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How to find the right setup for propagation in vertical farms

"Adopting vertical farms for vegetable propagation is used more frequently by propagators as the same m2 can be used trifold or even more. However, there are so many setup options that come with benefits or consequences. Let me explain to you why," says Hans van Herk, Founder of Propagation Solutions.

Firstly, Hans highlights that processes like germination, grafting, and pinching in vegetable crops like tomatoes, peppers, cucumbers, watermelons, or eggplants can all be done in a vertical farm. The main benefits of propagation in vertical farms are the high plant density per square meter, total control of climate, light, and humidity, protection against sun radiation, and strict planning capabilities.

"However, despite its many benefits it has drawbacks as well, Hans emphasizes. "Low usability and higher costs than traditional nursery methods using tents." Despite these 'negatives', vertical farming is gaining importance for risk management, particularly as growers plant more in the summer. This is crucial for crops like tomatoes, whose prices typically increase as Christmas approaches. "Therefore, it all depends on your setup, product and the client request," Hans points out.

Hans van Herk

The germination process
Germination of vegetable plants typically occurs in a germ room, which is a simple setup with heating and misting systems. This dark room maintains a temperature of around 22°C and humidity between 85-90%. Seeds usually crack after 2-4 days, requiring timely removal to prevent quality issues. Regular daily checks are essential, as some varieties germinate faster than expected. To ensure uniform temperature and germination speed, trays should be spaced 1-2 cm apart, which can be managed using racks or bamboo sticks.

"Keeping CapEx in mind, these systems can be relatively simple and aren't that expensive. However, strict daily control is needed as varieties can surprise you really by being much faster than the standard."

Various set-up options
All plants in one room
In a large room, trays on trolleys can be wrapped with stretch foil to maintain 100% humidity, or tunnels can be created on each layer. Each grafting day requires separate acclimatization. Fixed lights can be placed near the trolleys, or lights can be installed on each layer.

Fluorescent lights, although commonly used for their low light output, generate heat, limiting the number of layers and causing uneven evaporation and plant growth. LEDs are preferable as they remain cool and provide a blue spectrum (15-20%) to encourage plant compactness. "However, humidity affects compactness more significantly than light spectrum, making fast acclimatization crucial for promoting compact growth," Hans adds.

One room per day
According to Hans, using a dedicated room for each grafting day "is initially more expensive but offers long-term efficiency benefits. At least six rooms are needed, with an extra room for flexibility or Sunday grafting. While the initial cost is often scrutinized, ongoing labor and material costs for daily acclimatization in a single large room are frequently overlooked. Separate rooms improve logistics, growth control, and hygiene, making the overall process more manageable and efficient."

Propagation Solutions | LinkedIn

What number of lights and layers do I need?
With advancements in LED technology, the number of layers in vertical farming can be significantly increased. Recent tests have shown success with 7-8 layers using LED tubes per layer. In contrast, early 2000s fluorescent lights limited layers to 4-5 due to heat production, causing uneven temperature distribution and non-uniform plant batches.

Fluorescent lighting
Fluorescent lights were used previously to provide the necessary light levels but were limited to 35-40 cm height between layers because of the heat they generated. This limitation meant only 4-5 layers could fit on a Danish trolley, clearly restricting the system's efficiency. The fluorescent lights caused uneven heating, with a 1.5-2°C increase directly below the lights and only a 0.5-1°C increase at the borders. Over 5-6 days with 18 hours per day of lighting, this temperature difference accumulated significantly, leading to non-uniform plant batches. Consequently, trials with 6-7 layers were soon discontinued due to these issues.

LED lighting
In contrast, LEDs do not produce significant heat, allowing for more layers and increased plant density. They are suitable for both daily and all-in-one vertical healing farms. LEDs are effective in high-humidity environments (95-100%), which are essential for the healing of grafted plants by preventing evaporation and promoting callus formation.

However, Hans points out that despite the advantages of blue light in keeping plants compact, "if the high humidity level stays too long, despite a high level of blue light, the plants will stretch." Therefore, while the right spectrum is crucial, other climate conditions must also be considered to maintain plant compactness and ensure optimal growth.

Simple or highly extended healing rooms?
Healing rooms can be made quite simply with sandwich panel walls, a concrete floor, a heating system, and a humidifier. Using Danish trolleys adds flexibility and simplicity. However, this basic setup may be insufficient for the high value of plants on a trolley. With 6 layers per trolley, 4 trays per layer, and 120 plants per tray, one trolley can hold 2,880 plants. During peak season, with 20 trolleys per day, that's 57,600 plants daily—a substantial number.

Given the high and rising costs of seeds, labor, and materials, investing in a high-tech facility is sensible to protect these valuable plants. Hans emphasizes the importance of such investments: "With high-value batches of tomato plants in summer, it's important to secure things." A high-tech facility offers better risk management, particularly during heat waves and thunderstorms, which are becoming increasingly common. This reliability in difficult conditions justifies the investment in more advanced healing rooms.

Humidity: an enemy or friend?
"Humidity in the context of plant propagation is both an ally and an opponent," as Hans likes to see it. Initially, high humidity (98-100%) is crucial after grafting or pinching to halt evaporation and allow plants to join. This period, lasting a few days, requires maintaining a temperature of 22-23°C and providing minimal light (40-50 Joules per 24 hours).

However, as plants strengthen and the graft heals, maintaining high humidity becomes detrimental. If the humidity remains too high, plants cannot evaporate properly, leading to issues like guttation, and oedema in tomatoes, or even Botrytis. Excessive humidity also causes plants to stretch, which complicates handling and increases labor needs for additional support. Stretched plants are also less visually appealing.

As Hans points out, "Plants tell you that the 'canal' is open, and the roots are pumping the water into the top" when they start showing vertical leaves, root growth at the tray bottoms, and guttation. This indicates it's time to acclimatize the plants by gradually reducing humidity. Opening or cutting the plastic in small increments (1-2%) allows plants to adjust without wilting, preparing them for transplanting over 2–3 days.

Every vertical farm will require different actions based on actual conditions, making it essential to adapt protocols accordingly. Hans emphasizes that while protocols are useful, ultimately, "plants are 'playing the final card'" and dictate the necessary adjustments.

For more information:
Propagation Solutions
Hans van Herk, Founder and director
[email protected]
+ 31 6 2091 8100