no longer had to rely on smashable glass panes and the heavy structural components they required for support.
This meant that both commercial and backyard growers now had the opportunity to build or buy their own cost-effective growing structures—and they could be virtually any size, shape or design.
The development of new plastic technologies also sparked a rapid expansion in the market that saw a wide range of prefabricated greenhouse kits becoming available for the first time. While this sudden market boom gave gardeners a huge choice as far as protected growing structures was concerned, it also led to a lot of them making basic mistakes when it came to selecting the correct greenhouse design for their particular local climate.
Why Different Greenhouse Designs for Different Climates?
Not all greenhouses are created equal—a design that works well in a cool climate with long cold winters, snowfall, low light and high winds will not be the best design for a humid, tropical climate with variable light intensity. Different greenhouses are characterized by the level of protection from the outside environment they can offer and the capability they can provide growers to control the inside environment to a specific set of conditions.
The level of protection required depends on the type of crop being grown and the local climate. The objective with building any greenhouse is to find a design that will allow the grower to overcome the most limiting climatic problems in their particular area and obtain the maximum growth rates possible from their crops.
Climate Types and Suitable Greenhouse Designs
Dry tropical or desert climates
In dry desert environments, temperatures can be extremely high—hot enough to frazzle most plants inside a greenhouse structure unless cactus is the only crop being grown. Temperatures of well over 100°F (38°C) year round combined with low humidity are typical in this sort of climate. The main environmental threats are high winds carrying dust or sand, which can blast both crops and greenhouses.
A proven type of greenhouse structure for this type of extreme climate is actually just a simple tent with poles set deeply into the ground, constructed with high-tensile steel wires to form a basic framework over which a single layer of fine insect mesh is stretched and secured around the edges.
This forms a shaded and insect-proof structure that allows adequate air exchange to prevent heat buildup. Inside, the humidity can be increased by fogging or misting, which also acts to reduce temperatures—often to levels well below those of the outdoor environment. Low humidity levels allow for the effective use of evaporative cooling, which is the main feature of cropping in this kind of dry, arid climate.
Air movement is essential inside this type of structure to maintain good levels of transpiration within the crop, as this is another method of natural plant cooling. More advanced high-tech, computer controlled and air conditioned structures are also in use in climates like this.
Subtropical desert and Mediterranean climates
Desert areas in the 30 to 35° latitude range can experience much lower air temperatures during the winter months—although summer temperatures are still quite high, with very low rainfall. For year-round production of many commonly grown hydroponic crops, a structure that can be heated but still maintains a cool environment in summer is necessary.
In this type of climate a suitable structure is the ‘pad and fan’ cooled plastic greenhouse with top vents and heating. The pad and fan system both cools the air and increases humidity as water evaporates when air enters the greenhouse structure. Along with shading over the outside of the greenhouse, this produces an ideal environment during dry summer conditions.
As temperatures drop—which they can during the night, even in summer—the moist air can be vented through the top vents and the interior of the greenhouse can be heated. This process reduces the humidity in the greenhouse to outside levels and therefore prevents condensation from forming when temperatures cool outside.
Condensation is one of the major threats to greenhouse crops—droplets falling onto plants create a level of leaf wetness, which allows many fungal and bacterial pathogens to aggressively attack, creating disease outbreaks that are often difficult to control as new infections can occur every night.
Humid tropical climates
Tropical areas experience hot, muggy conditions both day and night for much of the year and are characterized by heavy downpours on a regular basis. In lowland areas the humidity can be extreme. Light levels can fluctuate from being high on bright sunny days to being rather low under overcast conditions, particularly during the rainy season.
Tropical areas close to the equator also experience a consistently short day length and this—combined with continually overcast conditions—can reduce light levels available for crop growth to below optimal levels. Insect pressure in tropical climates is often very high as well, requiring the use of insect mesh over vents and on open-sided structures.
Some tropical growers prefer to site their greenhouses in highland areas where temperatures are typically cooler and humidity levels less oppressive.
Good tropical greenhouse designs can be as simple as a rain cover or plastic roof with open or roll-up sides covered with insect mesh. In larger greenhouses, the structure is best designed with a ‘sawtooth’ roof layout—which allows good venting of the hot air inside the greenhouse on clear days.
