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  • Writer's pictureJefferson Landscape

Dealing with Allergies

The pollen count has to elevate considerably before I notice it, because my allergies are mild. That’s not the case for members of my family, whose reactions are much worse. Myriad and sundry symptoms appear with little notice in varying degrees: swollen eyes, a runny nose, a cough, migraine headaches, a rash, a scratchy throat, sneezing and wheezing. It's the type of misery only relieved by introducing anti-histamines or some other prescription.

Allergy symptoms occur when the immune system overreacts to an allergen. It may be harmless to most people, but if you’re one of the unlucky ones and your body is “allergic” or sensitive, it will trigger a reaction. Seasonal allergies are typically caused by pollen, an airborne allergen picked up and carried by the wind, created by various trees, weeds and grasses. If you’re allergic to pollen, your immune system considers it dangerous and releases a chemical called histamine to counteract it. The release of histamine can cause a variety of symptoms.

In some folks, pollen allergies cause severe reactions, making it uncomfortable and/or difficult to breathe. Hay fever (rhinitis), happens when the mucous membrane inside the nose becomes inflamed. The inflammation results in large sums of mucus, commonly producing a runny nose, a stuffy nose and/or a post-nasal drip. It also affects the eyes, causing watery, reddened, itchy, puffy eyes. Furthermore, asthma attacks can be triggered and indeed worsened for those who have allergies, because allergens increase the inflammation of the airways. In general, inhaled allergens cause allergic asthma. Thus, if you have asthma and can avoid coming in contact with an allergen, you may also be able to prevent the symptoms of an asthma attack.

Whether your allergies are of the irritating variety, or those that are more severe, Jefferson Landscape and Design is happy to assist our clients in selecting trees and plants with less pollen, both in the effort to inform those who need the information, and to design planting schemes less troubling for those whom allergies require careful consideration.

It’s always helpful to be prepared for when allergies are more apt to rear their ugly head by having access to and using the National Allergy Forecast. They even offer alerts for those who wish to sign up using their email alerts.

According to Thomas Ogren, a horticulturist who invented the Ogren Plant Allergy Scale (OPALS), pollen is the most common allergen we are exposed to. Ogren argues, we can control it by adjusting our landscapes to reduce plants that cause allergies. His books, The Allergy-Fighting Garden: Stop Asthma and Allergies with Smart Landscaping and Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping, list over 5,000 plants and rank them by allergy.

There are three common types of flowering systems that we should be aware of when choosing plants: perfect-flowered plants, monoecious plants, and dioecious plants.

Perfect-flowered plants: In perfect-flowered plants, male and female parts are in the same flower. Because the pollination process is relatively contained, these types of plants are often not highly allergenic.

Monoecious plants: In monoecious plants, male and female flowers grow on the same plant. Monoecious plants that are wind-pollinated are likely to cause allergies.

Dioecious plants: These plants are separate-sexed. Some are male, and some are female. The male trees produce pollen, and are often highly allergenic, while the female plants produce fruit or seeds and are not allergenic.

If you are interested in planting a dioecious plant and are prone to allergies, make sure it’s a female (easier said than done). The main thing to look for when determining the gender of a plant is pollen. If a plant has pollen it is a male (or part male in the case of monoecious and perfect-flowered plants). If a plant has seeds or fruit, it is typically a female (again, unless it is monoecious, a mixed gendered plant).

The OPALS Allergy Index Scale ranks plants from 1-10. Plants ranked 1 or 2 have very little potential for causing allergies, while plants ranked 9 or 10 are the most allergenic and can often cause hay fever and asthma.

OPALS consider a variety of factors when ranking how allergenic a plant is: the amount of pollen produced, potency of the pollen, how much of the year the plant is in bloom, size of the pollen grains, gravity of the pollen grains, stickiness and dryness of the pollen grains, type of flowering system, effect of sap, and smell of flowers, among other things. It's Quite a List!!

Jefferson Landscape and Design recently installed a project for a family whose daughter had severe allergies. The following is a partial list from that project and their corresponding OPALS Index number.


Pyrus Calleryana (Bradford Pear) - 4

Lagerstroemia (Crape Myrtle) - 5

Arbutus Unedo (Strawberry Tree) - 3

Persea Americana (Avocado Haas) - 2

Prunus Kwanzan (Flowering Cherry) - 5-6


Camelia Japonica (Camelia) - 3

Anisodontea Capensis (Cape Mallow) - 3

Viburnum Opulus Roseum (Viburnum) - 4

Viburnum Opulus Sterile (Viburnum) - 4

Abelia Grandiflora (Abelia) - 4


Hydrangea Varieties (Hydrangea) - 3

Hydrangea Quercifolia (Hydrangea Oakleaf) - 3

Azalea Varieties (Azalea) - 3-4

Hibiscus Varieties (Hibiscus) - 3

Leptospermum (Australian Tea Tree) - 4

Penstemon Varieties (Appleblossom) - 2

Salvia Varieties (Salvia) - 1-4

Plumbago Auriculata (Cape Plumbago) - 3


Verbena Canadensis (Verbena) - 3


Ophiopogon japonicus (Mondo Grass) - 3

If you or a loved one have an allergy concern, finding this stuff online can be tough. Jefferson Landscape and Design recommends buying one of Thomas Ogren's books, if for no other reason, it will likely be the only source/s available to access the OPALS Index.

Succulents are almost always ranked as a 1 (completely pollen-free), and Ogren breaks down several monoecious plants (where male and female flowers grow on the same plant), i.e., the Clematis vines, where males rank an 8 and females only rank a 1. The trouble is, it is virtually impossible finding a nursery, wholesale or otherwise, that can order a plant based on gender.

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