Mid-September 2018, we began a new round of trials in our tomato demonstration greenhouse at the Delphy Improvement Centre. In this project, Merlice vine tomatoes from De Ruiter Seeds are cultivated under a combination of Philips high pressure sodium (HPS) and Philips GreenPower LED grow lights. We try to prove that a hybrid lighting solution can give growers a flexible way to apply high light levels to the crop right through early spring, while still taking advantage of the radiant heat provided by HPS lights at colder times of the year. The goal is to harvest over 55 kg/m² at the end of week 18.

After 6 years of trials with 100% LED lighting, we chose to carry out a trial with a hybrid solution that included HPS lighting. More and more companies are looking to increase light levels for their crops and are limited by the radiant heat produced by HPS lighting. At the same time, many of them find 100% LED lighting a step too far because they still want to use at least some of the radiant heat produced by HPS lighting. They are also interested in reusing their existing infrastructure to manage costs.

In this trial, our starting point was a 180 µmol/m²/s HPS installation, which uses 108 Watt/m². Half of the HPS installation (54 Watt/m²) has been replaced by LED lighting, combined with 90 µmol/m²/s Philips GreenPower LED toplighting and 55 µmol/m²/s Philips GreenPower LED interlighting, resulting in a total light level of 235 µmol/m²/s. The Philips GreenPower LED toplighting module was chosen to provide optimal light distribution over the crops. The interlighting module was chosen because it provides the maximum vertical light distribution over the crops, which produces the best crop results.

Together with our partners, Delphy Improvement Centre, De Ruiter Seeds, Cultilene and Gremon Systems we have been searching for the right cultivation strategy to achieve our goal. Each week our team is also joined by three passionate growers to provide practical insights based upon their expertise.

On 12 September 2018, we planted at a stem density of 3.3 stems/m² with a plant that had been headed at the third leaf. In previous years we started with 2 heads per plant and kept a shoot that appeared 1 or 2 weeks after planting. By starting with 3 heads per plant, we extend production by 5 weeks from this third shoot to further optimize the winter production. The 3-header we started with was a good quality plant with 3 strong and equal heads. One week after planting, the first truss was flowering. In week 49 another shoot was kept to increase the plant density to 4.0 st/m² to finish the season.

Halfway through November the crop was getting too vegetative, but the trusses were not as strong as desired. We had to put aside our goal of making it through the winter without bracing the truss and braced the weaker trusses. To see how the interlighting would affect a weak truss, we decided not to brace the trusses in one row.

After a thorough evaluation of the climate, we decided to reduce watering and the crops immediately achieved the right balance. This hybrid set-up produces less radiant heat which also gives us more control over the climate compared to a full HPS set-up. Usually a grower with full HPS would turn off the HPS lighting in early spring as the sun heats up the greenhouse. With this hybrid set-up the grower can apply grow lights much longer to extend the season.

With an average weekly production of just above 2 kg/m2 we are on target to surpass the goal of over 55 kg/m² in week 18 and arrive at an end production of around 107 kg/m2 at the end of August. The weaker trusses have also been harvested in the meantime with an average fruit weight of 155 grams which meets our quality standards. In the end, the trusses that we did not brace also became strong enough to produce acceptable quality crops.

As we approach spring, we can further exploit the extra advantages of this lighting installation. When the outside temperatures and daylight increase, we can continue to apply 100% lighting to achieve a maximum amount of light for the crops. It is only when outside radiation exceeds 450 Watts that we will switch off the HPS lighting and use LED lighting alone.

Based on the results we have seen so far, we believe that a hybrid lighting set-up offers definite advantages to growers who are looking for the flexibility to use LEDs to increase light levels, extend their illumination period and improve crop results, while still taking advantage of the radiant heat provided by HPS lights.