Times Past

The trees of San Luis Obispo: What’s allergic, and what’s not

The padres always planted their olive trees outside of town, away from the missions.

Even then, they were aware that olive pollen made many people sick. Last Sunday’s column about allergy-free landscaping has been generating a lot of interest. In this column, with help from professor Matt Ritter and horticulturist Tom Ogren, we’ll continue to look at a few of the most unusual trees in San Luis Obispo.

At 274 Chorro St. is the biggest and only Beechnut tree in the whole town. Beech trees have beautiful, smooth gray bark that unfortunately seems to invite carving names in their big trunks. Beech are not considered allergenic. Also on Chorro are several large red-flowering eucalyptus trees (eucalyptus ficifolia).

Most eucalyptus shed quite a bit of pollen, and tiny insects live on this pollen. The insect dander is more allergenic than the pollen. These red-gum trees, however, have very large, sticky pollen grains, and do not trigger allergies. There are many Chinese pistache trees along Chorro. Pistache are separate-sexed, and the female trees make lots of small, red berries. Related to poison oak and poison sumac, the pollen (only from the male trees) is quite allergenic.

The berries on the female trees vanished a few weeks ago as migrating cedar waxwings ate them all. Marsh and Higuera streets have many large ornamental fig trees. While our Higuera Street is named for the family of Tomas Higuera, the name means “fig,” the precious fruit of the Mediterranean world.

These old evergreen trees shed almost no pollen. Speaking of fig trees, there’s an old fig tree at the mission. This productive tree is a clone from the original black mission fig. Other clones of this tree are growing in many yards in SLO. None make allergenic pollen, although their white sap can trigger rash or itch.

California Street near Cal Poly is lined with big, old Canary Island date palms. These are separate-sexed, and it is easy to spot the female trees, the ones with the small bright orange fruits. These dates are not good to eat, alas, but the trees are majestic. Pollen from the males is allergenic, but there is a good balance here of the sexes; most of the pollen is trapped by the female trees.

At 237 Del Mar Court is the largest and oldest dawn redwood tree in California. Thought to be extinct before 1954, it was planted by botany professor Robert Rodin. Across from this address is a fine, large all-female deodar cedar tree, planted by Professor Ralph Vorhies. At the very end of the street, is the largest California buckeye tree in the county.

The 125-foot tall Mexican fan palm at Pacific and Garden streets is the tallest palm in SLO. Mexican fan palms are perfect-flowered, and allergy friendly. Orioles, kestrels and barn owls all like to nest in these palms. These tall palms were brought by the padres to serve as landmarks. Palm Street in SLO was named for the palms.

At 1060 Pismo St. is an amazing old western redbud tree in bloom right now. Redbuds are insect-pollinated and allergy-friendly. Across from this location is Mitchel Park, home of the largest evergreen ash tree in the city. This tree, a male, is in bloom now and shedding pollen.

There are over a thousand ash trees planted in SLO, and all of them, except a small handful, are allergenic material. In the front yard at 975 Broad St. is probably the largest loquat tree in town. Related to apples, loquats are perfect-flowered and not allergenic. Locquats were introduced by the padres.

There are many fine orange trees in SLO, and at the Jack House is perhaps the largest pomelo tree in town. Pomelo, citrus maxima, has the largest fruit of all citrus. Citrus trees rarely cause any allergies, and most of the seedless types are pollen-free. Our last article drew attention from far and wide. New Yorker, Dr. Clifford Bassett, frequent expert guest on Good Morning America, and described by Dr. Oz as “America’s Allergist,” weighed in via email.

“We are seeing a global rise in pollen-allergies and allergic asthma. We need to use fewer male plants and more females. And, especially in communities and near schools, we should utilize OPALS, the Ogren Plant Allergy Scale.

Dale Sutliff, landscape architect and professor emeritus at Cal Poly writes: “I encourage the City of SLO to explore how our city might accomplish a more livable community by creating a more allergy-free environment. For a long time, our focus has been on drought-tolerant and sustainable.”

I would add that given the ongoing epidemic of allergies and asthma, our current type of landscaping is no longer sustainable. To quote Professor Walter Lewis, author of “Medical Botany,” “It makes no sense to plant allergenic trees or shrubs where we live, and our children play.”

Tom Ogren, Matt Ritter and Dal Sutliff contributed to this article.

This column is by Liz and Dan Krieger. Liz is a retired children’s librarian, and Dan is Professor of History, Emeritus at Cal Poly State University, San Luis Obispo. He is past President of the California Mission Studies Association, now part of the California Missions Foundation. He can be reached at slohistory@gmail.com.
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