The purpose of this publication is twofold. It will first explore general information on plants and their allergenic properties, and second, it will be a quick ready-reference to allergenic plants that could create a problem for you. Many people still believe that allergies are all in someone’s mind. However, many are found in our own backyards. It is important to remember that pollens cause not all allergies. Dust, molds, mildews, and animals cause many.
There are many misconceptions in the Southern Nevada area about which plants do and do not contribute to allergy problems. One must first understand allergies and how people are affected. Determining what causes an allergy may be difficult for most people, as the reactions may take some time to surface. Most people have immediate reactions that show within thirty minutes. Others can have delayed reactions, taking many hours for symptoms to appear. Some allergenic reactions to newly introduced plants may take years before showing up.
Such was the case during World War II when US farmers began growing caster beans for the war effort. It was only after a few years that the real impact of the pollen from these large plantings of caster beans began to affect the people around the areas where they were growing. It has been known for years that olives and fruitless mulberries cause allergy problems in southern Nevada. Both of these trees grow well in the southern Nevada climate and soils. However, much of the population suffers from the allergens they produce. Part of the problem is that these trees have been overplanted, much like the WWII plantings of castor beans, without taking care to select those that would have less pollen. Now they have been banned or highly discouraged in many communities.
Recently there has been an effort to find and use a pollen-free olive (Swan) but no known effort to find low-pollen producing fruitless mulberries. Male trees are often planted because people think they will require less care, as they do not produce messy fruit to clean up like the female trees. But the pollen they produce can be more of a problem for allergy sufferers. Some references suggest that trees and shrubs, such as ash, poplar, and willow, should not be planted because of the pollen the male trees produce. They overlook the fact that the female tree of the species would be just fine. With few or no males planted in the area, the fruit product is reduced or eliminated.
Part of the problem is that we keep searching and selecting what we think will be a maintenance free tree or shrub. By doing so, we often cause more problems. A dioecious tree or shrub is one that has male or female flowers on separate plants. Because the female and male flowers are found on separate trees, the males produce flowers with large quantities of pollen to insure pollination of the female plants. This factor makes our maintenance-free plants now an allergenic problem. Add this to all the other air pollution problems, and the combination becomes a serious problem in urban areas. Without good horticultural planning, city and state parks and developers can create problems that result in the suffering of many people.
The answer is not banning plants, but understanding how and why plants cause these problems, from a sound environmental horticultural perspective.
Allergy sufferers should not have to live in sterile landscapes with few if any plants. Landscapes can and should be beneficial to the total environment. When buying plants one should always buy by scientific name and check a good reference book. Sir Francis Bacon said that “Gardening is the purest of human pleasures" and so it should be, but will not be as long as one cannot go outside because of allergies.
Allergies will continue to be a problem as more of the population ages past 50. Those without allergies now will begin to develop them. This problem will continue to escalate as the population continues to age and people continue to be exposed to chemical pesticides, diesel fumes, and waste gases. Even the rubber particles that wear off auto tires contribute to the problem. It is interesting that these rubber particles can cause allergic reactions to the pollen of plants that are related to the true rubber tree in the landscape. There also seems to be a correlation between many of the desert legumes that are planted because they make their own nitrogen. They can also contribute to allergies with peanuts. To much of anything may cause problems.
Avoidance is the key to allergy relief. There still are those who say there is little we can do to change this problem, as most air-born pollen is from weeds that come in from undeveloped areas. It may seem unlikely that reducing the number of allergenic plants in a single yard would make a difference. However, a survey in San Luis Obispo, California, (by Thomas Leo Ogren) showed that the expected weeds that are blamed for allergies such as ragweed were not to be found. The allergy problems were from common landscape plants. This is true of Southern Nevada. It was found that a male pepper tree (Shinus mollie) in a yard exposed the homeowner to ten times the pollen than one in the neighbor's yard. Eliminating allergenic plants close to homes can reduce symptoms. What you plant does make a difference. Just as a new urban forest begins with a few trees planted in one yard, so does an allergenic free environment begin in one yard, and spread to an allergenic free city.
For more in-depth information, check your local library or bookstore for “Allergy-Free Gardening,” by Thomas Leo Ogren, or the internet “Allergenic Plants in landscapes.”
Criteria to look for when buying plants for the landscape (what not to buy)
- Most plants that have off-white and greenish colored flowers depend on wind rather than insects to be pollinated.
- Most male plants produce more pollen than complete flowered plants or monoecious (bi-sexual) plants.
- Monoecious plants where the male flowers are below the female are also a problem.
- Never plant medium and high rated allergenic plants near windows, especially bedroom windows.
- Plants to look for:
- Plants that have both female and male flower parts on the same flower (perfect flowers) or that have large or sticky pollen grains.
- Flowers that are trumpets shaped (they contain the pollen deep in the trumpet) and depend on pollinators like humming birds.
- Monoecious plants that have the pollen above the female flowers (like corn). They depend less on the wind for pollination and more on gravity.
- Female plants (in dioecious plants those plants with male and female plants), have only female flowers. (Many of the banned plants are the males of these plants.)
- Flowers that are fragrant have less pollen, as they depend on insects that are attracted by the fragrance to transfer the pollen from one flower to another.
- Add diversity to your garden and encourage it in your community and parks. The main reason that many of these offending trees and shrubs are a problem is because of overplanting. Diversity is one way to alleviate this problem and creates better urban wildlife areas. Nature is diverse, and this helps prevent the rapid spread of destructive insects and diseases that have one host throughout communities. Even a medium rated allergenic plant can be a problem if over planted.
- Colorful flowers, as they are most often pollinated by insects or birds.
- Choose grasses that are propagated vegetatively, and are only female.
Landscape maintenance to help with the problem
The following list should help in achieving an allergy-free yard. As most pollens travel only short distances just landscaping your yard with low allergen plants will help. All species of grass that produce male flowers can be allergenic, but some like Bermuda are more prone to causing allergies.
- If the grass is in the landscape now, mow it often to remove the pollen-producing parts as they appear, and before they open. Allergenic grasses could be replaced with either non-flowering hybrids or female only cultivars.
- Natives such as jojoba and rabbit bush are very allergenic. Just because a plant is native doesn’t mean it is safe to use in the landscape.
- Succulents and cacti do not produce wind-born pollen and are excellent low allergenic plants. They will also reduce water use.
- Fruit trees are insect pollinated and are not allergenic to most people. However, nut trees are wind pollinated, and can be allergenic.
Other ways to reduce allergenic problems in the landscape
- Work in your yard only when pollen count is down. Cool, cloudy, windless days are best. Another good time is just after a rain, or when the humidity is high.
- If you use an antihistamine nasal spray, do so prior to going outside.(Be sure to follow directions on the spray.)Wear goggles and / or a respirator mask to help.
- Keep your hands away from your face (especially your eyes) while working in the yard.
- Remove all flowering weeds.
- Water soil and organic mulch to keep dust down.
- Trim allergenic plants to help prevent flowering.
- Use organic mulches such as wood chips and bark to cover bare soil.
- Remove work clothes as soon as you finish the yard work. Place them in the washer. Then shower, including shampooing hair.
For a complete list of "low, medium and high allergenic plants" use the link below to download the PDF version of this factsheet.