Can green also be added to a Dutch town’s iconic blue and white colours? Delft’s parks and recreation department candidly admits that it is not easy to turn the inner city’s omnipresent hardscaping into a lush green oasis.
While tracking the right metrics can contribute to more meaningful urban green spaces, Joost van der Werf, the city arborist in Delft, believes that the core of his work lies in helping city trees to live longer by promoting species-rich, climate-resilient, large, and healthy trees, ensuring proper planting practices, and carefully maintaining and protecting existing trees.
In the run-up to the annual UN Climate Change Conference (COP30) in Belém, Brazil, the Dutch city of Delft hosted a ‘Healthy Trees for a Liveable and Sustainable Community’ lecture. The event was held in De Veste Theatre in Delft in November 2025, featuring Joost van der Werf, whose official job title reads as ‘Delft Urban Greenery, Water and Recreation Advisor’.
Van der Werf studied landscape architecture in Wageningen. He worked for a Boskoop-based tree nursery, Wezelenburg, and as a manager at the Green and Open Space Services of the cities of Boskoop and Alphen aan den Rijn. In 2024, he joined the municipality of Delft. His technical team comprises six people who together seek to translate the city’s lofty visions in the fields of climate change adaptation, urban greenery, biodiversity, physical activity, and space for play into meaningful tree care and maintenance.
How Green Is Delft?
Situated halfway between Rotterdam and The Hague, Delft is a charming town with a population of 110,000. It is famous for its Delft Blue pottery, featuring intricate blue-and-white patterns, and is also the hometown of Johannes Vermeer, one of the most celebrated Dutch artists of the 17th century. Today, many people tout the city as a Tech Hub, as it hosts one of the world’s most prestigious engineering and technology universities. Despite these attributes, the city is not widely associated with urban greenery.
Asked about the quality and quantity of the city’s green lungs, van der Werf says, “In 2021, Delft ranked as the second-greenest city of the Netherlands in the Green City Challenge. It was also awarded the ‘Tree Cities of the Netherlands’ status in the ‘large cities’ category in 2020. So, the starting point of our city trees analysis appears to be good. However, there’s a clear divide between the different neighbourhoods. Public green spaces make up around 13 per cent of the city, and most of that is made up by the urban forest ‘De Delftse Hout’, Delft’s greenest hotspot in the north east, and swaths of urban forestry across the city covering a total area of one million m².
“Districts such as Tanthof and Buitenhof have incorporated traditionally nature-based solutions, including urban forestry and large trees in their design. In other places, problems arise with, for example, towering Populus spp. (poplars), planted 60 to 70 years ago. They were put into the ground according to the principle of thinning, a silvicultural operation to reduce tree density in a stand [Editor: a group of trees growing together] to improve the quality and growth of the remaining trees.”
Van der Werf explains that thinning can also aim to alter the species composition of a stand, for example, by removing Populus after 30 years to give way to a more sustainable, slow-growing understory [Editor: younger trees growing beneath the main canopy] comprising Acer, Fagus, or Quercus.
The problem is that Delft’s poplar stands have never been properly thinned, creating potentially dangerous situations. A study by the International Society of Arboriculture (ISA) reveals, for example, that poplar trees are among the species most often cited in urban tree failure reports. Unstable branches or falling trees, altogether, can pose serious risks.
Van der Werf recalls, “One particular incident sticks in my mind—when in a residential area (Molukkenstraat) a large poplar fell on a house, causing huge damage. So, we examined the failure potential of the city’s 2,600 poplar trees and found that 600 pose a potential fall risk and need to be removed within two to three years.”
Green Guidelines
Van der Werf is enthusiastic about the contributions of trees to our landscape and highlights the benefits of urban trees, including year-round visual appeal, cooler streets, cleaner air, improved mental and physical health, better stormwater management, and greater biodiversity.
While scientific literature provides overwhelming evidence that trees are good for people and the planet, without specific guidelines on which numbers, species, sites, and care and maintenance to focus on, city councils, national governments, and international organisations often lack a clear framework for urban greening.
