Nothing to Sneeze At: Your Guide to Allergy-Free Gardening

Hold onto your hankies, dear readers: 2013 is shaping up to be one of the worst years on record for allergy sufferers. Not only that, pollen counts are expected to double by 2040.

Last winter’s heavy precipitation in particular and climate change in general are contributing factors. But there’s another cause, one hidden in plain sight and rustling above our heads, that has people reaching for the Claritin.

Turns out, it’s the men in this town — male trees in our public spaces, to be exact — that are causing an uptick in allergic and asthmatic reactions.

“Male trees produce no seeds, no seedpods, no fruit, so they’re ‘litter-free,’ tidy, clean,” says Thomas Leo Ogren, horticulturist and author of Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping. “We fill our cities with these male clones as a ‘modern’ way of landscaping, and the end result is everyone who lives close to them is constantly overexposed to allergenic pollen. The end-end result is an ever-increasing amount number of people with allergies and asthma.”

Thomas is also the creator of OPALS, a 1-10 scale that rates plants based on their allergenic potential, used by organizations like the American Lung Association. (For example, a “completely pollen-free tree like Red Maple ‘Autumn Glory’ ” receives a rating of one. On the other hand, Southern California’s ubiquitous bottlebrush gets a code-red rating of nine.) He was also kind enough to share some of his pollen-allergy wisdom with us here at The Horticult.

Below, he reveals what plants to embrace and what to avoid in your landscape, names the cities that are becoming more “plant-considerate,” and shares the incredibly romantic story of how he got into his line of work. Spoiler: it involves Yvonne, his wife of 46 years.

How prevalent are these male trees we’re talking about?

In some cities I’ve studied, male trees make up more than 80 percent of all the planted trees, and the situation with shrubs is the same, or maybe even worse. In my own city of San Luis Obispo, California, we have some 9,000 ash trees, and all but three are males. We also have huge numbers of planted Chinese pistache trees, almost all males; we have thousands of juniper bushes and most are male; here, as in most of the state, we also have many thousands of Xylosma shrubs and trees, and 100 percent of these in the state are male clones…it goes on and on.


Lombardy poplars like the ones above pump allergenic pollen into our shared spaces. (Ditto all male clones of the willow, the tree pictured in the topmost photo.)

So, these males are planted — and then what?

[In] dioecious species, that is, separate-sexed species, the pollen from the males must sometimes travel a considerable distance to reach the females. As such, the males of these species produce huge amounts of pollen, and it is usually very small, very light, and buoyant in the air. It is exactly the kind of pollen that is the most allergenic.

We fill our cities with these male clones as a ‘modern’ way of landscaping, and the end result is everyone who lives close to them (proximity pollinosis) is constantly over-exposed to allergenic pollen. The end-end result is an ever-increasing amount number of people with allergies and asthma.

 What’s the worst example you’ve seen?

Perhaps the worst landscaping, from an allergy point of view, is found in state schools, especially the elementary schools, where the most common shade tree planted is the “fruitless” mulberry tree. It is, of course, a male clone, and produces vast amounts of extremely allergenic pollen. Childhood asthma is now the number-one chronic childhood disease in the country.

Wow — that is truly unsettling. If someone wants to transform their space into an allergy-free haven (or rally on behalf of their school’s space, for example), what are some plants you recommend?

First of all, I always recommend that people use OPALS as a guideline. Any plant with a ranking above five or six you really don’t want in your own gardens or yards.

I strongly recommend the use of any and all female plants. Female lawns (for example, UC Verde Buffalo grass) produce no pollen. Flowers on female plants produce much more nectar than do male flowers, and are wonderful for hummingbirds, butterflies, bees, et cetera. Female plants produce fruit and seeds, and this feeds a large numbers of native songbirds. These same birds eat up large amounts of insect pests, leaving the whole garden much cleaner, and with considerably less mold. Female plants have flowers that carry a negative electrical charge — airborne pollen has a positive charge — and thus mutual attraction comes into play. What this means is that female plants trap pollen, lots of it, and they are in effect nature’s finest air cleaners.


Orchids very rarely cause pollen allergies, so they are a good choice for healthy gardens and bouquets.

What are some specific species that you like? (Here, you recommend orchids, banskia roses and Hydrangea macrophylla for brides…) For example, what do you grow in your space?

I collect female plants, and have females of close to a hundred species in my own collection. In my own yards I heavily plant fruit trees, guava, plums, peaches, nectarines, oranges, tangerines, apricots, pears, apples, and avocado. If it makes fruit it normally is not an allergy problem (exceptions are sweet cherries and almonds).

Some of my fruit tree selections, such as Fuyu persimmon, are all-female plants. All of my citrus are seedless, parthenocarpic [i.e., “virgin”] fruit, and these too are all pollen-free. I also always grow a few things like tomatoes and peppers.

