Impact of allergenic plants

Barcham Trees secured another great ‘catch’ for its series of arboricultural seminars when it lured Tom Ogren from California to present a seminar at its Cambridgeshire nursery. He is one of the leading experts in the field of allergenic plants and their impact on people and communities. He devised the OPALS (Ogren Plant Allergy Scale) which is now used by, among others, the American Lung Association and the USDA Urban Foresters. Colin Hambidge reports
The scale measures the allergy potential of all garden and landscape plants within a range of one to ten; the higher a plant’s rating the higher its risk of being an allergen. For example, Acer rubrum Autumn Glory is rated at one, while Callistemon citrinus (bottlebrush) is rated at nine. More than 130 different criteria are used to develop allergy rankings for plants. Each factor is either positive or negative. All factors are not weighted the same because some are more important than others. Most plants will have a combination of positive and negative factors that are computed to determine their OPALS ┬áranking.
Some of the factors used to rank a plant include the amount of pollen produced, the potency of individual pollen grains, length of flowering, the size, density, shape and weight of the pollen grains, sex if dioecious, average rankings in actual skin scratch, patch and sniff tests, and cross-reactivity to food allergies. Additional information regarding the factors used to build OPALS ┬ámay be found in Tom’s book Safe Sex in the Garden.
Tom Ogren’s main objective now is to help drive a robust, healthy trend toward allergy- and asthma-friendly gardens and landscapes in urban areas. For 30 years he has been researching the connections between the planted urban landscape and human health, in particular pollen-allergies and asthma. He has published three books on this subject, the latest being The Allergy-Fighting Garden.
His interest in the subject began around 30 years ago when his wife, to whom he has been married for 50 years, developed asthma and allergies. At the time, Tom read a book which
claimed allergies were actually psychosomatic; this was a theory he was happy to accept at the time, but as his wife’s problems increased he grew to realise the theory was not true. “Allergies affect how we perform in our lives and can have major negative impacts on us”, says Tom.
Dioecious plants – those which are either wholly male or wholly female – are of particular interest to him. Dioecious trees include willow, red maple, holly, yew, juniper, mulberry and bay. While urban trees, and especially those in public areas, are predominantly male and therefore shed no fruit onto pavements, female trees produce no pollen and so may be termed ‘allergy-free’ or ‘allergen-friendly’.
Tom Ogren is a great advocate (one might almost say ‘evangelist’) of female trees and of female plants in general. He is an avid plant collector and claims to have the largest collection in the world of female cultivars of trees, shrubs, vines and grasses that are useful in landscapes because they are totally pollen-free. He is also a consultant to states, cities, counties, schools, hospitals, and has worked for pharmaceutical companies.
He lectures frequently to healthcare professionals, city planners, arborists, landscapers and nurserymen, and now has several partners in a commercial venture to get many more pollen-free plants available to the public.
So how big is the problem of allergies? Tom told us that in 1950, it was thought that two to five per cent of the population of the USA had pollen allergies. This figure had grown to 12 per cent in 1988 and had rocketed to 38 per cent by 1999. Asthma is the most common chronic illness of childhood in the USA, and developing pollen allergies more than doubles the chance of a child developing asthma.
In 2017 the charity Asthma UK ( reported there are currently 1.1 million children (1 in 11) and 4.3 million adults (1 in 12) receiving treatment for asthma. The condition also increases the risk of developing heart disease and leukaemia, while women who have airborne allergies are at an increased risk of ovarian and breast cancers (Cell Biology, 2010 Study). Tom also mentioned more women than men suffer from asthma.
Another charity, Allergy UK (, reports that 44 per cent of adults in Britain suffer from at least one allergy, and the number of sufferers is on the rise, growing by around two million between 2008 and 2009 alone. Almost half (48 per cent) of sufferers have more than one allergy (Mintel, 2010).
While pollen is capable of travelling considerable distances, Tom told delegates most lands very close to where it originated. For twelve years, he taught landscape gardening in a California Youth Authority maximum-security prison, and he has also worked with the University of California Cooperative Extension, to establish community gardens in the Los Angeles inner city. While he is an advocate of using female plants in landscapes and gardens, he does not feel they should be used to the exclusion of male and monoecious plants (those which have both sexual reproductive systems on the same plant). “All plants are useful – in the right place”, he says. “”I would now never plant pollen-producing plants near doors, windows or too close to any part of a dwelling, but they can still be grown elsewhere”.
It disappoints Tom that so many landscape and garden designers limit themselves to around only 12 different plants, which they use in most of their projects. “It’s boring, and I much prefer plenty of diversity in planting schemes. Diversity of planting acts as a safety net or insurance policy”.

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