That is, until Dutch Dr Cecil Konijnendijk, co-director of the Nature Based Solutions Institute, launched his 3-30-300 rule to connect science, policy, and practice in urban forestry.
Van der Werf elaborates, “Delft uses the formula. However, when planting medium-sized trees in the city centre, you need more than a couple of years before reaching the 30-metre mark. Regarding the required 300 metres to the nearest park, we are currently transforming a former downtown parking lot, Gasthuisplaats, into a city park. Gradually creating a less car-centric and more people-friendly city centre allows the community to re-imagine their streets with more urban greenery.”
Van der Werf says that while the 3-30-300 rule provides comprehensive guidance, it also risks becoming outdated—particularly the 30 figure—as technology advances at an unprecedented pace.
More recently, the Netherlands Tree Standardisation Institute has switched from square metres to cubic metres when measuring tree crown volume using LiDAR, satellite, aerial, or drone imagery. Measuring the three-dimensional space occupied by a tree crown helps quantify its size, shape, and density, and better estimate a tree’s ecosystem services, such as carbon sequestration, oxygen production, water interception, and air purification.
The Netherlands Tree Standard strives for 2.2 m³ tree crown volume per m² land area. How does this work in practice? Imagine a 5,000 m² plot of land ready to be planted with trees. Adhering to the tree crown in cubic metres principles, you must seek to achieve 11,000 m³ tree crown volume.
To reach this goal, for example, you can use an eight-metre-tall plane tree (Platanus hispanica) with a 30-35cm stem circumference. Because such a tree provides a tree crown volume of approximately 35 m³, you will need 315 of them. If you have only young trees at your disposal with a 14-16 cm stem circumference, you will need to plant 2,000, as each young tree represents a tree crown volume of only 5 m³. [Source: Van den Berk Tree Nurseries]
Van der Werf adds, “For the greatest benefits, it is recommended to plant species of larger trees wherever possible. Understandably, a plane tree with a large and dense canopy performs better than a small decorative Malus tree. Recently, the province of South Holland launched the aptly named ‘One large tree equals 100 small trees’ campaign. As a tree grows bigger, the provision of benefits increases.”
For the first time, Dutch consultancy firm Cobra has mapped the country’s tree crown cubic volume (m³) per m² and per municipality. Such maps allow city councils and government officials to prioritise areas that will benefit most from additional tree planting and identify areas that need more conservation efforts for existing trees.
Based on Cobra’s Bomennorm Map, the municipality of Delft has a tree crown volume of 2.4 m³ per m², while the national standard is 2.2 m³ per m². The segmentation model groups the country’s municipalities into one of five categories: very strong, strong, median, weak, and very weak. Delft is classified as strong.
At the same time, Delft conducted a tree census using a data model that clearly defines what constitutes a tree. Van der Werf elaborates, “In line with our definition, a tree stands individually in a street or park, the remainder are defined as urban woodland or hedgerows; groups or stands of what in Dutch is called ‘boomvormers’ [Editor: small, young trees that tend to evolve into a tree with a single main trunk], and shrubs [Editor: with multiple stems emerging from the base of the plant].”
At first glance, stand-alone trees and urban woodland look similar. The difference in Delft is that isolated trees undergo a security and health check every three years, and urban woodland every six years.
Van der Werf continues, “Maintaining urban forestry mostly entails thinning the ‘boomvormers’ with genera such as Alnus, Quercus, Fraxinus, Betula and Salix being cut a few centimetres above ground. In doing so, more light can enter the understory, boosting biodiversity and a species-rich vegetation. Frankly speaking, lines can be blurred. Take the Prunus serrulata ‘Amagogawa’ trees near the railway station. The species typically grows with a columnar habit when young, becoming vase-shaped with age. It reaches a maximum of only six metres. Yet, as they are individually planted, they are classified as trees.”
