Implementing allergy-friendly tree planting

Wendy Batenburg, Henry Kuppen, Anne Overduin - de Vries and Letty de Weger

Figure 1: Corylus avellana in flower.

As climate change accelerates, cities across Europe are experiencing higher temperatures, more frequent heatwaves and increasing pressure on urban liveability. Trees are widely recognised as an essential part of the solution.

Through shading, evapotranspiration and radiation interception, trees can significantly cool their immediate surroundings and mitigate urban heat stress (Grilo et al., 2020). When they are well managed and allowed to mature, trees also deliver a wide range of additional ecosystem services, including improved air quality, water buffer, carbon sequestration, enhanced biodiversity, reduced noise pollution and positive effects on human health. Therefore, it is essential to both plant more trees and care for those already growing in urban areas.

However, trees are not universally beneficial in every aspect. Some species produce allergenic pollen that can trigger allergic reactions and exacerbate respiratory conditions such as allergic rhinitis and asthma. This health issue deserves more attention in tree species selection. To support informed decision-making, we developed the Tree Pollen Guide, an evidence-based guide to the allergenic potential of tree pollen.

Tree pollen and allergy

When specific allergenic pollen is inhaled or comes into contact with the eyes, susceptible individuals may develop symptoms such as sneezing, a blocked or runny nose, itchy or watery eyes, coughing and mucus production. They react to certain proteins in the pollen, called allergens. The amount of pollen produced also influences the severity of the allergic reaction.

These symptoms are often accompanied by fatigue, sleep disturbance, headaches and concentration problems, which can have a substantial impact on quality of life and everyday functioning. In urban environments, such complaints could lead to requests for the removal of problematic trees. From both ecological and policy perspectives, removal is an undesirable outcome. A more sustainable approach is to consider potential health effects at the stage of species selection and planting design, both in new designs and when replacing trees which have been removed. Enhancing species diversity within existing and new tree groups would also significantly mitigate pollen pressure, thereby reducing the impact on allergy patients.

Figure 2: The most prevalent tree pollen counted at the Leiden University Medical Centre.

Figure 3: The Tree Pollen Guide for allergenic pollen. This is a simplified version of the guide, leaving out the specific regions where the scientific evidence for allerginicity originated from. This is described in detail in De Weger et al. (2024).

Why regional classification matters

Although various international lists exist that classify tree species according to pollen allergenicity, these classifications vary considerably. Comparative research has shown strong discrepancies in allergenicity ratings for many species, partly because pollen production varies geographically and partly because population sensitivity differs between regions (Sousa-Silva et al., 2021). Trees that are highly allergenic in one climate zone may cause far fewer problems elsewhere because less pollen is released in those locations, inhabitants have lower susceptibility or just because the tree species with allergenic pollen is not present in the region due to climate or geographical isolation. This makes it essential to develop regionally specific, scientifically substantiated guidance.

The Tree Pollen Guide provides such a classification for the Netherlands, based on a combination of international literature, local pollen-monitoring data and local patient-sensitisation data. While developed for Dutch conditions, its underlying methodology and principles are relevant for tree professionals throughout northwestern Europe and the UK.

Scientific basis

The Tree Pollen Guide focuses on the most commonly used tree species in the Netherlands. For each species, existing scientific evidence on pollen allergenicity was reviewed. In many cases, information was only available at the genus or family level rather than for an individual species. Added to this existing scientific evidence was ambient air pollen counting. Leiden University Medical Centre and Elkerliek Hospital in Helmond have been collecting and counting pollen for more than 50 years. They have longterm data starting in 1969, from monitoring stations on the roof of the hospitals.

These measurements show that the highest annual pollen concentrations originate from the birch family (Betulaceae), in particular alder (Alnus) and birch (Betula) (Fig. 2). They are followed by members of the cypress family (Cupressaceae), then the oak genus (Quercus), ash genus (Fraxinus), willow genus (Salix), the pine family (Pinaceae) and the poplar genus (Populus). In addition, clinical data from a cohort of nearly 500 allergy patients in the Netherlands revealed that approximately 50% reacted to birch pollen allergens and other Betulaceae family members, such as alder and hazel. About 25% of the patients reacted to pollen from the Oleaceae family, such as species in the Fraxinus and Olea genera. The full methodology and results are described in our scientific publication (De Weger et al., 2024).

Categorising allergenic risk

By integrating pollen exposure data with clinical sensitivity, the Tree Pollen Guide groups tree species into five categories of allergenicity, ranging from low to very strong (Fig. 3). Birch occupies the highest category, as it combines extremely high pollen production with strong allergenic effects in a large proportion of patients. Alder and hazel, which both belong to the birch family, are classified as strongly allergenic.

