In a few decades, indoor city farms or vertical farms have become popular for producing healthy food year-round in urban environments and harsh climates. Leafy greens and herbs are the most common crops in these facilities, but growers are looking to expand to a much wider variety of vegetables and fruits. In April 2019, we began a long-term series of research studies on high-wire tomatoes at our Philips GrowWise Centre. Our goal? Find the optimal high-wire cultivation strategy, light recipe and best-performing tomato varieties for city farms.

To develop an effective high-wire indoor farm, we built on our years of know-how and experience from both greenhouse growers and vertical farms. Over the past decade, tomato production has been optimised with high-tech automation and data management. We can use this tremendous amount of knowledge and adapt and implement the same vision and technology in an indoor farm. The same construction, irrigation and cropping systems can be applied and we can even use the same kind of bumblebees for pollination. However, we should not blindly copy every aspect from a greenhouse as there are significant differences between an indoor city farm and a conventional greenhouse.

The most critical differences between a greenhouse and an indoor high-wire system, are perhaps that the latter uses active cooling and dehumidification instead of venting and uses only LED lighting instead of mostly sunlight. It is by excluding the effects of seasonal differences in temperature, humidity and light that the optimal growing environment can be created to produce a premium product year round.

We conducted three trials. Firstly, we wanted to learn from our experiences in greenhouse tomato cultivation, to see what learnings on climate conditions we could repurpose and then optimise. Secondly, we wanted to ensure that we were growing the best-suitable varieties for growing indoor tomatoes, so we planted 20 different varieties. Finally, in our third trial, we optimised for the five most suitable ones to find the optimal growth recipe for indoor growing.

We equipped one climate-controlled chamber with Philips GreenPower LED toplighting linear and a single row of Philips GreenPower LED interlighting. This combination of top- and interlighting is a proven solution for a uniform vertical and horizontal light distribution within the canopy. The Philips GrowWise Control System is used to plan for the right light levels throughout the growth phases of the plants.

Growing successfully indoors is all about finding the right balance between light, temperature and yield. Increasing the light intensities might increase the yield, but that does not happen linearly. In The GrowWise Center we can achieve up to 1200 µmol/s/m2, but plants do not use all that light equally efficiently. With the Philips GrowWise control system, we can optimise the light intensity to the needs of the plant and seek the optimal point where the extra yield is worth more than the extra energy input from both the light and the climate system. Hence, we do not just focus on yield maximisation, but rather try to achieve the most efficient system.

To steer the plant into the right balance between vegetative and generative growth it is important to adjust the temperature accordingly. Plants can be steered into a more generative state by increasing temperature. However, overcharging could result in too generative growth. The plants will put too much energy into the fruits and overload the plant, something that will cause a dip in production later on. Growing tomatoes is like walking a tight rope: correcting too fierce when in danger of falling off one side could result in falling off the other side. In a perfect strategy, only small adjustments are needed to keep the plants well balanced. Something that is undoubtedly easier without the variable influences of the climate outside.

The trial also helped us to better understand the partitioning of energy between the heat load from the LEDs and the transpired moisture from the plants. This allows us to go beyond the optimal light-use efficiency, and rather improve the total system efficiency.

Not all tomato varieties are suitable to grow in an indoor farm and not all are of interest for this form of high-tech cultivation. Some varieties are difficult to grow in a conventional system, but they might flourish in an environment with less prevalence of disease and a more stable climate. In theory, the highest production numbers could be reached with big truss and beef tomatoes, but these are certainly not suited for every market. Lower producing cherry and cocktail tomatoes or difficult to grow ‘Coeur de Boeuf’ are usually full of taste and visually appealing and therefore can serve high-end retailers or restaurant chefs. 

In the trials we have successfully grown over 20 cherry and cocktail varieties to select the best performing, best tasting and most robust varieties that require very little handling and pruning. This makes them ideal for vertical farmers who may not have a lot of experience in growing a certain variety of tomato and the reduced labour costs will increase the city farm’s profitability.

An important benefit inherent to indoor cultivation is that we have control over plant growth. The right light spectrum, optimal light intensities, temperatures, and CO₂ levels enable you to achieve an excellent balance of fruit load and vegetative plant growth. Having control over all these growth parameters means that we can really optimise for both production and taste. Moreover, the growth recipes that we have developed are reproducible. This creates a consistent cultivation environment that allows reliable yield prediction. It gives growers more flexibility and security over their production and enables them to steer yields to meet market demands. In addition, creating the right environment also provides the potential to actively steer specific characteristics, such as sugar content, acidity, vitamins and other compounds, to meet preferences for taste and nutritional value, respectively.

Building a farm in an area that already has plenty of greenhouse or open field production would, of course, not be very profitable. Yet, there are many places where locally grown tomatoes are not available all year round because of the harsh climate or other factors. A farm in the Middle East, for example, makes perfect sense: lots of water can be saved and the close proximity to the consumer means that the product can be super-fresh and of high quality. Our trials show that it is possible to grow fresh, tasty tomatoes in an indoor, climate-controlled facility year round, without the use of pesticides.

Based on the results of the trials, we now have a blueprint for growing great-tasting high-wire tomatoes in a city farm with a fully controllable environment. This opens up new opportunities to grow fresh tomatoes year-round in extremely hot and cold climates. We can advise growers on how to set up their system and how to choose the best variety based on their customers and market. With the research baseline in place, we can now focus on trials for specific concepts and markets.

Please learn about our Philips GrowWise Center or read our other blogs on growing tomatoes.