According to van der Werf, the entire city of Delft has approximately 30,000 trees, of which the inner city has 2,646. Life is not easy for them, van der Werf elaborates, “The average tree in Dutch city centres does not become older than 35 years, and Delft is no different. Such a short-lived existence is regrettable, as after 35 years it has reached only 25 per cent of its crown volume. It is even more of a shame to acknowledge that planting a tree is not an instant carbon fix. A tree only reaches zero net after 30 years when carbon removal is high, and all inputs for its growth are offset. What’s more, most of the trees start contributing to biodiversity only after 30 years.”
Pests And Diseases
The reasons for the dire state of Delft’s inner-city trees are multifaceted. In Delft, the Salicaceae (Salix alba), Rosaceae (Prunus, Malus), Sapindaceae (Acer and Aesculus) and Malvaceae (Tilia) families are a common sight.
Yet, urban green spaces dominated by a few common species are less healthy than species-rich vegetation. With species homogeneity, pests and diseases pose an even more devastating threat.
Van der Werf explains, “When infestations of horse chestnut leaf miner (Cameraria ohridella) build up, the leaves become brown and shrivelled. Heavily affected trees drop their leaves early. Over the years, the pest has significantly weakened the tree. Delft’s horse chestnuts are also under attack by the bacterium Pseudomonas syringae, or bleeding canker. Another lethal threat is ash dieback (Hymenoscyphus fraxineus or Chalara fraxinea). Ash trees have been among the most planted trees, and over the past decade, we have seen massive dieback. It isn’t my task here to predict the next disease or pest, but to create species-rich settings and thus reduce the impact of the next phyto-threat.”
To critics who are saying that the city centre’s droopy trees are the result of deferred maintenance, van der Werf’s answer is utterly resolute. “With regards to tree disease, this is absolutely not the case. There’s no stopping ash dieback, and horse chestnut bleeding canker wreaks havoc across the country and far beyond. We have literally tried everything to conserve our chestnut trees, teaming up with universities and other Dutch towns to exchange knowledge. This disease has nothing to do with deferred maintenance. Consider that once the mature tree crown has been reached, a large chestnut tree doesn’t require much maintenance.”
Providing a guideline for more diverse urban greening is the 10-20-30 rule proposed by the late forestry expert Dr Frank Santamour, research geneticist at the US National Arboretum. The rule was first presented in 1990 and suggests an urban tree population should include no more than ten per cent of any one species, 20 per cent of any one genus, or 30 per cent of any family.
According to van der Werf, Delft stays well within Santamour’s criteria. Not without reason: the city has seen a massive dieback of ash trees, horse chestnuts, and Ulmus, which, up to ten years ago, still dominated the city centre. Since these trees have been replaced mostly by alternative new species, the city now follows the Santamour rules even better.
Tree Planning For A More Challenging Climate
On top of the biodiversity issues, climate change is taking its toll, prompting van der Werf and his team to plant species that can better withstand hotter temperatures and unpredictable downpours. He says, “In the 2024/2025 season, we have removed and replaced 427 trees, of which 26 are rarely used species and 29 are completely new, including Acer pseudoplatanus ‘Erectum’, Gleditsia triacanthos ‘Shademaster’, Platanus × hispanica ‘Malburg’ and Tilia tomentosa ‘Doornik’ and Liquidambar styraciflua ‘Moraine’. During one planting season, we have added eight per cent new trees to our tree portfolio.”
In Delft, there is a tendency towards larger trees and mixed-species greenery blending with the non-native. Gleditsia, for example, native to North America, China, Japan, and Persia, is one exotic species that could appear in greater numbers.
While many people have an innate aversion to anything new and non-native, alternative species usually thrive in urban environments, especially amid increasing heatwaves.
In determining the ecological value of ‘newcomers’, the Register of Ecological Trees of the city of Leiden proves useful, particularly in debates where the term native is used rigidly. Speaking from his own experience, van der Werf says, “When I plant an oak, people tell me I have done a good job without realising there’s a huge difference between planting an American oak (Quercus rubra), a summer oak (Quercus robur) or winter oak (Quercus petraea). An American oak hosts generally not more than 20 different insects, a summer oak more than 400.”
