Reading is an important habit.

When it comes to vertical farming, what does one read?

A quick Google search gives just two books: Dr. Dickson Despommier’s classic, The Vertical Farm; and Plant Factory, by Dr. Toyoki Kozai, Dr. Genhua Niu, and Prof. Michiko Takagaki.

I’m sure most people are familiar with The Vertical Farm by now. It’s the book that gets people interested in vertical farming. Dickson Despommier, the ‘father of vertical farming’, presents a sustainable way to grow high-quality food for everyone, solve our waste problems, and leave nature alone. Plant Factory, written five years later, is quite obscure in comparison. It is a comprehensive yet technical book on plant factories with artificial lighting (PFALs).

Dickson Despommier presents an ideal vision of the role vertical farms can play – something for us to aspire to. Plant Factory is more like a dry collection of academic reports, giving detailed information about plant factories in their current form. It gives us the numbers, as well as recent technological findings, business practices, and challenges for the next generation of vertical farms.

Plant Factory contains a lot of information on different disciplines. What you learn and look for will depend on your situation, and this article is by no means a summary. This article is a collection of some of the new things I learned from Plant Factory. It’s more a list with brief explanations than an article. I suggest you look up anything you find interesting or don’t understand.

General observations

Although the book is called Plant Factory, Toyoki Kozai explains a vision at the beginning of the book quite similar to Dickson Despommier’s. He outlines some of the current problems faced by humanity – like malnutrition, extreme weather, and waste problems. Ideally, plant factories would act as resource-management systems, working together with other biosystems cultivating mushrooms, insects, and fish. At one point he points out how 100 hectares of farmland could be replaced with a single hectare of PFAL and 99 hectares of forest, similar to what is described in The Vertical Farm. I enjoyed seeing this holistic approach in such a detailed book.

PFALs have a reputation of being for leafy greens or medicinal plants, but the book gives quite some attention to the production of seedlings. Less sexy I suppose, but important and relevant. PFAL-grown produce is also being used in processed/canned products in parts of East Asia.

The PFAL industry seems quite developed in countries like Japan, Taiwan, and Korea – and not just because of their high number or commercial prevalence. There are many organisations and events going on. There have also been workshops/trainings with participants from all over Asia. In Korea, PFALs are not subsidised, but policy does support their implementation. In Japan, most companies seem to be distributing to supermarkets. The retail price of a head of lettuce from a Japanese PFAL is about 1.8 USD.

Although it is only in the past decade that vertical farming has gained fame, the first research into PFALs started in the 1980s.

Biology

Fundamentally, farming is about manipulating biology. Nowhere is this more explicit than in PFALs, and so a good chunk of Plant Factory is on the biology of plants.

Bacterial load is a major factor in determining the shelf life of produce. Indoor-farmed vegetables have a lower bacterial load (under 300 CFU/g) compared to their soil-farmed counterparts.

Only 40% of a lettuce crop’s weight is actually salable. Roots and outer leaves can’t be sold. If we could find a way to keep root size minimal, that would improve the efficiency of PFALs.

Algae growth is a big problem in hydroponics systems. They hog oxygen and nutrients. What I didn’t know is that algae gives a place for other pests like fungi to grow.

Some contaminants like viruses are difficult to directly detect, so contaminant indicator bacteria can be used instead.

Nitrate levels in vegetables can be reduced by feeding the plants ammonium instead, one week before harvest.

When plants absorb positive ions, they release protons into the solution to make up. This makes the solution become more acidic over time. Likewise, absorbing more negative ions makes the solution become more basic.

There’s a whole chapter on tipburn. Tipburn is generally caused by calcium deficit, which can be caused by vapour pressure deficit (VPD) not being high enough. The uptake of calcium is driven by transpiration, so not enough transpiration is a bad thing. In environments with a high VPD, the stomata can close up. Plants grown in high VPD become more resistant to disease.

If a seedling gets leggy (i.e. it has a long hypocotyl), put the cube in which it germinated on its side for a while.

