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Selecting
Landscape Trees with Asthmatics and Allergy Sufferers in Mind
About 20 years ago horticulturist and author Thomas Ogren became
interested in allergy-free gardening because his wife suffered
from allergies and asthma. He began research and went back to
college to get his MS degree, focusing on plant/allergy connections.
He discovered that many 'dioecious male' trees triggered severe
allergies. Ogren lives in San Luis Obispo, CA. His books have
been reviewed in dozens of publications, been the focus of a CBS
Evening News special, and the Discovery Channel in Canada filmed
a documentary about his research results. Pollen is the main culprit in allergies (although molds are also
important). In 1972 a pollen scientist named Raynor demonstrated
the importance of locality in allergy. A simplification of his
findings boils down to: a male pepper tree in bloom in your yard
will expose you to ten times as much allergenic pollen as a similar
pepper tree in a neighbor's yard down the street. As gardeners,
we can control what we plant and dramatically limit allergens
in our own yards: the fewer allergenic plants in the garden, the
less chance of exposure. Ogren realized that the allergy problem was increasing because
male trees, which do not produce fruits, seeds, messy flowers
or old seedpods, were being planted by the millions in California
cities and landscapes. Perhaps we should digress and briefly discuss
the sex of plants, since most people don't realize that some are
male, some are female, and many are bisexual. Perfect flowered. Many flowers, such as apple blossoms
or roses, have both male and female parts all inside the same
flowers. These flowers may also be called bisexual or complete.
The pollen in a perfect flowered plant doesn't have far to travel,
and often, but not always, these plants cause few pollen allergies.
Monoecious flowered. In Latin monoecious means "one
house". In this system there are separate male and female
flowers; however, they are both on the same plant. Corn is a good
example of a monoecious plant. The tassels are male flowers and
the silks in the undeveloped ears are female flowers. The pollen
from the tassels drifts down to fertilize the silks and when one
silk is pollinated a kernel of corn develops. Oak and cypress
are two examples of monoecious flowered trees. They are pollinated
by the wind and each is capable of causing allergy. Dioecious flowered. In this system, plants have flowers
that are all male or all female, and the plants themselves are
either all male or all female. In most cases, the pollen from
male plants is carried to the female plants by wind. Dioecious
plants include ash, willows, poplars, hollies, pepper trees, some
maples, mulberry, all ginkgos, many palms, and others. Pollen
from dioecious males is typically quite abundant and causes a
great deal of allergy. Ogren's discovery. The relationship of plant sex to allergic
reactions bears repeating. Ogren discovered that many 'dioecious
male' trees triggered severe allergies. Male trees shed a lot
of pollen. All the flowering plants, except 'dioecious females',
have pollen. Through his intense research Ogren found several
trees to be pollen-free or close to it. He rated more than a thousand
species of plants on a scale of 1-10 with 10 being severely allergenic.
A few examples of trees with a rating of 1-3 are firs, cedars,
and strawberry tree (Arbutus unedo), as well as the fruit
trees like apricots, avocados, peaches, plums, nectarines, persimmon,
pineapple guava, but also almonds and flowering red leaf plums,
bronze loquat, and flowering pears. Also the female trees of ash,
Chinese fringe tree, Chinese pistache, junipers, some named maples,
palms, podocarpus, poplars, sour gum or tupelo, and willows. Some of the worst trees are acacia, alder, beech, birch, buckeye,
CA pepper, camphor, catalpa, Chinese evergreen elm, cypress, liquidambar,
mimosa, oak, olive, pecan, sycamore, walnut, and zelkova. Ogren's Best Plants. In general, the best plants for an
allergy-free garden are those that produce large, very showy,
lightly scented flowers, in which the male flower parts are either
few in number or deeply recessed within the blossom (these are
usually pollinated by insects and rarely cause pollen problems).
Then there are the separate-sexed species, the 'dioecious females'
that do not produce pollen. They are often commercially named
or are easy to distinguish from the male tree (because they contain
fruit clusters). Be aware that there are drawbacks to female trees.
When pollinated they produce fruits/seed capsules that can be
troublesome. Research your species before you plant as some trees
are considered messy, while the fruit of another species are not
a bother at all. Reference: Allergy-Free Gardening by Thomas Leo Ogren, 2000.
Ten Speed Press, Berkeley, CA November 6, 2003 |