Moderately allergenic species include close relatives of birch, such as hornbeam, as well as members of the beech family (Fagaceae), like beech and oak. Ash and its relative olive also fall into this category. Species classified as weakly allergenic currently cause limited problems in the Netherlands, but they are recognised as allergens elsewhere in Europe. Plane tree pollen, for example, is moderately allergenic in southern Europe but still affects relatively few people in northwestern Europe. Similarly, species from the cypress and yew families, including Japanese cedar (Cryptomeria japonica), are major allergens in countries such as Japan, but currently provoke few complaints in the Dutch context. The Tree Pollen Guide also provides information about the source of the data, making it possible to use it internationally with some adjustments for the prevalence of certain species.

Importantly, climate change may alter these patterns. Warmer conditions and longer growing seasons could increase pollen production or shift flowering periods for certain plant species, potentially raising allergenic risks in regions where they are currently low. For this reason, even weakly allergenic species should not be planted in large monocultures.

Species classified as low allergenic show little or no evidence of having relevant allergenic effects to date. In some cases, such as wingnuts (Pterocarya) or the Chinese varnish tree (Koelreuteria paniculata), scientific information on allergenicity is still scarce. Nevertheless, pollen from this group is currently considered the least likely to cause allergic symptoms.

Figure 4: Alder (Alnus sp.) in flower.

Figure 5: Alder pollen.

Figure 6: A pollen sampler on the roof of the Leiden University Medical Centre.

From species lists to smarter planting

A high allergenicity rating does not automatically mean that a tree species should never be planted. The Tree Pollen Guide is not intended as a blacklist, but as a tool to support nuanced, context-sensitive decisions. For some species, choosing female individuals instead of male ones can significantly reduce allergenic exposure, as female trees do not produce pollen. This option is particularly relevant for dioecious species, although female trees may produce fruits or seeds that require additional management. Another strategy to minimise airborne pollen is to choose insect-pollinated tree species rather than anemophilous (wind-pollinated) ones, as these release lower amounts of pollen in the air.

Equally important are planting location, diversity and spatial arrangement. Strongly allergenic species should preferably be avoided in densely populated public areas such as around schools or healthcare facilities. Planting large numbers of pollen-producing trees of the same species in close proximity should also be avoided, as this can lead to locally high pollen concentrations. Greater species diversity not only benefits biodiversity and resilience, but also reduces allergenic risk and supports the development of a healthy immune system.

A practical tool for professionals and citizens

The Tree Pollen Guide supports arborists, urban foresters, landscape architects and municipal decision-makers in making more health-conscious tree species choices without compromising ecological value. At the same time, it offers useful guidance to private individuals selecting trees for gardens or residential areas.

Ultimately, the goal is not to remove allergenic trees from the urban landscape altogether, but to design greener cities that are resilient, climate-adaptive and inclusive – for people with pollen allergies too. The Tree Pollen Guide was developed by Letty de Weger (Leiden University Medical Centre), Arnold van Vliet (Wageningen University & Research), Henry Kuppen and Wendy Batenburg (Terra Nostra) as part of the multidisciplinary research programmes ‘Managing the effects of climate change on allergy: from pollen to patient’ and its followup ‘Healthy adaptation to heat and pollen in a changing climate’, funded by ZonMW. The Tree Pollen Guide (in Dutch) is available at www.lumc.nl/bomenkompas and is incorporated in the Treetable (www.bomentabel.nl) from the Wageningen University & Research.

References

de Weger, L.A. et al. (2024). Method to develop a regional guide for the allergenic potential of tree pollen. Science of The Total Environment 926: 171575. www.doi.org/10.1016/j.scitotenv.2024.171575

Grilo, F. et al. (2020). Using green to cool to grey: Modelling the cooling effect of green spaces with a high spacial resolution. Science of the Total Environment 274. www.doi.org/10.1016/j.scitotenv.2020.138182

Sousa-Silva, R. et al. (2021). Strong variations in urban allergenicity riskscapes due to poor knowledge of tree pollen allergenic potential. Scientific Reports 11: 10196. www.doi.org/10.1038/s41598-021-89353-7

Wendy Batenburg and Henry Kuppen, are from Terra Nostra, The Netherlands, www.terranostra.nu/nl

Anne Overduin – de Vries and Letty de Weger are from Leiden University Medical Centre, The Netherlands.

This article was taken from Issue 213 Summer 2026 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.