Light

As long as photon flux density remains the same, pulsing light doesn’t give a lower net photosynthetic rate (NPR) than when light isn’t pulsed. As long as the photosynthetic photon flux density (PPFD) remains the same.

Green light may have a use, and is quite effective. It is reflected well and therefore scattered around. This means it penetrates the canopy much more than red or blue light would. As a result, green light has a canopy efficiency comparable to red or blue light.

Plants acclimatised to light with a high red:far-red (R:FR) ratio have a high resistance to biotic factors such as pests, diseases, and herbivores. We don’t quite know why this is, but it’s probably down to more than just morphological changes.

It turns out that having a power cut, up to an hour long, doesn’t really affect the yield, as long as you compensate by turning the lights on for a little longer.

Technology

Some interesting ideas in terms of layout. A greenhouse could be put on the roof of a PFAL, using some of the residual heat. There was also an example of a Chinese vertical farm (growing with sunlight in a greenhouse à la Sky Greens) having crops on top that can tolerate the more intense light.

Ion-selective electrodes are expensive at the moment and have a short lifespan. They use a membrane that is specific to certain ions. Artificial neural networks are another way to predict the concentration of specific ions.

Another observation is the diversity in systems. On the surface, they all look the same – racks with plants and LED lights. But there are subtleties. Some are more modular, others are automated, and so on. Each company has their claim to fame, which brings us to the next point:

Companies

Japan

Japan Dome House, Co., Ltd. uses polystyrene buildings, called ‘dome houses’. They are better insulated and more airtight than conventional buildings. I wonder why they chose a round shape for their buildings – perhaps to lower surface area, or just to differentiate themselves? Their PFALs cost 250 million yen (2.3 million USD) to build. One of their PFALs has a mushroom factory next door, used for CO 2 enrichment. Japan Dome House also has automatic loading systems for placing trays onto the PFALs.

Other companies mentioned: Spread Co., Ltd., Mirai Co., Ltd., Internationally Local and Company (InLoCo), Sci Tech Farm Co., Ltd., Berg Earth Co., Ltd.

Taiwan

Yasai-Lab Corp. runs the biggest PFAL in Taiwan, but uses none of the ‘essential 5’ parts that Kozai outlines in his book. It’s actually not a PFAL, but a plant factory with lamps. Their business model is high-volume, low price. This makes their competitors look bad, but also means that their farms aren’t as good of an investment as their competitors’.

Glonacal Green Technology Corp. is into the construction of PFALs. They do consulting as well as growing. Their design is unique in that it has moveable lights and less cables. This makes it easier to lease, and also decreases the odds of contamination.

Other companies mentioned: Cal-Com Bio Corp., Ting-Mao Bio-Technology Corp., Lee-Pin Corp.

In passing, the boopk mentions companies and organisations in other parts of the world, but these are the ones that are covered in depth.

Sustainability

Apparently PFALs are 50x as water-efficient as greenhouses, though I do wonder which greenhouses he is comparing them to.

Kozai mentions that building a PFAL emits a lot of CO 2 , since concrete and steel are the materials of choice. He raises the idea of using biobased materials such as wood instead.

Light use efficiency (i.e. the amount of energy stored in a plant compared to how much solar radiation is used) is higher in greenhouses and PFALs than on open fields. Why? Because the plant is growing under better conditions, so the light that the plant receives is used much more efficiently. This outweighs the fact that the original solar energy is lost through solar panels and LEDs. PFALs should then be more efficient in this regard than open field farming, and maybe more efficient than greenhouses. Of course, I don’t think Kozai took the energy costs for HVAC into account, but lighting is 70% of the energy cost anyway.

Challenges

This was one of my favourite chapters. Toyoki Kozai covers different areas that still need to be explored for the next generation of PFALs:

Light spectrum and the position of the lights.

Breeding.

Restricting root mass (as mentioned before).

Ever-flowering berry production.

Solar cells and renewable energy.