A farming tunnel is a structure, usually semi-circular, square or elongated, made of steel pipes and covered in polyethylene plastic in which crops can be grown, either directly in the soil or in containers with a soil-less growing medium (also known as hydroponics).

The interior of the tunnel heats up because incoming solar radiation from the sun warms the soil, plants and structures in the tunnel faster than heat can escape the structure. Tunnels thus provide a higher temperature and/or humidity than that which is naturally found in the environment.
Various crops can be grown successfully in tunnels such as tomatoes, cucumbers, sweet peppers, lettuce, spinach, cabbage, cauliflower, strawberries and even flowers.

Compared to open field farming, tunnel farming has many advantages and benefits. Listed below are some of the main factors.

Climate Control

Because tunnels are covered in plastic, they provide a controlled growing environment or climate. The main concept of growing under climate control is to create optimal growing conditions by controlling temperature (through ventilation, heating and cooling systems), humidity (through irrigation) and light (through plastic coverage). Tunnels also provide protection against the elements and extreme weather, like frost, heat, wind and hail.

Increased Crop Yield

A greater number of plants can be planted in a tunnel than on open land, because of factors such as space needed for tractors, harvesters etc. This means that even small spaces can be utilized for tunnel farming. Additionally to this, the protected environment created by the tunnel a higher yield can be produced.

Saves Water

Irrigation in the tunnels can be strictly controlled and applied only when and where it is needed, resulting in a lot less water wastage. This in turn means a financial saving for the farmer and also increases the sustainability of the crop. Because South Africa often experiences serious drought, farmers are at great risk of losing their entire crop. Farming in a tunnel means being able to control irrigation even in times of drought, and can be the difference between crop failure and success.

Protects against Pests and Diseases

The controlled environment of a tunnel provides protection against many natural pests such as birds and insects. Diseases and fungus are are also minimized by controlling moisture levels in the tunnel and by spraying certain pesticides (herbicide, fungicide and insecticide). 

Extends Growing Season

Perhaps one of the most important advantages of tunnel farming is that cops can be grown out of season. For example in some parts of the country tomatoes can only be grown in the warmer months. By planting in tunnels, extreme temperatures can be controlled. A farmer can thus plant tomatoes all year round, or at least for the largest part of the year. Being able to provide produce to the markets out of season means getting higher prices.

Substrate Quality

The substrate is the substance from which an organism (or plant) obtains its nourishment. In the case of tunnel farming, the substrate can either be the soil or some soil-less medium such as pine sawdust. The quality of the substrate can be controlled by fertilization, which can increase plant growth and productivity. 

Saves cost on Fertilisers

As with water, fertilizers can be better controlled in a setup. The fertilizer is mixed into the irrigation system allowing greater accuracy in dosage and distribution. 

In short, the combined benefits of these factors is ultimately healthier plants, higher productivity and better crop quality. By being able to control some of the risk factors associated with farming, tunnel farming can be more financially viable than other conventional methods, if managed properly.