Sex and Sneezing in Your Garden

Plant Gender, Allergies, and Pollinator Politics
Photograph By
Cheryl Corson, RLA, ASLA

Honeybees feeding on flower nectar transport pollen from flower to flower.

Plant selection is a lot like socially responsible investing: the more you know about it, the harder it is to do the right thing. Are native plants good or bad? That depends. Is pollen good or bad? That depends. Are plant-eating bugs in your garden good or bad? That depends, but usually better than you think. 

Add to the conundrums of bee colony collapse disorder, monarch butterfly decline, bat white nose syndrome, and dare we say it – climate change, is the precipitous rise in recent decades of allergies in the American human population. Allergies now rank fifth in all American chronic diseases Pollen allergies are connected to asthma, another serious chronic disease, especially in children. Allergy proliferation may be attributed in part to commercial ornamental plant propagation practices. Here’s how.

Plants and Sex

Before industrial scale agriculture and horticulture practices, street trees were grown from seedlings, a slow natural process resulting in approximately 50% male and 50% female plants. You may remember that some trees and shrubs are dioecious, which means male and female plants are separate individuals. Female trees tend to be called “messy,” meaning when people encounter their fruit, flowers, and seeds they don’t like all the cleaning up. If you live on Capitol Hill you’ll know that female Ginkgo trees shed their pungent fruit on sidewalks in the fall. Many people complain about this despite a long list of health benefits conferred by Ginkgo fruit (see Web MD), including as a remedy for asthma. 

According to the great description in his new book, The Allergy Fighting Garden (2015, Ten Speed Press), and a chapter called, “Botanical Sexism and Our Current Allergy Crisis,” author Thomas Leo Ogren explains how modern plant cloning and other propagation practices carried the U.S. Department of Agriculture’s mid-20th century recommendation to select for male plants to avoid all that female messiness. And on a massive scale, that’s just what happened, resulting today in mostly male street trees which produce, you guessed it, copious amounts of airborne pollen in environments with highly concentrated human populations. Ogren and others correlate this doubling of urban airborne tree pollen with the rise in allergies. His book makes a clear and convincing case.

Pollen and Pollinators

“But wait a minute,” you say, “I thought we were supposed to help bring back pollinators like the honey bee and the monarch butterfly. I thought pollen was good.” Keep in mind that, as Ogren says, “if you spend as much time looking at pollen under a microscope as I do, you’ll learn that all pollen is different, just like people.” He differentiates between predominately lightweight airborne tree pollen which is problematic, and the larger, heavier, insect borne pollen found in many flowering perennials and shrubs, which is not wreaking the same havoc on humans because it isn’t getting in our lungs. This is the pollen spread by all the beautiful and good invertebrates we are trying to bring back from the brink of extinction. Or at least we say we are.

It’s not that all airborne tree pollen is bad and all insect borne flowering plant pollen is good. That would be too easy. But in Tom Ogren’s perfect world, street trees would be female and thus, pollen free. He has developed a numeric ranking of ornamental plants by how much problem pollen they disperse, called, after himself, OPALS, the Ogren Plant Allergy Scale. His book ranks a number of trees, shrubs, perennials, and even bulbs according to this one to ten scale (where “one” is good).

Your own personal allergies are important to consider when purchasing trees for your garden, but urban school grounds where kids exercise are critical in terms of exposure to excessive amounts of airborne tree pollen. Male clonal cultivars in these environments may cause a disproportionate amount of harm to young people whose immune systems are still developing. And some of these male trees are native plants. Indeed, it is nearly impossible to purchase seed-grown tree stock these days now that cloning and tissue culture are the norm. It’s a thorny issue. 

If you purchase Ogren’s book, keep in mind that he is California based and many plants listed are known invasives in our Mid-Atlantic region. Perform due diligence on any low-OPALS rated plant to be sure it is not going to create problems in your yard or your neighbor’s yard. Running bamboo is a case in point. It has a low OPALS rating but I would never recommend it, even with a so-called root barrier. Still, the OPALS rating is the first of its kind and has been endorsed by the USDA, which is a tad ironic. 

National Pollinator Week

Now that you understand that we can dislike some pollen and not others, June is the month to celebrate the good pollen and the pollinators we love. National Pollinator Week is June 15-21 this year. And what would Capitol Hill-style Pollinator Week be without an Executive Memorandum? The “2014 White House Directive on Dwindling Pollinator Health” is our President’s effort to get federal agencies working together to address this serious environmental and economic issue. 

Mark Cason, the American Society of Landscape Architects’ liaison to the U.S. Department of Transportation on the pollinator action plan, is advocating for the “Highway BEE Act” in the current congressional session. This effort would allow willing state departments of transportation to change their highway maintenance practices to reduce mowing, which cuts off flowers needed by pollinators, plant pollinator friendly plants on roadsides, and use IPM, or integrated pest management in lieu of harmful herbicides and pesticides. Cason says the goal is to include pollinator-friendly language in the Surface Transportation Reauthorization Bill, MAP-21.

You can celebrate National Pollinator Week by attending several events at the Smithsonian’s Museum of Natural History including an outdoor Pollinator Party on June 17 from 10am-1pm. Or, you can attend a Capitol Hill Congressional briefing hosted by the Pollinator Partnership on June 20 from 3-5:30pm at the Longworth House Office Building. For more, see:

How to Attract Pollinators to Your Garden

You can find resources at the National Wildlife Federation to support pollinators in your yard. See: When purchasing plants, be sure they have not been treated with a prevalent new class of insecticides called, “neonicotinoids.” These are, according to the Xerces Society, “systemic chemicals absorbed by the plant, and dispersed through plant tissues, including pollen and nectar.” See: This means you can buy a native Aesclepias tuberosa, or butterfly weed, and if it’s been treated with a neonicotinoid, it will not do your monarch butterfly a bit of good. Home Depot is now labelling their plants treated with this class of chemicals. Ask your local garden center before you buy plants, so you can be assured you really are providing the help you intend.

It’s not easy to do the right thing, but knowledge is power. Homeowners can buy wisely by reading books and labels. Landscape architects and designers can specify plants carefully. Garden centers can eliminate harmful chemicals from their supply chain. Governments can be mindful of unintended consequences of their policies. And we can all take some time to learn more than what we think we know. Happy pollinating!

These checkered-fringe prominent caterpillars (Schizura ipomoeae) hide from hungry birds by disguising itself as a dead leaf.
Yellow swallowtails in a feeding frenzy on native Joe-Pye Weed (Eupatorium purpureum)

Cheryl Corson, RLA, ASLA, is a local licensed landscape architect working on Capitol Hill and beyond. The more she learns, the less she thinks she knows. With her designs, she tries to “first, do no harm.” See:

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