Laidback Gardener Tip of the Day

Why Grow Trees That Make Us Sneeze?

Knackiger Winter für Pollenallergiker ideal
It’s the inconspicuous flowers, like these birch catkins, that cause spring hay fever.

Everyone blames ragweed (Ambrosia artemisiifolia) for hay fever (allergic rhinitis) and it is true that at the end of the summer, ragweed can make the lives of people who are allergic to it really miserable. And grasses are the main cause of hay fever during the summer months. But in the spring, it’s the tree pollen that causes most of the trouble. I’ve seen ragweed eradication campaigns and medias reminding us to mow our lawns before the grasses start to bloom and produce pollen, but you never seem to hear any suggestions about what to do about tree pollen. That’s because authorities simply seem to believe that trees are just too abundant in nature and therefore, there is little we can do to prevent their pollen from ruining people’s health. But is that really true? Couldn’t we be doing something to reduce the ravages of tree pollen? The answer to that is yes and is in fact quite simple: we could be planting a greater percentage of trees that are non-allergenic.

Allergenic or non-allergenic?

Showy flowers, like those of this crabapple, are almost never allergenic.

In general, trees with showy flowers (crabapples, mountain ash, magnolia, etc.) are pollinated only by or mostly by insects. Their pollen is simply too heavy to be carried by the wind from the flower to people’s noses. So insect-pollinated trees (entomophilous is the scientific term) simply don’t provoke hay fever… unless maybe you stuck your nose right into the flower!

There are a few exceptions to every rule, of course, and there are a few trees with showy flowers that are both insect-pollinated and wind-pollinated, the horse chestnut (Aesculus spp.) being the best known. Pretty as its flowers may be, they are mostly wind-pollinated and are highly allergenic.

Male pussy willows, with their insignificant flowers, are allergenic.

Most trees without showy flowers are wind-pollinated (anemophilous). Their pollen is light and produced in great abundance, far more than that of insect-pollinated trees. These are the trees that cause hay fever during the spring months.

However not all the wind-pollinated trees are highly allergenic. Pines (Pinus) and spruce (Picea), for example, have a waxy pollen that rarely causes reactions. The pollen of birches (Betula), poplars (Populus), ashes (Fraxinus) and most maples (Acer), on the other hand, provokes a very strong reaction in most people with seasonal allergies.

Where things become interesting is when your realize that, even among wind-pollinated trees, not all produce pollen. Many wind-pollinated trees (Manitoba maple, ashes, willows, poplars, mulberries, etc.) are dioecious, that is to say that male and female flowers are borne on different plants. For these trees, there is an easy solution to the pollen problem. Simply plant female trees only. They receive pollen, but never produce it.

Botanical Sexism

21050502DThe tragedy is that, nowadays, it is precisely the opposite that happens: we purposely plant almost exclusively “seedless” trees (i.e. male trees), especially in urban and suburban settings. This vast overuse of male only trees is called botanical sexism.

Why are male trees so popular? It’s because they’re see as “clean”. Male trees produce only pollen, which is usually invisible. They never produce seeds that fall to the ground and accumulate, potentially blocking gutters and drains and needing sweeping up. In our urban mindset, seeds are “dirty” and therefore undesirable.

Female trees don’t produce pollen, but instead all those “dirty” seeds. That’s why urban planners generally avoid female trees, and why you almost never see female trees in towns and cities any more. But by choosing cleanliness as the ideal criteria for tree selection and thus planting almost only male trees, cities are making the hay fever situation much worse. According to on survey done in Canada, the Canadian Urban Energy Audit (there are similar studies for Europe and the United States), the urban forest is 74 to 97% male, depending on the city. Compare that to the situation in natural forests, where the forest is about 50% male and 50% female. In some cities, tree pollen levels have more than tripled since the 1970s… entirely because of botanical sexism!

A Quick Fix

When planners stop planting seedless male trees and start upping the total of female one, the pollen count starts to drop dramatically. Of course, that’s only logical, as with fewer male trees, there should be much less pollen in the air. But that’s not the only factor to consider. That’s because female trees are not just passive receptors of pollen, they actively filter the air, catching and absorbing pollen, causing pollen counts to plummet and alleviating the suffering of people with allergies.