I also have a great many roses, including some pollen-free selections, and also rose from my own hybridizing. I grow a large amount of flowers, mostly perennials, and with these I only use ones with low allergy potential, such as salvia, ivy geraniums, tuberous begonias, et cetera. I plant low-allergy perennials such as Cuphea for the honey- and bumblebees, and Asclepias for the butterflies. I use bird baths, suet feeders and selected plants to encourage as many songbirds in my yard as possible.

For groundcovers I use red apple iceplant, which is a terrific plant for honeybees; I also use an all-female form of wire plant, Muehlenbeckia complexa, as a ground cover, and also some all-female spider plant. I also use some tall fescue, which if mowed regularly never produces any pollen.

On the other side of the spectrum, what plants are the most problematic? 

In California, the worst ones for urban landscapes are: any and all male clones of willow, poplar, aspen, cottonwood, ash, pistache, cedar, mulberry, juniper, yew, red maple, silver maple, box elder, Podocarpus, holly, coyote bush, sedges, restios, pepper trees.

In addition, there are some monoecious trees [i.e., having both male and female reproductive organs] that are also quite allergenic and greatly overused. These would include: olive trees, some species of eucalyptus, oaks, sycamores, acacia and cypress.

What might surprise people about the connection between city planning and their allergies? 

In general, city planning has only made allergies worse. The more “modern” or “educated” the landscape is, the more allergenic it is — but this is changing! In some cities city planners are starting to pay attention. In Canada, city planners from both Edmonton and Toronto are now using OPALS to plant more allergy-friendly city landscapes. Likewise in New Zealand, the city planners (all of whom came to the talks I gave there) in both Auckland and Christchurch are also now using OPALS and are planting as many new allergy-free trees as they can.

Are similar improvements being made here in the U.S.? What kind of cultural shift needs to happen here?

In some cities I’ve worked with, for example Albuquerque, New Mexico, they now have a pollen control ordinance on the books. There it is illegal to plant or sell male clonal trees and shrubs of many species. Likewise, other cities such as Las Vegas and Tucson have banned the planting of olives and male mulberry trees.

In California, the State Department of Public Health has endorsed using the concepts of allergy-free gardening as a way to combat the epidemic of allergic asthma. Things are starting to change, if too slowly, but there is some light at the end of this tunnel, I think.

In the meantime, millions of extremely allergenic trees and shrubs are being sold and planted all across the U.S. For this to change, it will need to become trendy to plant allergy-friendly plants instead. Education is the key, and those with allergies need to help spread the word. Likewise, people need to get involved more on a local level, and we need to get our own cities to start to develop urban forests that are friendly for all of us, including those with allergies and/or asthma.

What’s the most surprising pollen behavior you’ve observed?

I have seen case after case in California where people who had a male Podocarpus tree or shrub planted right next to their bedroom window started to experience symptoms that mimicked chemotherapy treatments. These people often developed a persistent cough, then headaches, felt generally miserable for months on end.

Podocarpus is a poisonous species and the pollen is also toxic, especially when inhaled. The individual pollen grains are so small that a thousand of them could pass through each little square hole in a typical window screen, simultaneously. Podocarpus is a close relative of the true yews, the Taxus species, from which the chemo drug Taxol (used for breast cancer) is derived. Likewise there is also another chemo drug made from Podocarpus, and it is used for people with leukemia.

Once we’ve IDed the problem, the male Podocarpus is removed, and is replaced with a female Podocarpus, or any other allergy-friendly shrub. And…bingo! The person is cured.

On the East Coast I find the exact same scenario happening with those who have a male yew planted next to their bedroom windows. When these species are in bloom, you can simply flick your finger on the male plants, and a little cloud of “smoke” will arise….it is of course, pollen — pollen that is not only quite allergenic, but also poisonous.

How did you get into this line of horticulture?

I have always been extremely interested in plants — been planting seeds, on my own, since I was five years old. I was trying to graft trees by the time I was nine. (I’m 66 now.)

I’ve been married to the same girl for over 46 years, Yvonne, and she’s always had bad allergies and asthma, too. She was 15 when I met her. By the time we bought our first house, I wanted to re-landscape it so that nothing in our yards would trigger her allergies or asthma. By then I knew they were connected, as often she’d come down with bad pollen allergies and it would turn into asthma — dangerous, scary asthma.

In the beginning there was next to nothing to read on this, so I just started to study the plants themselves, and their flowers and pollen production, plant by plant. A great many years later, that evolved to be the book Allergy-Free Gardening.

How is Yvonne doing now?

Nothing I’ve planted causes Yvonne any allergies. Her allergies are better, and her asthma has now been under control for quite a few years.

I can’t control what my neighbors plant, alas. But in the long run my aim is to get people to be “plant considerate.” Even if you don’t have allergies yourself, there’s no reason when selecting a new tree or bush, not to be considerate of those who do.