Despite the benefits of greenhouse technology, most farmers face numerous challenges. They include:
TEMPERATURE AND HUMIDITY
Many farmers fail to get good profits from greenhouse crops because they cannot manage the two important factors that determine plant growth and productivity. Greenhouse temperatures below 13 degrees Celsius and above 30 degrees Celsius in the case of dry air or higher than 30 to 35 degrees Celsius in cases of high air humidity affect growth and productivity of most crops. The optimal temperatures for production of greenhouse tomatoes, pepper and eggplant should be 15 to 30 degrees Celsius and not beyond 35 degrees Celsius.
The temperatures should be maintained at around 16 to 30 degrees Celsius during the day and 13 to 18 degrees Celsius during the night.
Some tomato farmers close the greenhouses the whole day without ventilation resulting in excessively high temperatures beyond 40 degrees Celsius that lead to reduced performance of the crops. Generally, small-scale greenhouses are prone to overheating when the sun is too hot.
Farmers should, therefore, open the doors and sides of the greenhouse from 8am to 5pm for ventilation and close them at night to reduce heat loss. Greenhouses with only insect nets on the walls are unsuitable for areas with low temperatures like the highlands of Kericho, Mau Narok, Molo and Mt Kenya regions but may have positive effects in hot and warm areas such as Mombasa, Kitui, Kisumu, Machakos and Garissa. Ideally, farmers should have a thermometer for measuring temperature inside the greenhouse for effective management.
High relative humidity resulting from crop transpiration, water evaporation from the humid soil or other growing media and condensation of water vapour on the different greenhouse surfaces is another challenge.
Relative humidity above 80 per cent coupled with high temperatures increases incidences of bacterial and fungal diseases like bacterial wilt and botrytis (gray mold) while greenhouse conditions with relative humidity below 60 per cent and high temperatures create favourable micro-climate for rapid multiplication and development of mites and insect pests.
Humidity in the greenhouse should, therefore, be regulated by ensuring adequate ventilation, maintaining high temperatures at night, using plastic mulching on planting beds and avoid wetting of the greenhouse floor.
LOSS OF SOIL FERTILITY
This is a common problem as most farmers plant one crop continuously without rotation. This can be overcome by rotating crops, for example growing onions or melons, pepper or eggplant after tomatoes and the use of both organic and inorganic fertilisers to replenish soil fertility.
Farmers can also replace greenhouse soil by bringing quality soil from outside. Adopt the technology of growing crops in pots or soilless cultures. The soils when used continuously with same crop will have a buildup of diseases, especially bacterial wilt, bacterial canker, fusarium and verticilium wilts.
Greenhouse farmers are, therefore, encouraged to treat the soil by sterilisation, solarisation or fumigation using fumigants such as Dazomet, Metam Sodium and Chloroptin.
Greenhouses get contaminated by people visiting the structures or by use of infected planting material, water and farm tools. Have a foodbath at the entry.
LACK OF QUALITY WATER SOURCES
Many parts of the country are deficient of good water sources and rivers or boreholes may also be absent. In addition, available water could be saline, chlorinated or contaminated with diseases like bacterial wilt, hence it is of poor quality.
Contaminated water is common in areas where farmers in sources of water plant crops like potatoes, tomatoes and water flowing downstream is collected by farmers for greenhouse farming. This introduces bacterial wilt in greenhouse tomato.
PESTS AND DISEASE
Management of insect pests and diseases is the biggest challenge in greenhouse farming. This depends on the type of the crops that are planted. However, generally, pathogens and insects can be established in a greenhouse very fast. They are very difficult if not impossible to get rid of effectively. For greenhouses that are covered with plastic, the use of ultraviolet-absorbing plastics can reduce insect problems.
Dr Samuel Nyalala and Carol Mutua, Department of Crops, Horticulture and Soils, Egerton University.