Presentation at the Australian Society of Arboriculture Conference (May 2nd and 3rd, 2017)
By Thomas Leo Ogren
Australia is considered to be one of the five healthiest countries in the world, but rates of allergy are high there, and asthma is epidemic.
The focus of this paper is on urban areas, not rural. There has been an explosion in the percentages of people who now have allergies, and most of the allergy is in urban areas. Urban areas present unusual allergy problems: air pollution, mostly from cars and trucks, is worst in cities; cities have a great deal of hard surface, and on this pollen lands, and then often becomes airborne again. The cities also have increased temperatures, known as the "urban heat island effect."
Most notable about cities and allergy is that the city-made air pollution can and does combine with pollen grains, and the net result is that the pollen becomes much more allergenic. Sometimes, as happened last year, again, in Melbourne, thunderstorm activity causes pollen grains to rupture, to explode. Inside these tiny pollen grains are thousands of even smaller starch granules. The microscopic particulates of this exploded pollen can be inhaled deeply into the lungs, and the result is asthma.
But allergy and cities have another problem that has not been properly addressed: when the cities were planned and the city trees were planted, little, if any, thought was given to how allergenic or not those trees were. Unfortunately, large numbers of allergenic trees were planted throughout all the largest Australian cities. Likewise, when lawn grasses were chosen, again, no thought was given to how much allergenic grass pollen these might shed. The shrubs of the cities are also suspect, and bushes such as cypress, boxwood and male yes and male junipers were used...all of them potentially allergenic.
Commercial nurseries have long propagated and sold large numbers of woody plants that are said to be "low-maintenance" or "litter-free." Sometimes these are sold as "non-blooming" or as "podless" or "seedless." What these actually are is male clones, and all of them are quite allergenic. This phenomenon, which is worldwide in all industrialized nations, is now called "botanical sexism." The worst problem with this botanical sexism is that there are too many male trees and not nearly enough female ones. Male trees always produce pollen; female trees produce no pollen, ever, and female trees also trap and remove pollen from the air. In most cities though, there is little or no balance of the sexes, and allergy and asthma are the result.
It has often been written in newspapers or journals or on-line in Australia that the native Australian plants don't cause allergies....the allergies (supposedly) are triggered by non-native trees. In truth, both native and non-native trees can cause allergies. Australia is home to many hundreds of species of Eucalyptus, and most Eucalyptus pollen does cause allergies. Eucalyptus pollen is large, heavy, clumps up, and is triangular shaped, so it doesn't travel far in the air. Most of the pollen from Eucalyptus trees falls quite close to where the trees are growing, and this is the zone where allergies to Eucalyptus take place. It should also be noted that in California, Eucalyptus pollen more often than not has tiny insects living in and on the pollen. These insects (some sort of microscopic thrip it is believed) are allergenic themselves, as is the insect dander from them.
Even though there are said to be more than 800 species of Eucalyptus in Australia, no one has ever done a thorough study of how allergenic, or not, each of these species is. We do know that in some species, Eucalyptus ficifolia, red flowering gum, that the pollen is extremely sticky, and thus never does go airborne to cause allergies. More work, per allergy potential, needs to be done on the entire genre of Eucalyptus.
Silver Wattle trees are native to Australia and beautiful when in bloom. It has been written numerous times that pollen from this species (Acacia dealbata) is not allergenic because the pollen grains are so large, and because the trees are heavily visited by pollinators. However, wattle trees produce both perfect-flowers (with male and female parts inside of each flower) AND also many all-male unisexual flowers. As such the trees produce considerable pollen and it is often a problem for those in the immediate vicinity of the trees.
With the Corymbia (genus) many trees are naturally all-male, androdioecious. There are no all-female trees of Corymbia. As a result, this genre of Eucalyptus-like trees has great potential for triggering allergies.
White cedar, Melia azedarach : In 2006 Metropolitan Tree Growers Pty Ltd advertised to have about 1000 sterile White Cedar trees available. This selection was discovered in a local streetscape. The flower forms to a bud without opening, resulting in no pollen and ultimately no seed. These would make excellent allergy-free trees for city streets.
