Unlike roses, the protected vegetable sector in Kenya is still quite small-scale. On a recent trip there I set out to find some larger nurseries growing vegetables under plastic. Ultimately I found one of 0.5 hectares; most of the others I visited were between 1,500 and 3,000 m2. In Uganda and Kenya I am involved in a project in which we are using sensor technology to make growers more aware of the climate in their greenhouses.

The basic questions we ask are: how hot and humid is it in their greenhouses and what temperature and RH are they aiming for. When we introduce this sensor technology, I notice time and again that people don’t have the faintest idea what the temperature is during the day or night, let alone how humid it really is. If I ask how hot it gets during the day, the answer I usually get is 40°C and that it doesn’t drop below around 20°C at night. But after taking measurements for a while, we discover that the night-time temperature often drops to the ideal level of 17°C and that during the day it never really gets above 37°C and is more likely to be around the 30-32°C mark. So in fact it’s a pretty reasonable climate for growing tomatoes and other fruiting vegetables.

Humidity is much more important, of course. The sensor readings show that with a little wind during the day it soon gets too dry and the vents have to be closed. Many of these greenhouses have side walls that can be opened and closed. The trick is to keep the humidity inside without letting the temperature rise too high. An additional problem is that a lot of polytunnels here are much too low: they are often still built with gutter heights of as little as 2-2.5 metres. Luckily, as European influence spreads, heights of around 4-5 metres are catching on quite quickly. However, you also have to bear in mind that they don’t have cherry pickers, so everything is done by ladder.

Other physical laws come into play too, such as the fact that heat rises to the top of the greenhouse, so you need vents in the ridge as well as in the sides. Local growers and especially greenhouse builders often still need convincing of this. In the meantime, a number of smart new greenhouses are shooting up in various places in East Africa, in which they are growing excellent tomatoes. As a consultant, it makes you proud to see these success stories; passing on knowledge is so satisfying.

Herbert Stolker.
Senior Consultant Africa.