The top of the vents sometimes features a roll-up plastic cover stretching down to the gutters to prevent rain from entering the greenhouse during downpours. Since temperatures are warm day and night for much of the year, heating and insulation aren’t required and vents can remain open, covered with insect mesh.
Misting systems and air-movement fans can be used to cool the environment inside this type of structure and movable thermal screens can be employed to reduce incoming sunlight on bright, cloudless days and—pulled back—to allow maximum light penetration under overcast conditions.
Since high winds from tropical typhoons or hurricanes can be a major risk in this climate, these types of greenhouses are designed so that the wide insect mesh side covers will come away from the building before causing any damage to the framework.
Temperate climates are home to most of the world’s commercial hydroponic production. They are characterized by seasonal variations in temperature and year-round moderate rainfall. The temperature variation is enough that environmental modification is required for at least part of the growing season when the outside climate is unsuitable—usually when it’s too cold for optimum plant growth. Efficient heating of the air inside the greenhouse and insulating and maintaining this heated air is the main consideration.
Growers wanting year-round high growth rates and maximum yields in these environments usually select greenhouses featuring fully clad side walls, roof and side vents, allowing large ventilation areas and computer control of environmental equipment such as heaters, shade or thermal screens, fogging and vents.
Temperate zone greenhouse design often makes use of plastic cladding ‘twin skins’ where the space between the two layers of plastic is inflated, offering improved insulation and better environmental control.
Cold temperate climates
Cold temperate climates in latitudes above 45° are characterized by large variations in day length and temperature, especially in continental regions. Daytime temperatures can be below freezing for a large part of the year with very short day lengths, while coastal regions have short, mild summers and extended day lengths. Daytime temperatures during summer can be very high in central areas of places like Canada or Russia.
Greenhouses for this type of environment need solid walls and strongly constructed, comparatively steep solid roofs to carry snow loads that would collapse plastic film structures.
These greenhouses are often double insulated by installing plastic film on the inside walls and positioning retractable thermal screens across the eaves at stud height. To prevent heat loss, vents are often kept closed during the winter months. This—together with short day lengths—means supplementary carbon dioxide injection and lighting will be necessary for hydroponic crops.
Structural Greenhouse Design
Most modern hydroponic greenhouses for all climates these days feature a stud height of at least 10 feet and sometimes much more—an increase in height from early greenhouse designs that were often so low to the ground it was hard to stand up inside them.
Regardless of the type or design of the greenhouse or what crop is being grown, a tall greenhouse structure provides a better environment for plants and a larger buffer against minor changes in external temperatures.
The resulting improved capacity for air movement is a necessary aspect of modern greenhouse cropping that has been shown to benefit numerous crops by improving transpiration and reducing disease.
The volume of air that needs to be heated in cooler climates can be reduced by pulling thermal screens across the greenhouse roof at night and heating only under the screen—this creates a large insulation layer above the screen and under the greenhouse roof, thus slowing the rate of heat loss through the cladding.
For the serious hydroponic grower who wants to maximize yields from a backyard greenhouse the best option is to select a scaled-down version of a commercial greenhouse of the correct design for the local climate.
Some design faults to look for with backyard greenhouse kits include the structure being too small, which restricts airflow and can lead to a rapid buildup of heat.
Also check for a fairly large top vent—the structure should have at least a manually opening window for venting or a large rollback vent in the roof. Avoid greenhouse kits that rely only on a small door opening to vent the house, as this rarely works under warm growing conditions.
Only high-quality plastic film should be used as cladding material—it should be UV stabilized, with a suitably long life (at least three years) and you should make sure that you can get replacement claddings for the greenhouse when required.
Many keen hydroponic growers have come to know the joys of a well-designed and highly productive greenhouse structure. However, getting the basics sorted before construction takes place is essential for the future health and productivity of your plants.
Determining the type of outdoor climate you have, understanding the limiting factors that particular climate imposes on crop production and finding the best type of greenhouse structure to overcome those limitations is what all growers—large and small—need to do first.
The same principles of greenhouse design apply to both large commercial installations and the smallest backyard structure—getting the basics right during construction will go a long way toward making any greenhouse a productive place to work or play.