Conifers: Allergy-Friendly Evergreen Trees

By Ronald Elardo

Read about an allergy-friendly way to garden and landscape with conifers and evergreen trees.

An allergy-friendly conifer, the female Chilean pine (Araucaria araucana)
An allergy-friendly conifer, the female Chilean pine (Araucaria araucana)

Allergies and asthma have been rising at an alarming rate among children and adults worldwide. In the United States alone, 50 million Americans have nasal allergies, in what CBS News (May 11, 2015) has called a “pollen tsunami." And, according to a Rutgers University study, “[b]y 2040, the pollen count will more than double levels in 2000.”

Further, “air pollution and allergies airways disorders through the induction of inflammation and oxidative stress in the lungs [will increase].” The National Center for Biotechnology Information states that: “[e]mission reduction efforts and federal air quality standards have been insufficient to shield children from potentially serious health damage.” Pollen and pollution have become double whammies in the assault against our ability to breathe to sustain life properly.

So, what does all of this have to do with conifers?

A Coniferous Solution to Allergenic Plants

Thomas Leo Ogren has, for the second time in his writing and research career, postulated that, through studying the pollen output of certain plants and trees, harmful pollen can be reduced by planting more female plants, among them many genera of conifer. Why female trees?

Ogren reports that, in the 1940’s, the US Department of Agriculture sought to reduce the amount of seed pod and fruit “pollution” by promoting the production of male plants. The nursery industry responded to the call so that “clonal trees and shrubs became the rule." The industry, with the government’s blessing and encouragement, manipulated the environment.

Since female trees are “messy”, and male trees are not, male plant production went into full swing. Male trees live longer and grow larger, and thus produce more pollen. Ogren writes that: “4 out of every 5 top-selling trees in the US are male trees.” In order to “reverse this unhealthy process,” male trees can be top-grafted in winter, thus giving them a sex-change over to female trees.

Indeed, the nursery industry has a large financial stake in their current inventory of “male trees and clones." But that can be remedied.

Another allergy-friendly evergreen, the common juniper (Juniperus communis)
Another allergy-friendly evergreen, the common juniper (Juniperus communis)

The Ogren Pollen Allergy List Scale (OPALS)

In addition to sex-change grafting, one other way has been that places like Albuquerque, New Mexico, Las Vegas, Nevada, Toronto, Ontario, Edmonton, Alberta, and the State of California have enacted ordinances to curtail the production and planting of allergenic pollen plants. Because your yard affects you, Ogren presents an Ogren Pollen Allergy List Scale (OPALS), which ranks trees, shrubs and plants according to their nefarious pollen effects, or lack thereof.

An OPALS of 1 is the best kind of plant, 10 the worst in pollen production. In the case of conifers, however, pollen grains have qualifiers. Tom points out that many monoecious plants will have one sex on one branch and another sex on another. In many cases, as propagators take cuttings, they take them from the lower, male branches, thus creating male trees. Tom cites Italian cypress as one prime example.

Tom, in his further research, is always looking to connect with growers in search of female selections (cultivars), like Cephalotaxus, for example. Had Ogren his way, Ginkgo biloba female trees would be preferable for planting. Male ginkgos would be reduced in number, even though the industry produces and promotes and sells the exact opposite.

Ogren Pollen Allergy List Scale (OPALS) for Evergreen Trees

Since conifer genera, for the most part, are monoecious, cross-pollination and pollen-production are far less of a problem. For example, here are the conifer genera/species cited by Ogren and their OPALS rating:

Abies 2

Araucaria 1 (for female trees); 7 for male trees Note: Grow them in containers, and far less pollen is produced.

Callitris 9

Cedrus atlantica 2

Cedrus brevifolia 2

Cedrus deodara 4

Cedrus deodara ‘Pendula’ 5

Cedrus libani 2

Cephalotaxus 1 (female trees); 9 (male trees)

Chamaecyparis 8

Cryptomeria japonica 10

Ginkgo biloba 7 (male trees); 2 (female trees)

Juniperus (with berries) 1

Keteleeria 3

Larix 2

Metasequoia glyptostroboides 4

Picea 3

Pinus 4; Pinus contorta 8 Note: “Although pines shed enormous quantities of pollen grains, the grains are waxy and not highly irritating to mucous membranes. Their potential for allergy is rather low and, when it occurs, not usually severe.”

Platycladus orientalis 7

Podocarpus 1 (female trees); 10 (male trees) Pollen is also toxic.

Pseudolarix kaempferi 3

Pseudotsuga 3

Sciadopitys 7

Sequoia sempervirens 5

Sequoiadendron giganteum 5

Taxodium 5

Taxus 1 (female trees); 10 (male trees) Note that pollen is also toxic.

Thuja 8

Thujopsis dolabrata 9

Tsuga 3

Wollemia nobilis 4

Many of us like to plant Japanese maples in our gardens and landscapes. On the whole, Acer species (with the exception of Acer rubrum ‘Festival’ OPALS 1) have high OPALS ratings of 5 or more.

An allergy-friendly conifer, the female brown pine (Podocarpus neriifolius)
An allergy-friendly conifer, the female brown pine (Podocarpus neriifolius)

The Allergy-Fighting Garden

Thomas Ogren has a Master’s degree in agricultural science with an emphasis on plant flowering systems and their relationship to allergies. His research has spanned 30 years. He has written in many publications and authored Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping and The Allergy-Fighting Garden: Stop Asthma and Allergies with Smart Landscaping.

Their cost is relatively low. He is the creator of the Ogren Plant Allergy Scale (OPALS), the very first plant-allergy ranking system. He has been a consultant to the American Lung Association, the USDA, the city of Christchurch, New Zealand, the California Department of Public Health, Allegra, and Johnson & Johnson. His voice has been heard on the Canadian Discovery Channel and as radio-show host. His email is tloallerg[email protected].

The Allergy-Fighting Garden: Stop Asthma and Allergies with Smart Landscaping is printed by Ten Speed Press, Berkeley. It offers 244 pages, jam-packed with information sections. In Part I, Tom begins the discussion of plant biology, fighting allergy-causing agents in your neighborhood and city, understanding plant sex and allergies, eliminating mold spores, and allergy-blocking hedges. Part II introduces the OPALS concept and, most importantly, plant rankings according to their OPALS.

A Useful Conifer Guide for Garden-Owners and Landscapers

Plants are listed according to genus and species. Common names are also provided with reference to botanical nomenclature. This is an extremely important listing. Ogren also provides a Glossary of Horticultural Terms such as bract, monoecious and dioecious, and so much more. Tom provides a listing of Recommended Readings to educate further his audience.

The book also provides Useful Websites for readers to consult. There is a Pollen Calendar, which lists plants by genus with their pollen production months, and USDA Zone maps. The Index assists the reader in searching the text. Thomas Ogren’s work is a must-have reference book for all who are sensitive to allergy-causing pollens and who landscape either for themselves or for their clients. Tom’s work is also a great reference work for communities which are trying to clean up the air their children and adults breathe in.

This article was originally published in the Summer 2016 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.


Scott Gilmore

Do you have members around north Texas area? I would join if I could connect with others in the area to gain local knowledge. Thank you for the help.