Sample Applications > Vegetables

New Light Developments For Tomato Growers

Assimilation lighting for tomatoes has been tried and tailed in the past. Now, with more knowledge and experience this type of cultivation is being revived and the potential gain looks much more promising.

By Harry Stijger, HS Communication

Two years ago, Hortilux Schréder, of Monster , The Netherlands, initiated plans to revive the concept of lighting vegetable crops. Earlier experiments with assimilation light and tomatoes had been unsuccessful. However, due to new lighting techniques and production methods, such as planting in between plants and growing in gutters, success this time seemed feasible. Last year, based on experiences in Canada and Scandinavia, and model calculations made by the Dutch Testing Station for Flowers and Greenhouse Vegetables (then the PBG, now the PPO), the company came up with a practical solution for growing tomatoes with supplementary lighting. Light is the most limiting link in the growth factor chain and therefore the first aspect to cause a loss of growth. This means that it is often necessary to top-up natural daylight because during the dark period of the year there is a deficiency in both light intensity and length of daylight. In countries which lie closer to the equator, the light intensity is sufficient but can still benefit by lengthening the daylight hours. The question is how much extra light is required? If this is based on only plant physiology, the first measure must be to ensure that the plant appears satisfied. However as Figure 1 demonstrates the level of assimilation lighting required, from a practical point of view, cannot be achieved. For the optimal light level, it is necessary to consider how much plant growth can be achieved by a certain number of hours of assimilation lighting. In order to make a comparison the extra growth achievable by having assimilation lighting has been calculated as though it came from natural light. (See Table 1).

Equating with Natural Light

In Holland, during the winter month of December, the average amount of radiation coming from outside, measured with a Kipp solarmetre, is 183 J/cm2 (see Table 1, column 4). When the yearly light requirement is 10,000 lux and 3000 hours, the assimilation lighting has to be on for 18 hours per day (column 5). This provides 162 J/cm2 PAR (growth) light (column 7). Further calculations indicate that this is equivalent to radiation from outside of 492 J/cm2 (column 8, "outside"). This gives an increase over natural light of 269% (column 9) and gives a total daily sum of 675 J/cm2 (column 10). This means that in December it is possible to provide light during the day which is comparable with a natural day of light in October or in early spring at the end of February, beginning of March. The same table shows that if the assimilation lighting is 15,000 lux then the total daily sum is 922 J/cm2. This is a better value for tomatoes because there are some dark days in winter when the amount of radiation received is just 83 J/cm2. The daily total is then 100 J/cm2 less at 822 J/cm2. This value is very close to the minimum value required for tomatoes.

A rule of thumb is that in addition to a maintenance requirement of 100 J/cm2 for each plant, 100 J/cm2 is also required for each tomato cluster. That means, for an average of seven clusters, 700 J/cm2 is needed. However, in all the trials carried out previously, the minimum value of 800 J/cm2 was not supplied so the results were not satisfactory.

Putting into Practice

Calculations based on a model made by PPO, show that tomatoes cultivated in the Netherlands, lit with 10,000 lux can produce 92 kg/m2 annually, and with 15,000 lux it is possible to produce 104 kg/m2. When production is unlit the harvest Is about 59kg/m2. This is assuming that there is a six hour dark period during the night and that the lights go out when they produce more than 200 W/m2. Based on favorable results achieved in practice so far, the experimental phase would now appear to be over. This year several different tomato growers, involving around 8ha in Holland and 9ha in North America, have set up large lighting programs. Cultivation techniques such as the number of plants per m2, the lighting period, lighting level and the climate control need to be further optimized in order to maximize the potential gain. The key to this will be knowledge and experience. The assimilation lamps give off warmth, which therefore produce a slight temperature increase in the greenhouse, The rule of thumb which says every 1000 lux produces a one degree temperature increase is only valid for an empty greenhouse or one in which the crop transpires only slightly. The radiation from the lamps activates the plants so that the evaporation energy, independent of the Leaf Area Index; consumes 30-40% of the energy from the lamps. Thus there is 60-70% over and therefore 1000 lux produces a temperature increase of 0.6-0.7 degree.

