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By Thomas Leo Ogren
Some urban tree species cause an inordinate amount of asthma and allergy problems, while other tree species cause little or no health
problems. A large part of the problem is that the arborists and landscape professionals, who plant these trees, often don’t know the
difference.
The type of trees (and shrubs) used in modern city landscapes has changed dramatically in the past three decades. In the past, the majority of
street trees used were perfect-flowered, insect-pollinated trees, such as the once so common American elm tree. Today though, many of the most
widely used city trees are wind-pollinated species. Most of these species are unisexually flowered (dioecious and/or monoecious) and further
compounding the problem, thousands of popular cultivars sold today are touted to be “seedless,” “low-maintenance,” “pod-free” or “litter-free.”
These fruitless, seedless trees are male plants, all male, and male trees produce prodigious amounts of allergenic pollen. Female trees produce
NO pollen what so ever.
In dioecious-flowered trees such as most ash, willow and poplars, it is easy to propagate male only trees because they are separate-sexed.
Monoecious trees, which in Nature always have both sexes (male and female flowers) on the same tree, also usually produce abundant pollen. It is
possible to have all-male trees from the monoecious species. On many species the sexes will be born on separate branches, such as on a Honey
Locust tree. If you take cuttings, or budwood, only from the branches with male flowers, then, you'll get an all-male tree. Lots of monoecious
Acer spp. cultivars are male-only plants.
In a somewhat different way, there are also numerous monoecious species where only the top or only the bottom will have either male or female
flowers. For example, the bottom half of a mature Italian Cypress for example is all-male. Female wood is found only at the top of the plant.
Thus, scion wood taken from the bottom usually produces "seedless" plants.
The terms “dioecious,” and “monoecious,” are botanical terms, not horticultural terms. We could perhaps say that a manipulated, asexually
propagated all-male cultivar, taken from a monoecious species, is now “dioecious,” but this is not quite correct. A proper dioecious tree is one
that in Nature is separate-sexed. These modern engineered trees are never found in Nature.
Interestingly, the first reference in print I ever found to this single sexing-out with monoecious street trees, was in a USDA booklet, from
1982, called “Genetic Improvement of Urban Street Trees.”
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