Pollen: Friend or Foe

The incidence of allergy is increasing at an alarming rate especially in urban areas (D’Amato et al, 2013). There are several reasons for this, one of the most potent and troublesome triggers being pollen. The most common allergenic pollens are derived from grass and trees. This article will focus on trees, through which the key positive and negative impacts of pollen in the modern metropolis will be explained. It highlights significant research papers and projects, and outlines the basis of sustainable solutions for safer and healthier public and private outdoor space, through inter-professional collaboration and ‘designing out’ allergy by judicious planting and landscaping.

Pollen positive

The world needs pollen and industrious pollinating insects – particularly the bees – which help to restore the imbalance of monocultures and concrete jungles. Biodiversity of both flora and fauna is critical to safeguarding the planet. Getting the right mix of plants – from majestic urban trees to delightful nectar-rich flowers – is critical in the creation of outdoor spaces for healthy living. The benefits are extensive, as minimising allergenic pollens alleviates symptoms in individuals, with significant impact on educational attainment (Muraro et al 2010) – and thence the productivity and wealth of nations. The right pollens in the right place are an undervalued asset.

Characteristics of pollen

Pollen grains carry male gametes (i.e. male DNA) to the female parts of a flower. Pollen requires both resilience and assistance to protect the gametes on their journey and reach their goal. Air-borne (anemophilous) pollen is particularly light, fine and produced in huge amounts, while pollen dispersed by insects (entomophilous) is less likely to cause allergy because it is usually heavier,stickier and produced in smaller quantities. Examples of Species producing air-borne pollen include Birch, Alder, Ash, Plane and Lime – familiar in many streetscapes.

Sexism in plants

There are three primary classifications of flowering plant: perfect, monoecious and dioecious. Perfect-flowered plants (sometimes referred to as bisexual or hermaphrodite) have male and female parts in the same flower. Monoecious plants have separate (unisexual) male and female flowers, both growing on the same plant; not all are allergenic, but many are wind-pollinated. Dioecious plants are separate-sexed; each separate plant is either male or female. Males produce pollen, which makes them highly allergenic, while females produce seeds or fruit and no pollen. For commercial reasons the horticultural industry clones most trees and shrubs asexually. In the case of dioecious species the male selections outnumber the female ones.

Local authorities in their street planting schemes often favour male cultivars, because they cause less litter than female trees which drop their fruit and make a mess on the pavement. This sexist approach helps to economise on street cleaning maintenance and to mitigate against slip and trip hazards. The effect in many towns and cities has been to reduce biodiversity, and this is accentuated by the aesthetic preferences of many landscape architects for formality and symmetry, in planting a single cultivar (monoculture) in grids. The impact on health is often greater exposure to pollen and an increase in resultant allergies.

More felt than seen

It is worth noting that pollen in the air is virtually invisible. Allergic reactions to it can be immediate with symptoms such as sneezing. However the response is often delayed and persistent, resulting in ongoing inflammation. Fine air-borne pollen may be carried in the hair or clothes of a sufferer, who may be unaware of the exposure and therefore not link their cold-like symptoms to the plant source. Moreover, sniffles may occur earlier than expected; the pollen calendar demonstrates that air-borne pollen from one tree species or another is present from January until August, and grass pollen throughout the summer.

Closing the data gap

The National Pollen and Aerobiology Research Unit at the University of Worcester is the exception to the rule in capturing pollen data throughout the year (NPARU website). Across the UK most pollen counts and media-reporting take place between mid-March and late August. This corresponds to the months when the public at large expect hay fever to occur, and may influence the popular opinion that it is showy, colourful herbaceous flowers which are responsible for triggering spring and summer allergies. In fact, for the most part, this is a myth. The high visibility of these flowers attracts bees and other beneficial garden insects – good news for biodiversity as well as human health, because insect- pollinated flowering plants are invariably low allergen.

But what about the link between pollen and poor air quality caused by traffic pollution? Internationally, a number of scientists are currently investigating the potential for air pollution to increase the allergenicity of pollens. In March 2015 Science Daily reported on the Max Planck Institute research findings presented to the 249th National Meeting & Exposition of the American Chemical Society. A pair of air pollutants linked to climate change – nitrogen dioxide and ground-level ozone – could also be a major contributor to the unparalleled rise in the number of people sneezing, sniffling and wheezing during allergy season. These two gases appear to provoke a chain of chemical reactions that may increase the potency of certain airborne allergens (American Chemical Society 2015). Nitrogen dioxide has been implicated in recent media coverage of diesel- powered vehicle emissions, issuing wider public awareness of the harm to health caused by poor air quality, especially in urban areas. However the link to air- borne pollens has not yet been recognised beyond specialist aerobiological research labs and academic papers.

All too often the presence of allergic triggers such as pollen is not appreciated or taken into account when planning and designing both public and private landscapes. European Aerobiology Society (EAS) President Dr. Michel Thibaudon has said: “Highly allergenic trees are planted close to people’s houses and schools, because pollen emissions and their interaction with human activities are not taken into account when planning. We need to systematically collect this data to take informed decisions to protect our health from pollen emissions” (Thibaudon 2015). The results may help to close the pollen data gap and consequently raise awareness of how some street trees can both filter toxic pollution and generate a potent cocktail of air-borne pollens.