New Developments

Hortilux Schréder has developed a new lighting system that intercepts half as much light coming into the greenhouse from outside than current systems. With this new HS Remote System, the lamp and reflector are separate from the switch apparatus, the so- called Remote Body. This Remote Body controls the lamp and hangs in a C-profile with in the crop, above a hanging gutter or under the cultivation pipes, so that it no longer intercepts light from outside. Only the reflector and lamp holder hang above the crop. Because this is lightweight, the system can be held in place by two thin steel wires which further reduce light interception. When comparing this system with those currently available, plants receive about 2.5% more light from outside, when the light is 15,000 lux. Also, warmth from the Remote Body, about 45 Watts per body, is responsible for about 7% of the total heat production. Because the warmth is at a good height for the crop, it is more efficiently used. The only point of concern is that the steel wire support system does put some extra pressure on the outer walls of the greenhouse.

Advantages of Lighting

The extra productivity which comes from this sort of lit cultivation must amply pay back the investment cost of the lighting. Fortunately, reasonable payback would appear to be possible for an installation producing 10,000 to 15,000 lux. Actual figures are not being made public but testimony to the benefits must be the increasing number of growers who are installing lighting systems. There are several advantages: better energy utilization and energy use per kg of product; and a stronger crop and less chance of pests and diseases so it is easier for biological crop protection to be effective. There is more production per square meter and therefore more efficient space utilization. There is also more uniform quality and taste which gives better marketing possibilities. Conversely, to maximize the benefit a good marketing strategy is necessary. However, this kind of cultivation requires extra attention and it is a big change for growers to implement, a change which can be compared with the switch from growing in soil to growing in substrates.

Source: Fruit&Veg Tech – vol.1 nr.1 2001 - 39

Lighten Up!

Approximately two years ago Hortilux Schréder instigated trials on light in glasshouse crop production. Previous experiments with tomatoes had not been altogether successful, but new lighting techniques and developments like interplanting and 'gutter' cultivation promised at least a realistic chance of success.

Supported by Canadian and Scandinavian experiences and model calculations by the PPO research station at Naaldwijk in the Netherlands, an trial was set up. One of the preconditions was the availability of a suitable glasshouse which could be lit with approximately 11,000 lux and with climate control adjusted separately.

After the fist season, and despite a number of inadequacies, the results were positive and this prompted a number of growers to put the results into practice. This year about 8ha of tomato crops are being grown under this high light regime in the Netherlands. It is also being used on 2ha of crop in France and 9ha in North America (US and Canada).

But..how much light?

There was, of course, no argument about the importance of light to crop development. The most important link in the chain of growth factors, it is also the most critical factor if it is deficient. By increasing natural daylight with supplementary lighting growers in northern latitudes overcome seasonal declines in sunlight intensity and duration. But the technology also has an application in more southerly countries where extending daylength yields considerable benefits too.

The significant question is: how much supplementary light is appropriate in any given crop situation? If we were to approach this issue from the plant's physiological angle, our first aim would be to prevent saturation symptoms. The growth curve for tomatoes (see graph) illustrates that this level cannot be realized through assimilation lighting.

So what is the optimum lighting level for salad crops grown in north west Europe? To answer this question, we have to calculate the increase of the supplementary lighting sum, realized through assimilation lighting, for a given number of hours when the crop is lit. To facilitate the comparison we will convert the supplementary lighting sum of the assimilation lighting to natural light figures ( see table).

Tomato 6 hours of darkness / night lights out over 200W/m2 CO2 1,000ppm Cucumber 4 hours of darkness / night lights out over 200W/m2 CO2 800ppm Peppers 4 hours of darkness/night lights out over 200W/m2 CO2 1,000ppm

Yield benefit

The next question is, what how much more yield can be expected from the extra lighting. For this purpose Naaldwijk has developed an ECP (standing for energy CO2 production) model that predicts yields (in kg/m2) for different lighting levels.