CSI Tree
Helping Delft City Council decide what climate-resilient trees suit urban areas best is CSI Tree, a research project by Wageningen University and the Netherlands’ major tree growers. It involves a systematic investigation in which plant material is collected worldwide and then planted out in a greenhouse. The latter serves as a climate lab, observing plant adaptations and responses to changing climatic conditions over the coming decades. By manipulating variables such as temperature, humidity, and light, researchers hope to gain insight into how plants handle stressors, including drought, flooding or heat. The project necessitates forethought, van der Werf stresses. “How is the climate going to be in 50, 70 or 85 years? If you plant a tree now, you want it to be alive and kicking in the distant future. My message to the tree-growing readers of AIPH and FloraCulture International is to join CSI Tree. Imagine that within a few years, CSI Tree comes up with a list of 50 climate-proof trees. By then, they should also be on offer.”
When asked about whether there are currently signs of undersupply, van der Werf says, “At times, a specific size is not available, but for now we do not experience any availability constraints.”
A World On Its Own
Drought is probably the biggest challenge to trees worldwide. Yet, Delft’s inner city is a world of its own, where lowland geography affects trees, particularly those lining the scenic canals. Van der Werf explains, “In certain parts, the situation is bizarre with water levels that are 30cm below the soil surface. Here, we must do our utmost to create legroom for roots and protect them from ever-rising groundwater levels.”
A tightly compacted soil certainly does not help the situation either, nor does the fierce competition between trees and above-ground and below-ground infrastructure such as power lines, park benches, and playgrounds.
“Keeping fish in a tank that’s too small for them keeps them from growing to their full size, and trees are no different. It’s our responsibility to provide sufficient usable soil for root growth in line with the tree’s desired lifespan,” says van der Werf.
Falling Off The Growth Chart
Van der Werf, the city arborist, shows a growth chart of a horse chestnut tree that is clearly below its anticipated growth trajectory. The 77-year-old tree has a crown volume of 750 m³, while at that age, it should be 2,500 m³.
Such growth delays can be seen across the country. In Delft alone, half of the city’s trees struggle to survive and exhibit stunted growth.
The good news is that there is still room to extend the tree’s lifespan, provided the right methods and technologies are used. Nurturing and protecting nature seems to be the motto, with doubling tree crown volume across the city as a prominent objective.
Meanwhile, protection is not a luxury during municipal repairs and construction activities when the soil, and therefore the life in it, should be covered to protect it against compaction due to heavy traffic. Trees must be protected with planks and by fences.
Planting Location Assessment
It is also crucial to check the planting location. Van der Werf and his men examined the planting locations along the canals, presuming that in such a challenging, Venice-like environment, trees help one another by connecting through their mycorrhizal network. Surprisingly, they didn’t.
Delft has little choice but to come to the rescue of canal trees in the form of Amsterdam tree sand, a sand-based structural growing medium used when planting semi-mature trees in paved urban environments. Tree sand can be used under sidewalks and parking areas. Van der Werf explains, “It should be gently pressed to avoid cars, slabs and pavers from sinking away. If done neatly, adjacent trees can use it for furthering their root system.”
Special attention needs to be given to the depth of the sand layer. He adds, “The sand contains organic matter, so when it reaches groundwater level, it acts like a sponge, causing the root zone to be saturated with water. Also, we discovered that the soil under roadways lining our canals tends to be very wet with excess water constantly flowing towards tree roots. So, we used road kerbs to create a barrier between the road and the planting location, forming a channel along which surface water can be drained and creating additional rooting space. Additionally, we used drainage sand at the lowest point to stop the capillary action.”
In other places, Delft’s city arborists use high-pressure air injection to decompress soil. Permeable soils drain freely and allow air and water to move to and around the root system. Another example of thoughtful tree care can be seen along Delft’s new tramway under construction, which features suspended tram stop platforms, with nearly all of the soil beneath the pavement suitable for tree root growth.