Birch: friend and foe

The planting of trees in urban areas has increased in the past few years, as green spaces are proven to be more pleasant to live in, reduce crime, mitigate against global warming, attenuate storm water and filter carbon emissions. Silver birch is an attractive tree with wonderful bark and foliage that casts dappled light and is resilient to traffic pollution. Birch is now a common sight in city hardscapes where its fine pollen cannot be absorbed into soft surfaces and, instead, flies about invisibly. In allergy terms, birch pollen is particularly problematic due to the ‘birch oral allergy syndrome’ that links it to food allergies.

The case study (see below) of silver birch newly introduced to Christchurch New Zealand, is a useful benchmark. It demonstrates two critical factors contributing to the allergenic potential of a planted environment: proximity pollinosis and monoculture. It is worth noting that birch has a biennial trend of pollen production with alternating years of high and low catch.

Sustainable solutions for health

By adding people to the Right Plant Right Place principle of sustainable horticulture, it is possible to reduce exposure to pollens with judicious placement and combinations of species. To this effect, the Ogren Plant Allergy Scale (Ogren, 2015) that indicates the allergenicity of a plant on a 1-10 scale, is a very useful tool. Planting alone will not ameliorate the landscape, but it can help. Designer of child-friendly gardens Jackie Herald advocates that ‘a healthy and happy landscape is also about spatial flow and the appropriate density of low pollen planting, with a good balance of hard and soft surfaces. There’s such a wide choice of materials and planting options, it’s always possible to create a low allergen garden with plenty of WOW factor!’ She considers the allergy question to be integral to best practice: it is high on her standard list of criteria to establish the client’s design brief. Yet, more often than not, landscape architects’ and garden designers’ primary influence is aesthetic, the silver birch being a perennial favourite.

Guidelines for designing spaces with low allergy impact (Carinanos and Casares- Porcel, 2011), developed jointly by botanists and pharmacists at the University of Granada, Spain, include the following critical factors:

  • ?  Increase plant biodiversity
  • ?  Avoid massive use of male individuals of dioecious species
  • ?  Choose species with low-to-moderate pollen production
  • ?  Adopt appropriate management, maintenance and gardening strategies to ensure removal of opportunist and spontaneous species
  • ?  Avoid forming large focal pollen sources and screens by respecting planting distances
  • ?Obtain expert advice when selecting suitable species for each green area, and avoid fostering cross-reactivity between panallergens.


Trees – like people – in cities behave differently from less regimented ‘natural’ settings in the countryside. Statistically the incidence of allergy is much less in rural areas than urban zones. An MDT approach in both research and professional practice is required to ensure healthier environments to live in. This includes allergists and doctors, botanists and landscape designers, town planners and educationalists, and well informed politicians with a will to generate holistic change.

Case study: birch allergy in New Zealand

American allergy-fighting horticulturalist Thomas Leo Ogren (Allergy-free Gardening 2015) was contacted by a doctor in Christchurch, New Zealand, who was very concerned about the trees recently planted at the primary school which his daughter, then age 9, attended. She was suffering from both pollen allergies and allergic-asthma. Just prior to this the press had devoted many column inches to the thousands of non-native silver birch trees that lined streets of Christchurch, and to the terrible pollen-allergies these same trees caused so many people each spring. The birch planting had been continued directly through the school playground. The doctor requested that the local authority remove the birch trees and replace them with low-pollen trees. When his request was denied he took the authority to court. In the process Ogren was requested to submit an expert testimony, which he did. Ogren acknowledged that birch trees are beautiful, but they don’t belong in schoolyards. Trees are important, but children are more important. He advised the birches be removed and immediately replaced with pollen-free trees, using the largest feasible specimens. The court found in the doctor’s favour, the silver birches were replaced with good-sized Red Maple ‘Autumn Glory’, which is a female, pollen- free cultivar valued for its brilliant display of autumn colour, and already known to thrive in Christchurch. Today, these maples are mature and provide much- needed shade at the school. Best of all, the children are no longer exposed each spring to a bombardment of allergenic birch pollen in their playground.


Allergy-free Gardening, 2015. Allergy-free school yards. Available at: http://www.allergyfree-gardening.com/articles/50-allergy-free-school- yards.html [accessed 18 October 2015]

American Chemical Society, 2015. Air pollutants could boost potency of common airborne allergens. Science Daily, 22 March 2015. Available at: www.sciendedaily.com/releases/2015/03/150322080208.htm

Carinanos, P. and Casares-Porcel, M., 2011. Urban green zones and related pollen allergy: A review. Some guidelines for designing spaces with low allergy impact. Landscape and Urban Planning, 101:205–214

D’Amato, G. et al, 2013. Climate change, air pollution and extreme events leading to increasing prevalence of allergic respiratory diseases. Multidisciplinary Respiratory Medicine, 8:12

ISAAC (The International Study of Asthma and Allergies in Childhood) Steering Committee), 1998. Worldwide variation in prevalence of symptoms of asthma, rhinoconjunctivitis, and atopic eczema. Lancet:1225-32

Muraro, A. et al, 2010. The management of the allergic child at school: EAACI/GA2LEN Task Force on the allergic child at school. Allergy 65: 681–689

NPARU website http://www.worcester.ac.uk/discover/pollen-forecast.html

Ogren,T.L., 2015. The Allergy Fighting Garden. Berkeley: Ten Speed Press

Thibaudon, M., 2015. Quoted in More Europeans will be allergic to pollen if no measure is taken to reduce exposure. Press release available at: http://www.efanet.org/images/2015/EAACI_-_EAS_-_EFA_-_IRS_Joint_PR_- _International_Ragweed_Day_20150627.pdf

© Shenagh Hume 2015