In the coming years the gas prices are expected to rise. This will make tomato growers try to save more
energy in the growing process to keep costs down and keep the process profitable. Using means already
available it is possible to make a big impact. An important part of this is growing under permanent plastic in the first few weeks of growth. In the winter especially there is energy to be saved. Here are a few points to consider when growing tomatoes under permanent plastic.
Plastic cover
The perforated permanent plastic cover can be used as an additional cover under the movable screens.
Working with an extra plastic cover can have a number of benefits as well as lowering the energy usage.
Because there is always a permanent plastic cover over the plants, there is no longer a chill when the screens are opened. Therefore the plant temperature always stays the same, meaning faster growing times. This is a plus in the fight against pepino mosaic virus.
In the first weeks of growth of a winter planting, mostly in freezing weather, the climate can be 'too dry'. By using a permanent plastic cover the dampness and so the micro-climate in the green house can be improved. Growing can then be done at a lower temperature, because there is always a permanent plastic cover over the tomatoes, which makes it milder for the plants.
Points of concern
Working with an additional plastic cover also has a few points to look out for. The plastic takes away some light, but the plants can be grown faster, and so the distance between the plants must be adjusted. This means a bigger distance between the plants at the start of growing (which has the additional benefit of lowering the cost of plants) and faster growth. Ultimately growth is faster, resulting in earlier production, without using any more energy.
The temperature of the plants under the plastic cover is always higher at night than with plants without cover. This means that the pre-night temperature must be lower to achieve the correct temperature and this saves energy.
During the day this the opposite; the permanent cover diffuses the sunlight, making the plant temperature lower than in a situation without a plastic cover. Due to this a higher greenhouse temperature is necessary to achieve the same temperature, but the irradiation helps with this. When growing tomatoes under a permanent cover you must always check if the plants have enough darkness at the end of the day, especially when conditions are milder outside, if not then you must control the climate. Think about a slightly higher temperature in the middle of the day and limited air above the screen. But if it is 13°C outside in January you as tomato grower will not be happy that there is permanent plastic covering hanging over your tomatoes.
To control the moisture under the plastic, and to achieve low enough pre-night temperatures, the space
above the plastic can be ventilated. A moveable screen can also manipulated, for an optimal situation. If a permanent AC-plastic is present the plants can be less protected, especially in mild weather, by the
moveable screens. In this case the energy saved in tomato growing is much less in mild winters but in colder circumstances there is an energy saving to be made. It is expected that energy saving in the range of 1-4 m3/m2 gas is achievable, depending on the outside temperatures.
As the plants get bigger, it becomes damper in the greenhouse and the outside temperatures rise, the plastic covers must be removed. Using the plastic for too long can lead to weakness in the plants. This in
combination with the moist climate can lead to an early botrytis infection. In the period with two screens (starting from 11 weeks, week 50-8) we get 24000 J diffusion of which 4kg/m2 of tomatoes can be grown.
Due to the presence of plastic there is 10% less light in the greenhouse. This gives, over the whole period, a lower production potential of 0.4kg/m2. The production, due to the faster growing time will be quicker, therefore the expectation is that the lower production will be offset by a better price. Further there can also be savings in the cost of the plants due to possibility of more space between them in the beginning and faster growing times.
In mild weather plastic covering is not a real energy saver in tomato growing and can even get in the way. In colder weather the advantages are clearer. Unfortunately we realised this too late. Don't just look for energy savings, but look for savings in combination with other possible growing benefits.
