Don’t Grow Trees That Make You Sneeze

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Tree pollen is the major cause of hay fever in the spring. Photo: publicdomainpictures.net

The first warm days of spring, we all rush outside to take advantage of our outdoor spaces. So, off we go, puttering about in the garden, starting up the barbecue, reading a novel on Kindle on the deck. It’s just so wonderful to be outdoors, then… achoo! Yes, all the symptoms pop up instantly: sneezing, congestion, eye irritation, tearing up, asthma and more. It’s “seasonal allergic rhinitis,” more popularly known as hay fever, an ailment that affects 10–30% of the world’s population.

Pollen grains seen under a microscopic (artificially colored). Photo: Dartmouth College, Wikimedia Commons

The cause is, of course, the presence of pollen in the air we breathe. Our body, for reasons still poorly understood, reacts to this pollen, attacking it as if it were an infection, and voila! Months of suffering annually.

Of course, there are many pollens that cause hay fever. Early summer is the time for grasses to come into bloom and most are highly allergenic. In late summer, ragweed (Ambrosia artemisiifolia) takes over as the main cause of hay fever. But in the spring, it’s tree pollen that causes most of the trouble.

Now, we’ve probably all seen ragweed eradication campaigns and media reminding us to mow our lawns before 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, especially in cities, is yes and, in fact, the solution is 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 rarely allergenic. Photo: cps.proboards.com

In general, trees with showy or highly scented flowers (crabapples, mountain ash, magnolia, etc.) are pollinated only by or mostly by insects (some also by birds). 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 stick your nose right into the flower or stand under the tree when it is in full bloom and shake the branches!

These catkins are the insignificant flowers of a birch tree, a major cause of spring allergies. Photo: brilliantbotany.tumblr.com

Most trees with unobtrusive, insignificant, scentless 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, even though pines, especially, are among the most prolific pollen-producers of all trees. The pollen of birches (Betula), poplars (Populus), ashes (Fraxinus) and most maples (Acer), on the other hand, provokes a very strong reaction in people with seasonal allergies.

Where things become interesting is when you realize that, even among wind-pollinated trees, not all specimens 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

Yes, sexism even extends to our choice of trees! Ill.: Qwenu!

The 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 seen as “clean.” Male trees produce only pollen, which is usually so small humans don’t see it. They never produce seeds that fall to the ground and build up, potentially blocking gutters and drains and needing sweeping up. To 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 anymore. 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 one survey done in Canada (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

Yews (Taxus spp.) are dioecious, with seperate male and female plants. This yew shows, by its berries, that it’s a female and therefore will not produce pollen.. Photo: Southern Living

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. 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 owns 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, i.e. 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 trees, 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 airborne pollen counts. 

‘October Glory’, here showing its fall colors, is a male sterile cultivar of red maple (Acer rubrum) and produces no pollen. Photo: plantright.org

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 ‘Red Sunset’ are pollen-free females. It would be very interesting to see this kind of cultivar more widely promoted.

Sadly, few nurseries see any value in promoting non-allergenic trees and these very interesting pollen-free cultivars often remain hard to find.

In a Hay Fever Sufferer’s Crystal Ball…

I foresee a day when, for the health of their citizens, cities will actively promote the planting of non-allergenic trees. There’ll be posters and commercials, slogans and maybe even 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 offer any published material about pollen-free trees whatsoever. And the average garden center employee doesn’t have the foggiest idea which trees to recommend to an allergy sufferer.

Trees and Shrubs That Cause Allergies

Oak tree bearing pollen-rich catkins. Not a good choice for allergy sufferers. Photo: washingtonpost.com

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!

