Healthy Choices

How we plant and maintain our landscapes has a direct effect on the health of the people living, working or playing in these landscapes.

Green areas that produce high amounts of either pollen or mold spores will consistently make people sick, and landscapes that are clean and low in pollen and spores will make people feel better.

Allergies are now the most common illness in the United States, affecting some 40 percent of the entire American population. Asthma rates are growing so fast that they are now typically being called “epidemic.” Childhood asthma in the year 2005 has become the No. 1 chronic childhood illness in America. Sudden death from asthma, once considered very rare in the United States, is now increasingly common. Clearly all of us should be doing whatever we can to remedy this unhealthy situation.

Landscape contractors and consultants, groundskeepers, golf course superintendents and managers of hospitals, retirement communities, university campuses, amusement parks, city and county parks and many professionals involved with the planning or maintenance of large, commercial landscapes are in a position to make a difference. Increasingly, planned communities, hospitals, schools and universities are working toward making their own landscapes healthy places to be.

In the overwhelming majority of cases, whenever people are asked if they would prefer that their landscapes be as pollen-free as possible, their answers are a resounding “Yes!” There is no downside to low-allergy landscaping, other than that it initially takes a little more effort on the part of the landscape professionals in charge. Maintenance of a low-allergy landscape is no harder than that of a more typical high-allergy landscape.

As the public grows ever more sophisticated about allergy-free landscapes, green industry professionals will be expected to be ahead of the curve. You'll need to show that you are knowledgeable on the subject, and that in the future, instead of planting and maintaining high-allergy problem areas, you will want to be able to explain that you are, instead, doing everything possible to produce healthy urban landscapes.


We have two main concerns when trying to create healthy landscape areas where the air is fresh and good to breathe. First, our aim is to produce as little allergenic pollen as possible. Second, we want landscapes that generate very few allergenic mold spores. Both goals are completely do-able, if you know what you're doing.


With the addition of OPALS, (Ogren Plant-Allergy Scale), professionals now have the needed tools for allergy-free plantings of low-pollen and pollen-free landscapes. This numerical scale ranks all landscape plant materials on a simple 1 to 10 allergy basis. Plants that produce zero pollen, normally all-female cultivars, usually rank the best: No. 1. Trees that have abundant highly allergenic pollen, especially ones with long bloom periods, are usually ranked the worst: nine to ten. There are many trees and shrubs, however, that fall somewhere in between. Using a list of more than 125 factors, OPALS numerically ranks each species, and then further ranks individual cultivars. There are often dramatic allergy differences between two species of the same genus, and individual cultivars of the same species may also vary greatly.

The OPALS list was developed with two key questions:

  1. What do plants that are well known to cause allergy have in common?

  2. What do plants that are well known not to cause allergies have in common? Based on the above scale, it is now possible to state, for example, that Acer rubrum (Red Sunset Maple) is a No. 1 and causes no allergies. Most pine trees will rank at No. 4 and will cause some allergies. Most Platanus species (Sycamore) rank eight, and cause quite a bit of allergies. A large male (“fruitless”) mulberry tree, ranked one of the worst at ten, and will produce a super abundance of pollen and trigger severe allergic reactions to many living near it.

In my work with landscape designers, I've found that it's common for their goal to be creating attractive, interesting, diverse, easy-to-maintain landscapes where the average OPALS number is four or less. When higher-numbered (more allergenic) plants are used, they are normally used sparingly and are planted downwind and as far away from most human traffic as possible. One of the first commercial low-pollen landscapes using OPALS was the one designed for the new, “all-green construction” American Lung Association Regional Headquarters, in Richmond, Va., in 1999. Since the year 2000 and the publication of my book, Allergy-free Gardening (Ten Speed Press), low pollen and pollen-free landscapes have been used for many individuals' houses, housing developments, schools, parks, universities and hospitals.

Because some stray pollen will always blow in from other areas, we can never create landscapes that will always be 100 percent free of all pollen. However, this is not our aim. Our aim is allergen reduction. Allergies and asthma are most often triggered by over-exposure to heavy amounts of pollen. Pollen exposure is always greatest closest to the pollen source. Thus, a heavy pollinating male tree in bloom will shower allergenic pollen on everyone and everything directly beneath it's own drip line. The further away one is from the source, the less the exposure.