If a neighbor grows a pollen-bearing tree that causes you discomfort, you can easily reduce the pollen count on a very local level simply by planting a female tree of the same species near your window. It will filter the irritating pollen out of the air all on its own.

What About the Bisexuals?

Of course, a lot of wind-pollinated allergy-provoking trees, such as birch (Betula), oaks (Quercus) and alder (Alnus), are not dioecious, but rather monoecious or hermaphroditic. Both male and female flowers are borne on the same tree and therefore every tree can theoretically produce pollen. In creating a pollen-reduced oasis in urban areas, do we have to ban these “bisexual” species entirely?

Not necessarily! For these species, planting male sterile cultivars is an interesting solution. You see, in most populations of monoecious and hermaphroditic plants, occasional female-only plants are found. Or specimens whose pollen is deformed, never released or simply not produced. It would then be possible to multiply these male sterile trees through cuttings or grafting and to plant them in allergy-sufferers gardens or in cities in order to increase the share of non-allergenic trees and decrease the air-borne pollen.

For example, red maple (Acer rubrum) is usually monoecious (it generally has both male and female flowers), but dioecious specimens, both male only and female only, are fairly common and are easy to multiply. For example, the cultivars ‘October Glory’ and ‘Autumn Red’ are pollen-free. It would be very interesting to see this kind of cultivar more widely promoted. Sadly, few nurseries see any value in providing non-allergenic trees and these very interesting pollen-free cultivars often remain hard to find.

In a Hay Fever Sufferer’s Crystal Ball…

I forsee a day when, for the health of their citizens, cities will actively promote the growing of non-allergenic trees. There’ll be posters and commercials, slogans and theme songs. And tree labels in nurseries will actually indicate whether the plant is male or female, allergenic or non allergenic.

That day has not yet arrived, though. Few municipalities have any published material about pollen-free trees whatsoever. And the average garden center employee hasn’t the faintest idea which trees to recommend to an allergy-sufferer.

Trees and Shrubs That Cause Allergies

If you or someone in your family suffers from hay fever, here is a list of trees you would definitely not want to plant outside the bedroom window!

  • Alder (Alnus spp.)
  • Arborvitae (Thuja occidentalis)
  • Ash (male cultivars) (Fraxinus spp.)
  • Bayberry (Myrica pensylvanica)
  • Beech (Fagus spp.)
  • Birch (Betula spp.)
  • Catalpa (Catalpa spp.)
  • Douglas Fir (Pseudotsuga menziesii)
  • Elm (Ulmus spp.)
  • False Cypress (Chamaecyparis)
  • Fragrant Sumac (Rhus aromatica)
  • Gingko (male cultivars) (Ginkgo biloba)
  • Hackberry (Celtis spp.)
  • Hazel (Corylus spp.)
  • Hemlock (Tsuga spp.)
  • Hickory (Carya spp.)
  • Honey Locust (male cultivars) (Gleditsia tricanthos)
  • Hornbeam (Carpinus spp.)
  • Horsechestnut (Aesculus hippocastanum)
  • Juniper (male cultivars) (Juniperus spp.)
  • Kentucky Coffeetree (male cultivars) (Gymnoclada dioica)
  • Linden, Basswood (Tilia spp.)
  • Manitoba Maple (male cultivars) (Acer negundo)
  • Mulberry (male cultivars) (Morus spp.)
  • Norway Maple (Acer plantanoides)
  • Oak (Quercus spp.)
  • Poplar (Populus spp.)
  • Red Maple (male cultivars) (Acer rubrum)
  • Russian Olive (Elaeagnus angustifolia)
  • Sea-buckthorn (Hippophae rhamnoides)
  • Silver Maple (male cultivars) (Acer saccharinum)
  • Smokebush (Cotinus coggyria)
  • Staghorn Sumac (Rhus typhina, R. glabra)
  • Sweet Gale (Myrica gale)
  • Tree of Heaven (Ailanthus altissima)
  • Walnut (Juglans spp.)
  • Willow (Salix spp.)

The guru of allergy-free gardening is Thomas Leo Ogren, who wrote, among others, the book Allergy-Free Gardening. Definitely worth a thorough read!

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

2 comments on “Why Grow Trees That Make Us Sneeze?

  1. Margaret

    Fascinating stuff! Believe in science.

  2. Pingback: Trees that Won’t Make You Sneeze | Laidback Gardener

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