Tomatoes under lights
March 2, 2011 By Audrey Boulianne
Audrey Boulianne earned a bachelor’s degree from Laval University in Agronomy Science.
Audrey Boulianne earned a bachelor’s degree from Laval University in Agronomy Science. She began working at Les serres du St-Laurent in 1999 as a grower. She is now in charge of production for all of Savoura’s sites. She was a presenter during the Lighting Workshop held during last fall’s Canadian Greenhouse Conference in Toronto. The following are highlights of her talk.
|Production coordinator Audrey Boulianne
Savoura is based in Quebec and was established in 1989. We grow tomatoes under artificial lights on 16 hectares (39 acres) and we also have two hectares (five acres) of conventional greenhouses without lamps. We produce almost every type of tomato at six different sites, including beef, big TOV, cocktail and cherry. Our brand name, Savoura, is the most widely known brand in the Quebec market, with a recognition rate of 73 per cent.
|Savoura tomatoes, a market leader in Quebec. PHOTOS COURTESY SAVOURA
We built our first greenhouse in 1989 in Portneuf, near Quebec City, and it was at the time the biggest greenhouse with supplemental light in North America. It started at the same time in the Netherlands, but they now have nine times the surface area we have in Quebec for tomatoes (Fig. 1).
Photosynthesis reaction is the process behind production. Light, as much the sum as the radiation, is the catalyst to fix the carbon and make biomass and consequently tomatoes. As you can see in
Fig. 2, the natural light sum in Quebec is insufficient to meet plant needs in winter, which are around 1,500 joules/day. It’s necessary to meet those needs in light to ensure plants are growing well. The quality of set fruits is directly related to the quantity of light received according to the temperature.
Increasing the day length with supplemental light is the way to achieve a minimum flowering speed, around 0.7 clusters developed/week for beef, and 1.0 for the specialties. Those rates, even with lights, are 25 per cent lower than in the summer.
| Figure 1
| Figure 2
The Fig. 2 graph shows the quantity of supplemental light we add to the natural one with two kinds of bulbs, 400 W in the oldest greenhouse in a 10,000 Lux system installed, and 1,000 watts in the newest one with 12,500 Lux.
MANAGING SUPPLEMENTAL LIGHTING
Managing supplemental lighting is an art – it’s a big source of heat for a low contribution in light if we compare it to summer light levels.
We first need to fix the photoperiod. We start by providing light in September to increase both the day length and the light sum. We add three to four hours in the morning to get 16 hours per day. We turn them off around 250 W/m2, but we keep them on when it’s cloudy but not too warm outside – around in the low 14-15ºC range.
Otherwise, it’s too warm for the plant in relation to the light received, so you burn more sugars than you actually produce.
Then, from December to February, we provide supplemental lighting every day for 17 hours for the beef and 18 hours for the specialties. In Quebec, there are no laws with regards to light pollution yet so we can provide light any time and we don’t need dark curtains. There is no advantage in providing more than 18 hours of light – an excess of light might even result in a loss of yields.
TWENTY-FOUR AVERAGE TEMPERATURE
The 24-hour average temperature is the most important thing to deal with in winter production. This is the most effective tool to give the right balance and good energy to the plant in relation to the light it receives.
| Figure 3: Managing supplemental light
In Fig. 3, you can see a standard graph for a bright sunny day in winter. We turn the lamps on at 1 a.m. and turn them off at 6 p.m. We increase the temperature of the greenhouse before the start of the lamps to warm the plants and reach at least 17 to 18ºC under lights. We increase the ventilation settings earlier and quicker than the heating ones to avoid an excess of ventilation when the lamps turn on, because it increases the temperature by up to 3-4ºC in only 30 minutes.
If we do not vent, we take advantage of this surplus heat to warm the greenhouse and avoid cold air on the top of the plants, which can be very harmful for the crop, causing chlorosis and eventually a loss of foliar surface. It’s very important to make sure that the fruits are well heated before lamps are on to avoid condensation by a too fast increase in temperature. When it’s sunny, we use the power of the sun to make the boost; this helps to have a bigger DIF and start the pre-night from a higher temperature, with the result of having a very strong generative effect.
