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Room to grow

Inside a greenhouse you can create the ideal place to grow vegetables, germinate seedlings or even try your hand at growing exotic plants


By Gina Hartoog


A greenhouse provides protection against the elements and creates the ideal growing space for a myriad different plants. Anything from herbs and vegetables to orchids and tropical plants can be grown in a greenhouse. Light, temperature and humidity inside the structure are kept at an optional level for plant growth. If it gets too hot inside the greenhouse, a misting system is used to lower the temperature. If the temperature drops, a heating system can be used to raise it.


Back in time

The history of the greenhouse dates back to Roman times when the Emperor Tiberius took ill and palace doctors insisted that he eat one cucumber-like vegetable every day. In order to the keep up with the crop demand, palace staff built a house in which the plants could grow. Fires were kept burning outside the stone walls and this heated the air inside the structure. The cucumbers were planted in carts and taken out into the sun every day, before being returned to the warm stone room at night.


In the years that followed, countries like Italy, the Netherlands, France and England produced greenhouse-like structures. All had a common goal – to control the freezing northern temperatures and prolong the growing season.


Benefits of a greenhouse

Even today, the extended growing season is appealing to the suburban gardener. Seeds can be germinated far earlier than if they were planted outdoors and the growing season can be extended beyond the end of summer, even continuing through winter.


In simple terms, a greenhouse controls the atmospheric condition and keeps it at an optional level for good plant growth. Damaging elements like hail, frost or too much rain are eliminated. While a greenhouse won’t keep out all unwanted insects, it does offer protection against birds, moles and other small mammals that may feast on unprotected crops. Optimal control of temperature is important for greenhouse gardening. In summer, temperatures should not exceed 29°C and in winter, night temperatures should not drop below 7°C.


A greenhouse in your garden

While still available and an attractive option, modern greenhouses are rarely manufactured from glass. Advancements in technology, especially in the manufacture of plastics, have contributed to the development of better quality greenhouse coverings that offer additional benefits at a much lower cost. Coverings include plastic films, fabrics and solid sheets. Polycarbonate and polyethylene panels are cost-effective and a popular long-term choice today. The greenhouse frame may be manufactured from PVC, galvanised steel, aluminium, wood or fibreglass. The unit can be installed on a foundation or it can be a moveable, tunnel-like structure.


DIY greenhouse kits are available on the market. Size options vary and with modular structures, you can start off small and increase the size of the greenhouse as your budget allows. The greenhouse can be customised to your needs with roof vent panels, louvres and automatic openers for roof vents and louvres.


“Polycarbonate panels are durable, safe and lightweight,” says Linda Hill of Easy Greenhouses. “The polycarbonate walls used for our greenhouses are twin walled, either 4mm thick or 6mm thick. This assists with insulation and temperature control. Polycarbonate is highly transparent and assists with light transmission and diffusion.”


Tunnels are composed of steel hoops and covered in plastic. “The plastic is usually not a simple plastic – the better products have a number of unique properties like light diffusion and temperature regulation, and screen out a certain spectrum of light that ‘burns’ plants,” explains Kevin Black of Haygrove South Africa. “These structures offer the advantages of brick and glass greenhouses, but at a fraction of the cost. Some of them, like our Haygrove structures, are also movable.” The structure must be sturdy enough to withstand wind, so it must have an adequate anchoring system. “A very important consideration is venting,” says Kevin. “Check that the structure can be opened sufficiently to allow air movement. This is vital to control temperature and humidity.”


Setting up a greenhouse

Before choosing a product, you will need to consider your budget. “Big isn’t always necessarily better,” says Linda Hill. “With vegetables, even a small unit can produce a good yield for the family.”


Choosing the site

An ideal site for the garden greenhouse is a low traffic area on a level surface. Other points to consider include the change in the angle of the sun during winter and summer, existing trees in the garden as well as access to water and, if required for your set-up, electricity.

