Landscaping that's nothing to sneeze at

Thirty years ago, fewer than 12 percent of Americans had allergies. Today, a whopping 38 percent of us suffer from them. Not too many years ago, death from asthma was fairly rare. Today, sudden death from asthma is all too common and is now considered an epidemic. Asthma has suddenly become the number-one chronic childhood disease in America. Allergies are not just a nuisance but a major cause of suffering for many people.

To make matters worse, new data shows a strong connection between overexposure to pollen and mold spores and increases in other diseases such as heart disease, autism, pneumonia and reflux disease. However, these allergies aren't just an immutable fact of nature — they can be prevented. Much of the overexposure that causes allergies can be found in our own backyards. As landscapers, we have the ability to control allergens and improve the health of our communities through the plants that we use.

Blowing in the wind?

For many years no one paid much attention to the idea of allergy-free landscaping because as they said, “pollen blows.” Some claimed that you could go 100 miles out to sea in a ship and catch pollen in a pollen trap. So why bother with your own landscape if pollen can travel that far?

This old idea about pollen dispersal was partly right but mostly wrong. Yes, you could trap some pollen way out there in the ocean, but the pollen of most species would never travel that far.

In 1972, Gilbert Raynor, a meteorologist from New York, set up an experiment. He put pollen traps at close intervals starting right next to a large, pure stand of timothy grass being grown for hay. Timothy (Phleum spp.) pollen is known to be exceptionally light and buoyant — the sort of pollen you might expect to trap far out at sea.

At a mile from the field, Raynor was able to trap some timothy pollen. However, at a half mile from the field, he found that more than 99 percent of all the pollen had already landed and stuck. Closest to the field, he found by far the greatest concentration of pollen.

What exactly does this mean? Quite simply it means that the law of gravity applies to pollen. Research shows that with most trees, the largest amount of pollen falls out and lands within 30 feet of the drip-line of the pollen-producing tree. The closer you are to the tree, the more pollen you get.

Allergies develop from repeated overexposure to the same allergens. If your yard is full of pollen-producing trees and shrubs, you and your family are the ones who will be exposed the most. If the schoolyard where your children play is surrounded by allergy-causing shade trees, your children will be the ones most affected.

Maintaining diversity

Biodiversity is important in creating an allergy-free landscape. Almost any species of plants can eventually cause allergies if it is overplanted enough. Many landscapers tend to use the same plants over and over again. When residential houses are professionally landscaped with the exact same plant materials used to landscape banks, real estate offices and dentist shops, we all lose.

Lack of biodiversity isn't a recent problem. The landscape tree most common throughout most of America for many years was the stately American elm. This tree used to grace the streets of thousands of towns and cities. It caused a certain amount of low-level, early-spring allergy, simply because it was so common. When Dutch-elm disease (DED) started to spread and kill off these native elms, the insect-pollinated elms were most often replaced with wind-pollinated street trees. These replacement trees may have been more potent for allergens than the original elms. DED spread mostly from east to west across the United States, and so did the rise in allergy rates. It's possible to track the spread of allergy from the decline of the elms.

Battle of the sexes

Female plants produce seeds, seedpods and fruit. This “litter” falls on the sidewalks and creates a messy landscape. Unfortunately, plant breeders and propagators discovered how to “sex-out” many of these trees and shrubs. By using only male cultivars, they were able to create “litter-free” landscapes. They learned to use only male plants as the source for their scion wood for asexual propagation. First, they just used male plants from the dioecious (separate-sexed) species, but later they learned how to produce all-male clones from species that were never single sexed in nature. In the United States today, four out of five of the top-selling street-tree cultivars are male clones.

What exactly is the effect of using all male-cloned trees and shrubs? Very simply, this causes an excess of allergenic pollen. All-male flowers produce most of this airborne pollen. In contrast, unisexual female flowers produce no pollen.

Today though, most of the female plants are long gone from our landscapes. The pollen from the males floats about, and our mucus membranes, our eyes, skin and especially the linings of our nose and throat, now trap this wayward pollen.

