When growing crops in an open field, it is difficult for farmers to control the environment except by providing sufficient water and minerals for the growing plants.
Greenhouses allow farmers to control the conditions in which their crops are growing. The rate of photosynthesis can be increased using:
Artificial lighting also allows photosynthesis to continue after daylight hours.
The additional cost of providing extra heating, lighting and carbon dioxide has to be weighed against the increased crop yield and the extra income it will provide. The cost should not exceed the additional income it generates for the farmer.
In practice, the farmer will need to find the optimum growing conditions for the crop, given the costs of providing extra lighting, heat and carbon dioxide.
Polythene tunnels (polytunnels) protect crops from the effects of the weather, including wind, rain and extreme temperatures. They are large enough for people and machines to work inside and can provide warmer conditions than if the crops were grown outside.