  1. Acacia or wattle (Acacia spp.)
  2. Alder (Alnus spp.)
  3. Arborvitae, thuja or cedar (Thuja spp.)
  4. Ash (male cultivars) (Fraxinus spp.)
  5. Aspen (Populus spp.)
  6. Bald cypress (Taxodium spp.)
  7. Bay tree (male cultivars) (Laurus spp.)
  8. Bayberry (Myrica spp.)
  9. Beech (Fagus spp.)
  10. Birch (Betula spp.)
  11. Bottlebrush (Callistemon spp.)
  12. Boxwood (Buxus spp.)
  13. Buckeye (Aesculus spp.)
  14. Buckthorn (Rhamnus spp.)
  15. Cashew (Anacardium occidentale)
  16. Catalpa (Catalpa spp.)
  17. Chestnut (Castanea spp.)
  18. Cork tree (male cultivars) (Phellodendron spp.)
  19. Cycads (many species)
  20. Cypress (Cupressus spp.)
  21. Elm (Ulmus spp.)
  22. Euonymus (Euonymus spp.)
  23. False arborvitae (Thujopsis dolabrata)
  24. False cypress (Chamaecyparis spp.)
  25. Fringe tree (Chionanthus spp.)
  26. Gingko (male cultivars) (Ginkgo biloba)
  27. Hackberry (Celtis spp.)
  28. Hardy rubber tree (male cultivars) (Euccomia ulmoides)
  29. Hazelnut (Corylus spp.)
  30. Hemlock (Tsuga spp.)
  31. Hickory (Carya spp.)
  32. Honey Locust (male cultivars) (Gleditsia tricanthos)
  33. Hornbeam (Carpinus spp.)
  34. Horse chestnut (Aesculus hippocastanum.)
  35. Japanese cedar (Cryptomeria japonica)
  36. Japanese umbrella pine (Sciadopitys verticillata)
  37. Juniper (male cultivars) (Juniperus spp.)
  38. Kentucky Coffeetree (male cultivars) (Gymnoclada dioica)
  39. Linden or basswood (Tilia spp.)
  40. Manitoba maple (male cultivars) (Acer negundo)
  41. Maple (Acer spp.)
  42. Mulberry (male cultivars) (Morus spp.)
  43. Oak (Quercus spp.)
  44. Olive (Olea spp.)
  45. Palm (many species)
  46. Paper mulberry (Broussonetia papyrifera)
  47. Pepper tree (Schinus spp.)
  48. Plane (Platanus spp.)
  49. Plum yew (Cephalotaxus spp.)
  50. Poplar (Populus spp.)
  51. Privet (Ligustrum spp.)
  52. Russian olive (Elaeagnus angustifolia)
  53. Sassafras (Sassafras spp.)
  54. Sea buckthorn (Hippophae spp.)
  55. Silk tree or mimosa (Albizia julibrissin)
  56. Silverberry (Elaeagnus spp.)
  57. Smoke tree (Cotinus coggyria)
  58. Southern beech (Nothofagus spp.)
  59. Sumac (male cultivars) (Rhus spp.)
  60. Sweetgum (Liquidambar spp.)
  61. Tamarisk (Tamarix spp.)
  62. Tree of heaven (Ailanthus altissima)
  63. Tupelo (Nyssa spp.)
  64. Walnut (Juglans spp.)
  65. Willow (Salix spp.)
  66. Wingnut (Pterocarya ssp).
  67. Yellowwood and Buddhist pine (Podocarpus spp.)
  68. Yew (male cultivars) (Taxus spp.)
  69. Zelkova (Zelkova spp.)

For Further Information

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

Article adapted from one published on May 2, 2015.

2 thoughts on “Don’t Grow Trees That Make You Sneeze

  1. Date palms that were recycled from orchards that were displaced by the urban sprawl of Las Vegas were exclusively female. The females are more billowy and more appealing than the taller and leaner males. There are not many males in the orchards anyway, since each male pollinator can service many females. Without males in the landscapes, the females are not nearly as messy as they could be if they were to produce unharvested fruit. However, I am aware of at least one parking lot landscape that is outfitted with nice groves of female trees that get a bit of pollination from a random male tree in a nearby home garden. The pollen does not get far, so the ‘increased’ fruit mess is barely noticeable. It could be worse though. If that male tree were in my garden, I would just cut it down.
    Pampas grass cultivars are likewise all female. Because they can not produce viable seed, they are considered to be ‘sterile’. However, they can hybridize with naturalized pampas grass (of another species), which can be a problem. However, if the other species is already naturalized, a few hybrids does not make much difference.

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