Unhealthy trees, unhealthy people

Another major allergy problem comes from tiny airborne reproductive spores, which can be found on unhealthy trees. If you look up at a tree and the leaves look dirty, this is usually because they are indeed filthy. They're covered with insects and mold. Often a tree like this will produce incredible amounts of mold spores for months on end. These spores are usually much smaller than pollen grains and can be inhaled deep into the lungs. In mild southern climates, this mold formation can go on year round and continually shower everyone nearby with allergy-causing spores.

Mold spores grow when insects invade the unhealthy trees. Pests such as aphids, scale insects, mealybugs and whiteflies frequently attack unhealthy plants. These insects suck the vital plant juices. Waste matter secreted by insects is commonly called “honeydew,” which is nutrient rich. Almost immediately, molds will grow on this fertile substance and quickly start to reproduce with billions of tiny airborne spores.

The insects on the tree may not really be the cause of the tree's deterioration — they're just a reflection of a more basic problem. A tree may become unhealthy for several reasons, usually because it is not the best tree for that particular spot. Whatever the specific reason, a sickly tree is one that will soon be shedding mold spores and causing allergies.

On a scale of 1 to 10…

With the thousands of plants available to landscapers, some plants are bound to pose more of an allergy threat than others. To help establish which plants are more dangerous, I created an allergy-ranking system. On a scale of 1 to 10, 1 is the best allergy-free selection, and 10 is the worst. The sidebar on this page lists several plants and where they rank on this scale.

A hundred botanical and medical factors have been used to develop the ranking system. This system takes into account inhalant allergies from pollen, allergies from certain plant odors as well as contact allergies that cause skin rash.

Creating your allergy-free landscape

Allergy-free landscaping is really a type of NRM, natural resource management. We need to get back to landscapes that are naturally diverse, that use plants that will thrive and that use a blend of plants that are sexually balanced. Professionals in the green industry are the perfect individuals to start managing our landscapes to ensure that they are not just beautiful but healthy for everyone.

Landscapers can also profit from this new information. As customers become more informed, they will expect their landscapers and gardeners to know about allergy-free landscaping. They will seek advice from you, so be armed and ready with the right know-how.

Already a considerable number of new landscapes are being created where the plans call for either an allergy-free landscape or at least for a low-allergy one. Customers are increasingly hiring knowledgeable professionals who can remove the high-allergy plant materials and replace them with pollen-free choices. Presently, allergy-free landscaping has the greatest appeal for customers who are well-educated and financially well-off. Landscape consultants have been getting into this new market recently, and as with most things, those who get there first make the most money.

Thomas Ogren's allergy-ranking system can be found in his book, Allergy-Free Gardening, published in 2000 by Ten Speed Press, Berkeley, Calif.

Thomas Ogren, author of Allergy-Free Gardening, is a former landscape gardening instructor, nursery owner and gardening radio show host. You can visit his website at


Only two types of plant materials are pollen-free. Female plants produce no pollen, and they also trap pollen, making them the backbone of any allergy-free landscape. However, some types of plants are neither male nor female and do not have any male, pollen parts. Many formal double camellias, for example, have so many petals in each flower that there is no room left for reproductive parts. Plants like this are often hybrids and usually have fully doubled flowers. These are known simply as “pollen-free” plants.


The following plants are ranked 1-10, with 1 being the best allergy-free plants and 10 being the worst.

Acer palmatum (Japanese maple) — 5

Eucalyptus (gum trees, ironbark) — 6 to 8

Festuca elatior (tall fescue) — 3

Ligustrum (privet) — 8

Morus alba (white mulberry) — males 10, females 1

Pelargonium (geranium) — 5

Pinus (pine) — 4

Pyrus (pear) — ornamentals 4, fruiting varieties 3

Quercus (oak) — deciduous 8, evergreen 9

Rhododendron — azalea 3, rhododendron 4

Want to use this article? Click here for options!
© 2020 Penton Media Inc.

Interactive Products

Equipment Blue Book

Used Equipment Valuation Guide

Riding mowers, lawn tractors, snow throwers, golf carts


Grounds Maintenance Jobs

search our jobs database, upload your resume