Last Thursday I attended Circular Horticulture 2025, an event organised by Wageningen Research’s Greenhouse Horticulture business unit. Contrasting research done at the university, the business unit’s research is more practical and connected to industry. A lot of their research is done at test facilities in Bleiswijk, in Westland.
The purpose of this event was to present the business unit’s plans for circular horticulture, aiming for 2025. As soon as I saw the advert for this event, I contacted senior researcher Dr Wouter Verkerke, the event’s main organiser. It was all very interesting – many thanks.
In this article I will focus on Greenhouse Horticulture’s main presentation, given by Alexander Boedijn and Eric Poot.
Eric Poot 
Alexander Boedijn
Alexander and Eric gave the same presentation, in English and Dutch respectively. The topic was their vision and goals for future research within circular horticulture.
The presentation started with the United Nations’ sustainable development goals (for 2030), and how circular horticulture can play a role. The three main goals relevant to circular horticulture were:
- Zero Hunger (2)
- Responsible Consumption and Production (12)
- Decent Work and Economic Growth (8)
To most people reading this, how circular horticulture relates to the first two goals speaks for itself. Not only are there benefits to local food production, but state-of-the-art greenhouses save water. Outdoors, it takes 60 litres of water to grow 1 kg of tomatoes. In a Dutch greenhouse, this can be as low as 10 litres — whilst giving yields of 80-100 kg per square metre annually. As for the third goal, on work and economic growth, exporting Dutch knowledge can not only help the Netherlands but create new jobs, industries, and livelihoods elsewhere.
Next, three models of the economy and its resource flows were presented: linear, linear with feedback loops, and circular. The second model is essentially a linear model where certain flows are recycled, but it is still sort of linear.
Then came the main mantra of Alexander and Eric’s presentation. Circular horticulture should be three things: efficient, clean, and connected. These three goals are scale-dependent*. On the scale of a greenhouse, efficiency is the main goal. On a neighbourhood scale, it is important to stay clean and minimise environmental damage. Lastly, on the level of the industry or even a city, different processes should be connected, so that the outputs of one process can become inputs for the next, part of a product, or connected to natural processes.
Connecting different industries – or different production systems, like in an ecosystems-based approach – requires a different way of thinking. Alexander gave an interesting example of this. Companies are using tomato stems for packaging. This not only requires the process of turning stems into packaging to work well – it also means the grower may need to change their approach. Suddenly it becomes important to grow tomatoes with stem quality in mind, even if it is just a by-product.
This brings us to four (sequential, I think) main areas the Greenhouse Horticulture business unit wants to explore:
- Approach to the industry
- Redesign of greenhouse systems
- Cross-overs, or the connecting of different organisms and production systems, like in aquaponics-mushrooms-insects
- New business cases for the industry, presumably following on from the other three areas
Alexander and Eric then showed two Sankey diagrams of the water and nutrient flows going in and out of a typical tomato greenhouse.
The first diagram was quantitative, based on mass. The vast majority of mass lost from a greenhouse is the water lost through transpiration, 9.6 litres per kg of produce. 0.4 litres are lost through leakage and leaching, and 0.95 litres end up in a kg of produce (yes, tomatoes are 95% water, wasserbombe). In terms of nutrients, 67% ends up in the fruits and 28% in other plant material. Only 5% is leached.
The next diagram was a qualitative version of the first, based on environmental impact. Here, the sizes of the arrows were completely different. Whilst a big number, 9.6 litres of evaporation doesn’t really do much harm. On the other hand, that 5% of nutrients leached can be very damaging, especially if these nutrients come from finite sources.
Alexander and Eric’s presentations then ended with a look at some cross-over concepts. The first was an aquaponics system being experimented on in Bleiswijk, for data to validate a model, amongst other things. I liked how they acknowledged the need for unsustainable fish meal and supplementary nutrients in this system (aquaponics has its own inputs of course).
The second was less conventional. This is a 10-year-old concept that is being revived due to increasing interest in this whole circularity thing. The main systems are a pig farm, a tomato greenhouse, an arable field of sugar beets, and an algae pond. These systems are connected through nutrient flows, all through a biodigester acting as a central hub. In terms of energy, the greenhouse’s excess heat and solar energy can be harvested. Also, the sugar beets can be sent to the biodigester to heat other systems (which brings the food-vs-fuel debate to mind).
At the end, two other projects were mentioned: Kas2030 and Aqua ReUse. Kas2030 is a project (in Bleiswijk, of course) aiming for a zero fossil-fuel greenhouse whilst improving water use efficiency as well. Aqua ReUse is an organisation with many interesting projects. One of the projects mentioned involved sending all the leach water of a region’s greenhouse to an outdoor algae pond.
After this main presentation, there were pitches. Students were invited to join as well, and Wouter kindly gave me a slot to present some of the broad principles of ecosystems I like to write about on this blog. The five other pitches had some great gems of information – you’ll have to see in the next article!
* Nassim Taleb has a similar worldview that he applies to economic systems. I like this scale-dependent way of looking at the world.