In windy climates, wind protection, at least on one side of the greenhouse is advised. In very hot climates, your greenhouse would benefit from a position under a shady tree that will shelter it from the onslaught of the midday sun. If there are no trees to utilise, Linda suggests using shade cloth. “This will help to reduce heat inside the greenhouse in summer, while also providing insulation in cold weather,” she explains.



Some products offer a foundation which fits below the soil surface. You may also choose to build a solid foundation for your greenhouse and lay a concrete floor, but with this option, you must provide for water drainage to the outside.


If you choose not to have a solid foundation, finish the floor inside the greenhouse with a porous material like concrete stepping stones combined with gravel, pebbles or bark chips to allow water to drain away. Before finalising your flooring, consider the layout and incorporate a solid walkway inside the unit.


Controlling temperature 

Proper ventilation is essential for controlling temperature. Good air circulation through the unit will improve ventilation and increase evaporation, which lessens the chance of condensation forming. Condensation occurs when warm, moist air condenses on a cool surface. Inside the greenhouse, it increases the growth of mould and mildew, which can quickly spread. 


Place a thermometer near the middle of the greenhouse and monitor the temperatures at different times of the day. Window vents can be installed to provide ventilation. The hotter the greenhouse, the more ventilation you will require. “Most greenhouses should have vents that are equal in size to about 15% of the floor space of the unit,” says Linda. “A roof vent should open about 45° above the top of the greenhouse.” Louvres can also be installed on the side panels and fitted with automatic openers.

An alternative to extra vents is to install a thermostatically controlled exhaust fan system. The size selected will depend on the size of your greenhouse. With this option, air intake shutters are required at opposite ends of the structure.


Depending on the type of plants you want to grow and the climate in your area, heating inside the greenhouse may also be required. This is to prevent a sharp drop in temperature, especially at night and during the cold winter months. Various heating systems are available. “With more power outages on the cards, a gas heater is recommended as it is more reliable and less expensive,” says Linda. “With any system, ventilation is important to avoid a build-up of gas inside the greenhouse.” The size of your greenhouse will determine the correct number of BTUs (amount of heat) required.


Controlling humidity  

While shading and ventilation both play important roles in preventing overheating inside the greenhouse, cooling comes from evaporation or moisture inside the structure. Linda suggests a good automated system with misting nozzles to disperse water evenly around the greenhouse.” The amount of moisture required is determined by the amount of ventilation, shading provided and the climate in your area,” explains Linda. “Only apply misting during the day to avoid excess humidity at night,” says Linda. “You can install a timer and shut off the system about two hours before sunset.” Serious greenhouse hobbyists can install a system which controls temperature and humidity inside the greenhouse automatically.



Plants may be hand watered or you can install a sprinkler system or drip irrigation system inside the greenhouse. The drip irrigation system conserves water as it waters plants at the roots and helps to prevent overwatering. Rainwater can also be harvested from your house roof and piped to a tank for greenhouse irrigation. If you hand water your plants, first check if they do need water. Water only if the soil is dry to the touch.


Adding plants   

Shelving allows you to utilise all available space in the greenhouse. PVC shelving is available and easy to assemble. For greenhouses placed directly on the soil, crops and flowers can be planted directly in the beds. Plant stands, boxes and various other accessories can be added as and when your budget allows. Use a good quality potting soil for plants in containers. All pots must have drainage holes. Place stones or broken crockery at the base of the pot before you fill it with soil. Additional compost and fertiliser can be applied as needed by your individual plants.



A greenhouse does require some maintenance to keep the environment healthy and plants thriving. It is important to keep the greenhouse panels clean to allow sunlight in. Polycarbonate panels can be hosed down when necessary.


Regularly check plants for pests and diseases and take action quickly if they are detected. Because of the close proximity to other plants and optimal humidity and temperature, these can spread quickly. Whether you choose chemical or organic pest control products is a personal choice. Also consider companion planting. Many plants, especially herbs, can be used as excellent deterrents for unwanted pests. 

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