This calculation accepts the anomaly that yields are as high as they are without supplementary lighting. However theoretical calculations assume stable conditions, something that is exceptional in practice. And practical experience with tomato crops receiving supplementary lighting indicates that the calculated yield benefits are realistic. Results from the Belgian research station at Meerle suggest it should be possible to achieve a projected yield in tomatoes of 92 kg/m2/year. This was confirmed in trials in 1999 and 2000. On the basis of these results larger commercial projects started this year.

Other key questions, such as the optimum number of plants and heads/m2, the duration of lighting phases, lighting levels and accompanying climate settings, still have to be determined. These are key marginal factors.

Cost calculations

Clearly the benefits of supplementary lighting will have to outweigh the capital and running costs. The requirement for lighting usually occurs at times of relatively high cost. From what we know, however, it should be the case that the rewards exceed the costs. The panel opposite gives an indication of the costs of a lighting installation on a yearly basis in the Netherlands. Calculations have shown that Dutch conditions are favorable for a partial purchase of electrical energy from the mains and partial internal generation with a combined heat and power plant at a ratio of approximately 40:60. A precondition for this is that the heat utilization of the CHP is optimized. This means that when lighting in relatively mild weather conditions, heat from the CHP has to be transported to an unlit section of the glasshouse. A rule of thumb for each 1,000 lux, a lC temperature rise will be generated, provided the glasshouses are empty or contain plants with limited crop evaporation. The radiation of the lamps will activate the plants and the evaporation energy will consume 30 to 40% (depending on the leaf area index) of the energy of the lamps. This leaves 60 to 70% for the energy balance, which will result in a temperature rise of 0.6 to 0.7C/1,000 lux. This means that 'energy venting' will not be common for fast growing crops with a high level of evaporation.

Benefits

There are six key advantages of supplementary lighting:

• Energy is used more effectively and overall use/kg of product will be lower.
• Crop growth will be more vigorous leading to a lower risk of disease and aiding the process of biological control.
• Higher yields means a more efficient use of space.
• Fruit tends to be more consistent in terms of quality and taste.
• Full year-round production is possible in northern Europe.
• Labour planning can be improved.

Column 8 provides an overview of the values described opposite.

When added to the normal radiation sum and measured with a Kipp solarimeter (col 4), it provides the total daily sum (col 10). Example: in December the average radiation sum is 183 J/cm2. During this month, 10,000 lux of assimilation lighting with lamps on for 18 hours (a total of 3,000 hours annually, (col 5) adds 162 J/cm2 (col 7) PAR light. Converted to outside conditions, this is equal to 492 J/cm2 (col 8), an increase of 269% (col 9), resulting in a total daily sum of 675 J/cm2 (col 10). This allows us to recreate the conditions of a day in October or the end of February (col 4). If 15,000 lux were given, the total daily sum in December would be 922 J/cm2. This is a much more appealing figure for tomato production since in December days can typically register a radiation sum of 100 J/cm2 lower than 183 J/cm2. This means that the total radiation-sum is also 100 J/cm2 less while remaining just over 800 J/cm2, which is close to the standard minimum value for tomatoes. A rule of thumb states that, besides the maintenance value of 100 J/cm2, an additional 100 J/cm2 per truss is required. In earlier experiments with tomato lighting (Bakker) this value was never reached, which explains why these tests never yielded satisfactory results.

For growers adopting supplementary lighting represents a considerable change, comparable in some eyes to the shift out of the border soil and into substrates. Another important consideration is the social acceptability of extra lighting in glasshouses. The positive aspects of the technology -like energy efficiency, food safety, etc -will clearly have to be presented well. Answering the concerns of the public and the authorities will be best achieved if we do not to wait for the questions to be asked, but to be pro-active and initiate the information process. The first steps on this road have already been taken.

Further Applications for Vegetables

Here's a list of other Vegetable crops that can benefit from P.L. Light Systems products.

Vegetables

• Eggplant (Aubergine)
• Cucumber
• Lettuce
• Sweet pepper