CARBON DIOXIDE ENRICHMENT
CO2 enrichment is very important to get all the benefits from supplemental lighting. As we see on the photosynthesis process illustration (Fig. 2), carbon dioxide is a raw material for this reaction, so increasing the rate under low light conditions gives better yields. We begin enrichment 45 minutes to an hour after the lights go on, when we see that the plant starts to assimilate it, and we stop an hour before switching off.
Variety and plant density have a big effect on light interception and then, with 24-hour average temperatures, we can manage the crop to get growth and fruit size. We keep a density of 3.6 plants/m2 for specialties and 2.4 plants/m2 maximum for beef.
De-leafing is an important measure to maintain plant balance and growth. We very often remove a leaf from the heads, behind the setting cluster, to let the light penetrate the crop to improve fruit size
and quality. If we have too many plants/m2 related to the light, and we don’t adjust the settings in relation to that, then we have very small fruit size and hollow fruits because of the lack of energy to fill the fruits completely.
On top of that, thermal screens need to be managed. We need them because we very often reach temperatures as cold as –35ºC. On days like these, we keep them closed almost all day long. But when it’s milder, higher than –10ºC, we close them after the pre-night and reopen them when we turn on the lights or at sunrise if it’s a bit more chilly.
COSTS OF SUPPLEMENTAL LIGHTING
In our experience, it costs $13/m2 more in production fees for a lit greenhouse compared to a non-lit one. There has been an increase of 132 per cent in lighting costs over the last 20 years, rising from 28 cents to 65 cents per kw/hr.
IMPORTANCE OF INTERCROPPING
Intercropping is the method in which we put new plants in between the plants of the old crop. We top the heads of the old plants when we put in the new ones, so after two months, we finish harvesting from the old plants and we start on the younger plants. This method has allowed us to have uninterrupted production for the past six years.
Depending of the type of tomato, the phytosanitary health of the crop (meaning the presence of a virus or too much botrytis), and of course market needs, we do one or two intercrops per year.
In the last two years, we did only one crop per year, from July to July. But with the spring and the summer we had in 2010, with four periods of heat waves, we lost a lot of production. The plants were very long, about 11 to 14 metres. This meant that by the time the plant was able to pump water from the roots to the heads in the morning, we got wilting – the plant closes its stomata and this prevents growth all day long, resulting in BER, fruit abortion and, of course, a loss of production. So, we plan to change the crop in February for some of our sites for the coming winter.
Even if we don’t interrupt production, we frequently remove all the old stems and leaves on the ground and conduct monitoring for botrytis and fusarium every week in order to have clean greenhouses. We have been quite successful in the control of diseases.
We use rootstock to help with water uptake during the year and have more resistance against diseases. Over the last few years, we have achieved around 70 kg/m2 for beef tomato. Even if Ep Heuvlink, from Wageningen University in the Netherlands, calculates that we could harvest 127 kg/m2 under Quebec circumstances, we have never reached more than 75 kg/m2. There are many reasons for not achieving this goal, including diseases and uncontrollable outside weather such as heat waves.
However, according to my experience, it’s more related to the non-constant light sum we receive from day to day. On days you only have 30 joules of natural light, even if you add 18 hours of supplemental lighting, you only have 534 joules for production, which is only 45 per cent of what is needed. Even if we lower the 24-hour average temperature, it’s a lost day. In Quebec, we have lots of days like this.
With electricity prices rising quite quickly, and the forecast is for even higher increases, the future of supplemental lighting is to find an alternative to HPS (high pressure sodium). We recently signed a research and development agreement with GE for a project with LEDs. We will try to find the best way to light crops with LEDs, including the right colour, the best radiation levels, etc.
Intra-crop lighting is also a way that could improve yields and help us recover the cost of supplemental lighting.
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