Garden Guru: Thomas Ogren

By Cathy Cromell

Before Thomas Ogren wrote Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping, he wasn't particularly sympathetic to those who suffer from allergies, including his wife and other family members. "I had read that allergies were psychosomatic, so I believed that," he states. That erroneous belief started changing 20 years ago while Ogren was teaching horticulture to prison inmates in California. "I asked a big, muscle-bound gang member to pull Pride of Madeira weeds," he remembers. The inmate wanted a different job because he had gotten a terrible rash the last time he pulled the weeds. Ogren rubbed some of the prickly leaves on his own arm. "It flamed up, so I gave the inmate another job."

The next week Ogren drove to the Bay Area from his home in San Luis Obispo, on freeways lined with acacia in full bloom. "I had always liked the monoculture look of mass plantings, but when I arrived in Berkeley, everyone was sneezing except me," he describes.

These back-to-back experiences gave him the idea to do "sniff" tests with his class to determine if certain plants created problems. "We went outside and sniffed everything," he explains, "and a third of the class started sneezing uncontrollably from bottlebrush." Intrigued, he continued the sniff tests for a year with anyone who would volunteer, learning which plants typically triggered reactions. He stopped when a few people became ill.

"I felt so bad about the lack of empathy I had shown my wife that I replaced all the plants in our yard that seemed to cause problems." There was significant improvement in her allergies. Ogren's brother-in-law also replanted his yard. "For the first time in his adult life, he wasn't taking antihistamines," recalls Ogren. Fascinated by these results, Ogren talked to botanists, horticulture professors, and allergists, but there was little data available on plant allergies. His research led him to develop OPALS (Ogren Plant-Allergy Scale) to categorize plants numerically from 1 to 10. The higher the number, the more likely that the plant will cause allergy symptoms. Writing a book was the next step. "I ended up writing the book that I had wanted to buy," he states. "I received 326 book rejections over a 12-year period, but I kept working on it because the topic fascinated me."

Ogren became hooked on gardening at a young age. He started sowing seeds when he was 5 years old and remembers his grandfather giving him a strawberry guava tree when he was 7. He obtained a Master's degree in agriculture and horticulture from CalPoly-San Luis Obispo, later co-owning a retail nursery in Minnesota, and selling grape vines through a mail-order nursery in Los Angeles. Plants have never made him sneeze.

Unfortunately, allergies are on the rise in this country. In 1959, 2 to 5 percent of the population suffered from them. In 1984, the number was 12 to 15 percent. In 1999, it spiked to 38 percent. Ogren attributes these increases to major changes in our landscaping practices, in particular, the exclusive use of male plants, and monocultures - mass groupings of the same type of plant. Plant breeders developed male clones to avoid the clean-up required with growing female plants, which bear messy seed pods or fruit. However, males produce prodigious amounts of pollen, which causes allergy symptoms when the microscopic grains are inhaled. Monocultures aggravate the problem by increasing the opportunity to be overexposed to an allergen.

Ogren has some suggestions for reducing the allergens in your landscape:

  • If the plant variety you desire has female flowers on one plant and male flowers on another, choose the female plants because they will produce no pollen. Ask your nursery or garden center for female ash, willow, mulberry, juniper, and maple trees.
  • Plant a diverse garden so that you won't end up with large numbers of any one allergen-producing plant.
  • Select plants that are low-allergen producers. If you choose a high-allergen plant for some reason, install it at the edge of your property, away from windows and doors.
  • Choose plants that are well-adapted for your area and maintain them properly. Stressed plants are attacked by pests, such as aphids, which produce honeydew. Mold quickly grows on the nutrient-rich honeydew, producing spores that are allergenic. "Also, a stressed male plant will put out 2-3 times the normal amount of pollen before it dies in an attempt to reproduce itself," states Ogren.

For more information on allergy-free gardening, visit Ogren's Web site at

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