Healthy plants may equal a clean-air landscape. To create landscapes with low mold spore counts, the most important thing we can do is to make intelligent choices when we plant. Plants that fail to thrive often become magnets for insect pests. These sucking insects (whitefly, scale, mealybug and aphids) all secrete large amounts of “honeydew.” These copious insect secretions are sticky and very nutrient rich, and dry airborne mold spores land on them where they stick, germinate, reproduce and quickly explode in number.

Landscape trees and shrubs that look dirty are all too often under attack by insects and are covered with mold spores. All types of mold spores are highly allergenic and no landscape high in mold spores can be considered a healthy environment for the people working or living close by. When we choose landscape plants to use, we should always select ones that are very well adapted to our particular microclimate and soils.


Landscape plants that are under-watered will become stressed and will become targets for insect pests. Plants that are over-watered will become stressed and will also become targets for insect pests. Plants that are lacking in proper fertility may also eventually become stressed, and thus targets for insect pests. The more we limit insect pests, the fewer mold spores there will be in our landscapes.

Automatic irrigation systems that are not regularly attended to, especially ones set to over-water, will not only stress the landscape plants, but they will encourage mold growth in the soil and in the mulches used.

  • Mulch

    Organic mulches that decompose quickly often harbor mold spores, especially when they are over-watered. Increasingly we are recommending the use of non-organic mulches, especially smooth river rock or river gravel.

  • Air space

    Mold growth is encouraged by cool, damp, stale air conditions. Too often, young shrubs are planted with little consideration for their ultimate mature size. These plants may eventually overlap each other, smothering out the air space between them. Likewise, trees are often planted too close to each other, and many foundation plants are first set far too close to the buildings themselves. The more open and airy a landscape is, the better the air circulation and the less likely it will be to harbor mold spores.


To have healthy landscapes, that won't trigger allergies and asthma, use all of your best horticultural practices to keep them thriving and as insect- and mold-free as possible. Also, stop planting highly allergenic types of trees, shrubs and groundcovers. In particular, try to avoid using any male clonal selections, often sold as “seedless,” “podless” or “fruitless,” because all of these will produce large amounts of the worst kind of pollen. Whenever possible use female selections, as these will never produce any pollen. Formal-double flowered plants are also very useful as they normally produce little or no viable pollen.

Thomas Leo Ogren is the author of five published books and hundreds of articles on health and gardening. He is the author of Safe Sex in the Garden, Allergy-free Gardening and What the Experts May Not Tell You About Growing the Perfect Lawn. You can visit his Web site at


  • Red Maple (Acer rubrum ‘Red Sunset’) is a pollen-free, female tree. Ranked best: No. 1.

  • Southern Red Maple (Acer rubrum ‘San Felipe’) is a pollen-free, female tree, especially well-suited to warmer climates. Ranked best: No. 1.

  • Weeping mulberry (Morus alba ‘Pendula’) is a female, pollen-free small tree, easy to grow. Ranked best: No. 1.

  • Freeman Hybrid Red Maple (Acer X Freemanii ‘Autumn Fantasy’) is a pollen-free hybrid cross between red and silver maples. Ranked best: No. 1.

  • Juniperus scopulorum ‘Spearmint’ is a narrow, upright growing juniper that is female and pollen-free. Ranked best: No. 1.

  • Box Elder (Acer negundo ‘Variegatum’) is an easy-to-grow, small tri-leafed maple tree with highly colored leaves. It is a pollen-free female tree and OPALS ranked it best: No. 1.

  • Princess 77 Hybrid Bermuda grass (Cynodon dactylon hybrida ‘Princess 77’) is a newer, improved Bermudagrass, producing next to no pollen. Ranked: No. 2.

  • White Mulberry (Morus alba) “fruitless” is a highly allergenic male clone. Ranked worst: No. 10.

  • Common Bermuda grass (Cynodon dactylon) is a high-pollen-producing lawn grass. Ranked worst: No. 10.

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