Green Team 15: Low-allergy Landscapes

The battle of the sexes that produces human discomfort in springtime

After a cold and dark winter season, we welcome the sun in the northeast. Buds appear on trees, daffodils show their faces, and birds return from their southern migration. Having spent months indoors, we run to our parks, gardens, and waterfronts.

Our last post updated the work we did to ready New York’s beaches, post-Sandy, just in time for Memorial Day.

For seasonal allergy sufferers, this time of year is not all fun in the sun. Spring fever can mean hay fever and a season of congestion, headaches, runny noses, and watery eyes.  As predicted, 2013 is shaping up to be one of the worst allergy seasons on record in the northeast, amplified by the climate change that also brought us Sandy.

More from Metropolis

What can we do as landscape designers to help alleviate this escalation in allergies? Ask our campus clients, developers, and other large-scale landowners. Their requests have opened our itchy eyes to a plethora of misinformation associated with plant-based seasonal allergies and the species that are responsible.

Pollen–the fine powdery substance released by the male component of a flower to fertilize the female component–is the main culprit in seasonal allergies. In order to understand why some pollen causes allergies and other types do not, we must look at the reproductive and gender variations that occur from species to species.

In his book Allergy Free Gardening, Thomas Leo Ogren defines three sex categories for plants: perfect flowered (both male and female parts inside a single flower), monoecious (male and female parts on the same plant), and dioecious (male and female parts on separate plants). If you’re thinking this sounds more like a lesson on the birds and bees than a blog on allergies, you’re right! The pollen needed to pollinate a perfect flowered plant (like lilies) has a very short distance to travel, so it makes sense that these plants rank lower on the allergy scale. The pollen of monoecious and dioecious plants must travel farther to fertilize their counterparts, using pollinators like birds, bees, and the wind to carry the pollen.

Non-flowering plants like ash, willows, some maples, oaks, and cypress typically use the wind to pollinate, so they are more likely to cause seasonal allergies than insect-pollinated plants. By design, their pollen is light, small, and often barely visible, making it a nuisance to nasal passages and sinuses. Factors like flower position on the plant and whether the pollen has to rise or fall to fertilize can also influence a plant’s allergy potential.

Contrary to popular belief, plants with bright flowers like sunflowers and black-eyed Susans (Rudbeckia hirta) that use color to attract birds and bees are less likely to cause allergies. This pollen attaches itself to an insect pollinator and is transported to the female component, completing the reproductive cycle. Sticky and heavier than airborne pollen, it is less likely to find its way to your nose.

The bright yellow flower of the black-eyed susan Rudbeckia hirta attracts bees

Courtesy Mathews Nielsen

Now that we understand how plants transfer their pollen, it’s important to know when they produce pollen. Most trees release their pollen in early spring. Grasses release pollen in the late spring and early summer, while ragweed carries us into late summer and early fall. However, there are some plants, like the evergreen eucalyptus, that produce pollen year-round and are a continual problem out west.

When we look to identify the plants pollinating our nasal passages, we consider factors related to sex, gender, and season. But even with all this information, we still get it wrong sometimes. For years, goldenrod, with its bright yellow blooms, has been blamed for late summer allergies. It turns out that the heavy pollen of the goldenrod is transported by bees.The real culprit is ragweed, often found blooming alongside goldenrod. Ragweed pollen is so profuse it receives the worst possible allergy rating from Ogren.

Despite our understanding of pollen types and cycles, selecting plants is a complex process, and allergy potential tends not to be the most important factor when plants are chosen. Allergy potential, especially in New York City, is trumped by urban tolerance as well as disease and pest resistance. Many of the wind-pollinated trees, such as London Planetree (Platanus x acerifolia), are among the most frequently planted species in our city.

New York City’s most popular tree: the London Planetree, in Duane Park

Courtesy Mathews Nielsen 

In addition, we face a restriction in planting high allergy species like the female-ginkgo (Ginkgo biloba), a dioecious species and messy fruit tree. So, we’re limited to planting male ginkgoes; but pollen is produced by males, not by females. By exclusively planting male ginkgoes, we’re potentially increasing the amount of pollen in the city. This policy goes against Ogren’s theory of “no males, no pollen, no fruit, no allergies,” which hypothesizes that if we didn’t plant male trees, there would be no pollen, which in turn would cause the females to stop producing fruit, leaving us fruitless and with less tree pollen.

While we can’t avoid planting species with allergy potential altogether, we can focus on how these species are sited in a plan. At the Hunter College School of Social Work, we planted low allergy trees near the air intake vents. We designed with a palette of serviceberry (Amelanchier canadensis), summersweet (Clethra alnifolia), Japanese anemone (Anemone var. sylvestris), and vinca (Vinca minor).

The Hunter College School of Social Work courtyard and its air intake vents 

Courtesy Chuck Choi

Planting design is always a balancing act. It requires the consideration of factors like solar aspect, USDA zone, design, program, and maintenance requirements.

If recent trends are any indication, we’re going to be adding allergy potential to this list more frequently. 


In our next post, we’ll provide an in depth look at the various grades of stainless steel.


Johanna Phelps is a landscape architect at Mathews Nielsen Landscape Architects in New York City.  Since receiving her MLA from the University of Pennsylvania in 2004, Phelps has worked on urban campus projects in Manhattan, Brooklyn, and Philadelphia, a botanic research institute in Texas and a public plaza in Bilbao, Spain. 

This is one in a series of Metropolis blogs written by members of Mathews Nielsen’s “Green Team,” which focuses on research as the groundswell of effective landscape design and implementation. These posts explore design challenges the Green Team encounters and how it resolves them. Along the way, the team also shares its knowledge on plants, geography, stormwater, sustainability, materials, and more.

Recent Projects