How Much Is Delft Spending To Create Green Havens?
Naturally, suspended pavement systems and innovative technologies in arboriculture, such as a heat treatment against horse chestnut bleeding canker, require deep pockets. In Delft, the preliminary cost-benefit analysis of the latter technology signalled cost overruns and benefit shortfalls, including the risk of tree decline due to overheating.
This begs the question of how much money Delft City Council is spending to create green havens. Does it feel like a tree without leaves or like a lush green oasis?
The city’s parks and recreation department has an annual budget of €5,140,000 for the standard maintenance of green spaces. Van der Werf is happy that there is room to spend on new priorities. He says, “We have developed a 2024-2027 maintenance management plan, with the city attributing yearly an added €700,000, €300,000, and €200,000 for urban greening up, planting location assessment and improvement, and urban parks revitalisation, respectively. Considering it’s all extra money, I don’t see any reason to complain. It perfectly illustrates how committed Delft is to make its city greener, cleaner, healthier and climate resilient.”
In November last year, the liberal-progressive Democrats 66 (D66) won big in the Dutch General Election. Van der Werf considers this a positive development in a city where 25.3 per cent and 20.3 per cent of the votes were cast for D66 and GreenLeft/Labour (GroenLinks/PvdA), respectively. “Rob Jetten, the leader of D66, and the Netherlands’ new prime minister, has the ambition to build an additional ten cities, stressing these must be green and built according to the 3-30-300 principle. Perhaps more important is that many people, no matter which side of the political spectrum they vote for, have by now realised that to fight off upcoming heatwaves, we need trees. Delft is already a great city; we aim to make it even greener.”
The 3-30-300 Principle
Dutch Dr Cecil Konijnendijk, co-director of the Nature Based Solutions Institute, launched his 3-30-300 in 2021, and it has gained traction worldwide.
In the view of Konijnendijk, 3 is the minimum number of trees everyone should be able to see from their home. A room with a view of trees is good for people’s mental health and wellbeing.
30 references the tree canopy percentage every neighbourhood should have. The urban forest canopy provides better microclimates and air quality, reduces noise, and has positive impacts on physical and mental health. With this in mind, 30 per cent represents the minimum threshold for a canopy cover to be effective.
300 metres is the maximum distance from the nearest park or green space. This figure is more or less in line with the recommendations of the European Regional Office of the World Health Organisation, which advocates a maximum distance of 300 metres to the nearest green space of at least one hectare.
Intentional And Unintentional Damage To Urban Trees
Life on the city streets is not easy for urban trees. As if air pollution, climate change, pests and diseases weren’t enough, urban trees are prone to unintentional and intentional vandalism. Sometimes, people severely damage or wantonly destroy trees, breaking their trunks and branches or felling them entirely.
However, most of the time, people hardly realise how their actions can severely damage the tree’s structure.
In Delft, parking along canals presents particular challenges, with limited space between trees and waterways. Local shopkeepers report frequent manoeuvring in confined spaces, where trees at either end of parking areas are often subject to impact or disturbance.
Unintentional vandalism of nature was also observed at the Market Square. For many decades, the place was void of greenery and even served as a car park. At the turn of the century, the city council planted four large Acer saccharinum in front of the New Church as part of its greening strategy. Historically, trees also stood on either side of the church’s main entrance — the burial site of the Dutch royal family — as seen in archival photographs of royal funerals.
Now, the Market Square Acers are arguably the most photographed trees by tourists, although their performance has been less successful. Van der Werf explains, “At the time, the trees were planted according to all relevant standards, creating more than sufficient legroom for roots and using quality soil. However, even if you follow every possible tree-planting rule, outcomes can still be affected by local conditions. The Market Square turned out to be a very windy and, therefore, very hostile environment for trees. In addition, we discovered that during the weekly street market, residual water from seafood stalls, often containing salt, was regularly discharged at the base of the trees.”
