Pollen on a sidewalk from a male cedar tree. (Photo: Thomas Leo Ogren)

It’s the time year for watery eyes and itchy noses, and if you’re among the afflicted, you may be surprised to learn that decades of botanical sexism in urban landscapes have contributed to your woes.

Arborists often claim that all-male plants are “litter-free” because they shed no messy seeds, fruits or pods. In the 1949 USDA Yearbook of Agriculture, which focused on trees and forests, this advice was given to readers: “When used for street plantings, only male trees should be selected, to avoid the nuisance from the seed.” In the years following, the USDA produced and released into the market almost 100 new red maple and hybrid-maple-named clones (cultivars), and every single one of them was male.

These new male trees were soon joined by clonal male plants from a great many other dioecious tree species (having distinct males and females) such as willows, poplars, aspens, ash, silver maples, pistache, mulberry, pepper tree, etc. The wholesale growers also learned how to select male scion wood from trees that were monoecious (individuals with both male and female productive systems), and we started to see trees never seen before in nature, such as seedless cypress and pod-less honeylocust trees. If we take live wood (scion material) from a tree and grow it (asexually) from cuttings, grafting, or budding, it will be clonal, and will be the same sex as the tree it came from.

Red maple blossoming with pollen-covered flowers. (Photo: Liz West/Flickr)

Not satisfied with just the trees, the commercial growers then produced a flood of all-male shrubs, junipers, yew pines, fern pines, wax myrtles, alpine currants, plum yews, yews and more. In the past few years we’ve even seen all-male hanging basket plants like begonias. The problem is that while these trees and plants are “litter-free”, they all produce abundant allergenic pollen.

Prior to the 1970s there was only limited demand for new street trees, since almost every street in America seemed to be lined with those big, grand, long-lived stately American elm trees. But then Dutch elm disease struck and suddenly millions of our city trees started to die. By the mid-1980s many millions of elms had died and many streets were suddenly treeless. Enter the new modern, university-recommended trees: the clonal males. In short order millions of these wind-pollinated trees were grown, sold and planted to replace the old insect-pollinated elms.

It took a number of years for these new trees to mature enough to start to bloom, but eventually they did and with them came more city pollen and the “epidemic of allergy and asthma.” Many of these same trees are still alive and well and getting even larger, and the bigger they get, the more pollen they shed.

Pollen from male maple trees covering along the roadside. (Photo: Thomas Leo Ogren)

Despite what we often read about pollen blowing in from hundreds of miles away, in fact most pollen lands and sticks quite close to where it is shed. The greatest amount of pollen from a large tree will normally land within 20-30 feet from the dripline of the tree itself. This translates to what is known as “proximity pollinosis” or allergy that is triggered by the plants closest to where we live, work, play or go to school. Almost all urban allergies now come directly from the planted landscape–they are homegrown.

Allergies are rarely triggered by small amounts of an allergen; they are initiated by an overdose. Small amounts of pollen exposure are actually good for us, but if we have highly allergenic trees or shrubs in our own yards or lining our streets, we will soon enough be over-exposed. In order to put the brakes on America’s allergy epidemic, we need to reverse the trend toward male-dominated landscapes and stop selling and planting any more of the most allergenic trees, shrubs and grasses in our cities.

A male yew pine starting to shed pollen. (Photo: Thomas Leo Ogren)

A limited number of cities in the U.S. now have pollen-control ordinances banning the sale or planting of the worst of these clonal males. The Strategic Plan for Asthma from the California Department of Public Health recommends a statewide pollen-control ordinance. However, it has yet to be implemented.

For 30 years I’ve been researching the allergy-potential of all the commonly used horticultural plants. Sixteen years ago I first published the Ogren Plant Allergy Scale (OPALS(TM)), which has now been used by various cities and counties, asthma and allergy groups, lung associations and health departments, and even by the USDA urban foresters (UFORE, Urban Forest Effects Model). Nonetheless, use of OPALS is still the exception, not the rule.

With OPALS each plant is given a numerical ranking on a scale ranging from 1-10, where 1 is allergy-free and 10 is the most allergenic. Different cultivars of the same species are also taken into consideration, since there can be a world of difference between one and another where human health and wellness are concerned. Unfortunately, commercial horticulture has been very resistant to any kind of allergy-potential labeling of plants sold in nurseries.

Ecologists often tout urban trees for their ability to clean the air, for their particulate and pollution-trapping and removing qualities. Indeed big urban trees do trap and remove large amounts of toxic pollutants from air, soil and water. However, consider this: a tree, or any plant, will absorb many things besides just fertilizers. For example, plants will absorb xenoestrogenic compounds such as bisphenol A (BPA) along with a host of other toxins. But where do these toxins go once the tree has removed them?

They are now in and are part of the tree itself. If a large tree growing in a highly congested city that is packed with vehicular traffic were a female tree, it would shed seeds that would be contaminated with these same “removed” toxins. No one eats these seeds though, so they pose little threat to us. But what if that same tree is a clonal male, and each year it sheds vast amounts of airborne pollen? You can teach children at an urban school landscaped with large shade trees not to eat fallen seeds, but how can you teach them not to inhale pollen from the air around them?

Consider this: small children play hard and while playing they breathe in rapidly and are exposed to two to three times more pollutants in the air than adults are. It is also worth noting that schoolyards are often the places with the most allergenic trees. The highest official pollen count ever recorded in the U.S. (70,000 grains of pollen in each cubic yard of air space) was taken from the top of the administration building at an elementary school where all the mature trees, except one, were clonal males.

Children are not the only ones at risk, however. Women who have airborne allergies have been found to be at increased risk from leukemia, ovarian cancer and breast cancer, according to a 2010 study published in Cell Biology. It is time for a less sexually discriminatory planting strategy.

Female trees, even if they make seeds or pods, have much to offer us. A large female tree may easily have millions of individual flowers on her as she blooms. These tiny flowers are slightly sticky and feathery, and they produce a small negative electrical impulse. Pollen from male trees tumbles about in the air and picks up a positive electrical impulse. When you have negative and positive the result is mutual attraction. The pollen grains do not just get to the female trees by accident; rather they are drawn there by this mutual attraction.

Female trees produce no pollen, but they trap and remove large amounts of pollen from the air, and turn it into seed. Female trees (and female shrubs also) are not just passive, but are active allergy-fighting trees. The more female plants in a landscape, the less pollen there will be in the air in the immediate vicinity. By relying less on males and paying more attention to the allergy-potential of all the plants in our urban landscape, all of us may one day breathe easier.