Casuarina (he oaks, she oaks) is a genus of 17 tree species in the family Casuarinaceae, native to Australia, the Indian Subcontinent, southeast Asia, and islands of the western Pacific. These are common city trees in some parts of Australia. Casuarina is a dioecious species, entirely separate-sexed. The male trees, which grow quite large, are huge producers of extremely allergenic pollen, and really have no place in a city. Female Casuarina trees, which make seed cones that stick to the stems of the trees, the female trees produce no pollen, and they are allergy-fighting trees in that they also trap and remove pollen from the air.
Casuarina trees can be grown from cuttings, which grow on quickly, and it is advised that a great many should be grown from cuttings from selected female trees.
Birch trees: Silver Birch (Betula Pendula) are widely planted in many Australian cities, and everywhere they are used, they trigger allergy. There is a great deal of cross-reactivity between birch pollen and various foods, so that one triggered, a birch pollen allergy can quickly lead to food allergy (or intolerance) of more than a dozen foods, such as apples, pears, plums, melons, etc. Planting of any more silver birch trees should be stopped, as should the sale of them.
Olive trees are common in many cities in Australia, and olive pollen is a potent allergen. It is recommended that the sale and planting of olive trees within the city limits, be discourage, or better yet, banned.
Platanus, sycamore: There are many species of Platanus that are grown in Australian cities, and they do cause allergies. However, it is important for arborists to understand that when pollarded each winter, sycamore trees will not cause pollen-allergy. Large sycamore trees in the city should be routinely pollarded to limit their allergenic exposure.
Evergreen alder, Alnus acuminata: Popular street tree that sheds considerable pollen each spring. Alders too can be pollarded, and again, this will eliminate the pollen for that season.
Dodonaea viscosa, hopseed bush: Dodonaea are native and common in Australia. They are dioecious, separate-sexed, and the male plants produce clouds of pollen. Female plants look just as good, or better, and produce no pollen. Female selections of Dodonaea viscosa are quick and easy to grow from cuttings. A row of tall, female Dodonaea viscosa makes an excellent allergy-fighting windbreak or tall hedge.
Podocarpus species, mountain plum yew: Podocarpus, native Australian trees and shrubs, are all dioecious, separate-sexed. The leaves and bark of these trees is toxic if eaten, and the pollen is likewise toxic when inhaled. Exposure to the toxic and allergenic pollen of Podocarpus is common in close proximity to where males of this species are planted. Podocarpus is easily grown from cuttings, and it is strongly suggested that good female selections be made, and that the Australian cities would greatly benefit from the planting of many of these selections.
Grasses and grass-like plants: There are 50 species of Lomandra in Australia, and all of them are functionally dioecious. Pollen from males of these common landscape plants triggers allergy. Female Lomandra can be identified, and then propagated via division.
Kikuyu grass: There is a form of male sterile Kikuyu grass that already is popular in South Australia because of its drought tolerance and hard wearing qualities. It is a medium blade grass that can grow with very little water and thrives in South Australia's dry heat. Most Kikuyu varieties can spread by dispersing their seed into nearby garden beds making it extra invasive. Male sterile Kikuyu does not produce a seed, making it much less invasive. Male sterile Kikuyu grass is also pollen-free and allergy-friendly. Greater use of this grass should be made.
Buffalograss, Buchloe dactyloides: Native to the US, buffalograss grows low, needs little fertilizer, and is exceptionally drought tolerant. Buffalograss is dioecious, and there are some very fine all-female selections that can be purchased as plugs. Perhaps the very best one for Australia would be the female clone known as 'UC Verde'.
In the Melbourne area thunderstorm asthma has been triggered by excessive amounts of airborne grass pollen from perennial ryegrass. There are now a good number of very interesting and useful intergeneric hybrid crosses between perennial ryegrass and fescue grasses. Many of these new hybrids look almost the same as ryegrass, but they are male-sterile, and produce no pollen. These can be planted from plugs, sod, or from seed. There is a grass breeder in Oklahoma who is working on these, and if interested, I could make introductions.
Perhaps the whole idea of allergy-friendly urban environments boils down to this: It is something that can be done; we already have the tools we need to do this; great numbers of people would be spared their annual pollen allergy misery; some lives, especially of children, would be saved, and lastly, the financial savings from reduced allergy and asthma would be huge. Let's do it, Australia!